Bad Alternator Diode causing transmission/speedometer problems

Speedometer reads 20 mph in park. Transmission output speed sensor (vehicle speed sensor) codes. Transmission will not shift (2nd gear limp mode) from a bad alternator diode.
Clean, strong alternator output is critical for vehicle electrical performance. Without it, you can run into all kind of strange symptoms that you may not necessarily associate with an alternator problem, and this case study is a perfect example. I also supply some general guidelines for ripple testing that can be applied to any make or model out there.
Engine Performance Diagnostics chapter 1 page 27
Symptoms
- speedometer moves off of zero with the vehicle stopped and in gear
- check engine light (EGR fault code, unrelated)
- transmission shifting problems
Fix
- replace the alternator (failed rectifier diode)
Tests shown
- how to check for AC ripple using a lab scope with AC coupling
- identifying excessive AC ripple in a sensor voltage waveform
- how to eliminate a failed alternator by disconnecting the control circuit
- checking AC ripple at the alternator vs. at the battery
Tools used
- Autel MS908 MaxiSYS Automotive Diagnostic & Analysis System - USA Version
www.jbtoolsale...
- PicoScope 4425 4 Channel Standard Diagnostic Kit
www.aeswave.com...
Playlist
- (Chapter 1) Misfire ID, Compression Tests, Vacuum Leaks, Fuel Trim, Head Gasket, Restricted Exhaust, Low Power, DMM
• (Chapter 1) Misfire ID...
Related videos
- How to test an Alternator (alternator not charging from a blown fuse) - Mazda
• How to test an Alterna...
- How to test an alternator (Chrysler Dodge Jeep)
• How to test an alterna...
- Computer Controlled Alternator Testing - 2006 Mercury
• Video
- Battery keeps going dead (what to test before replacing)
• Battery keeps going de...
For more information on this topic, I have written a “field manual” called Engine Performance Diagnostics which is available at www.scannerdanner.com as an eBook or paper book.
Want even more diagnostic training? Whether you are a DIY trying to fix your own car, someone looking to become an auto technician, or a current auto technician that wants to get more into diagnostics, subscribe to ScannerDanner Premium www.scannerdan... There is a 14 day free trial.
On ScannerDanner Premium I will bring you right into my classroom at Rosedale Technical College. You will find page for page lectures taken right from my book as well as exclusive classroom type case studies. What is so special about these classroom case studies? I pull live problem vehicles directly into my classroom and we troubleshoot them in real time, using and applying the theory and testing procedures we learn during the classroom lectures. There is no better on-line training of how to troubleshoot automotive electrical and electronics systems anywhere!
Disclaimer:
Due to factors beyond the control of ScannerDanner LLC, it cannot guarantee against unauthorized modifications of this information, or improper use of this information. ScannerDanner LLC assumes no liability for property damage or injury incurred as a result of any of the information contained in this video. ScannerDanner LLC recommends safe practices when working with power tools, automotive lifts, lifting tools, jack stands, electrical equipment, blunt instruments, chemicals, lubricants, or any other tools or equipment seen or implied in this video. Due to factors beyond the control of ScannerDanner LLC, no information contained in this video shall create any express or implied warranty or guarantee of any particular result. Any injury, damage or loss that may result from improper use of these tools, equipment, or the information contained in this video is the sole responsibility of the user and not ScannerDanner LLC.

[@credilider1746]

Man this guy ain't no mechanic, he's a damn scientist.

[@660at7800]

How many people would have replaced the speed sensor, then the computer, then the wiring, then told the customer it cannot be repaired. What a great find. This is a great video for everyone, but the really good techs appreciate it the most because they see the "out of the box" diagnosing you come up with to find these problems. Fantastic!

[@jorg0370]

I just diagnosed and fixed the same problem on my 2001 Chrysler Town & Country LX with 3.3L yesterday. The alternator was never suspected until watching this video. Many thanks to ScannerDanner!

[@ScannerDanner]

sweet

[@wyattoneable]

I'm being re-exposed to automotive mechanics and a lot has changed since the 70's! Great video.

[@smittyfixes8723]

Hey Danner, im a mechanic for a truck repair center in mckees rocks pa. I recently purchased a class A RV with a ford V10. Driving home it set an output speed sensor code and would stall. I replaced the output and input sensor. No fix. I redid some grounds and back probed the sensor wires. No good. I found your video and ran the same test. Sure enough it was the alternator. Im pretty sure thats why it was traded in. The former owners couldnt figure it out. Thanks for the info and i love your videos!

[@ScannerDanner]

Awesome! Sucks to be them, great for you. Nice job man

[@tmolne117]

Thank you for the video. This content really helps out, even 10 years later

[@exmotorsports]

Absolutely Awesome stuff Paul... It's something most would overlook too. Example: 04 MB C230 komp weird things would be effected like Windows and sunroof stop working..to dash lights randomly come on and off.. All caused by a bad alternator with high ripple. Other shops wanted to change out window modules and dash cluster.. Thank goodness she declined that. Easy fix too. Anyways, just wanted to share my experience with a bad alternator. Cheers bro..

[@PhattyMo]

Another thing that I've seen make newer cars freak-the-heck-out was bad battery connections,grounds,or ground straps. Keep them clean and tight!

[@bobstrohl3147]

Had this exact problem with our 2003 dodge caravan. Replaced the speed sensor but problem didn't go away. Also, showed transmission control module and 2 other codes. Luckily we found your video, exactly what it was doing. We replaced the alternator now everything is fine with no codes coming up.Thanks so much!!!!!!!!!

[@ScannerDanner]

awesome! Thanks for watching

[@jexuellrodriguez6921]

ScannerDanner excellent video bro!!! I got a Chrysler voyager 3.3 2003, I got same problem with my van, should I change my alternator??? please bro help me with this, if the transmission is on (P) or if I'm on a stop sign it mark on the speedometer 20 -25 mph....it's crazy and 10 min. later the trans stay on first gear!!!

[@jexuellrodriguez6921]

Bob Strohl hey brother, u just change ur alternator and fix the problem?? I got same problem...

[@bobstrohl3147]

Yes, we replaced the alternator and all the codes went away and the van is running fine now. I couldn't believe it but that solved all the problems,

[@ScannerDanner]

Jexuell, unplug the alternator connector (Not the heavy battery cable part) and start it, see if your problems go away.

[@Ironhorserider366]

This video saved me thousands of dollars. My 2004 Pacifica was giving me similar issues. First I was having a parasitic battery drain over night. Second. When putting car in Drive or reverse it was aggress. Third. While idoling at traffic lights the speedometer would jump up from 10 to 20 mph depending if I had the HVAC going. Fourth. At times while driving it out not get out of 2nd or 3rd gear. Or it was aggressively switching gears. Fifth. Getting transmission codes. So after alot of online investigation and coming across videos like this. I replaced my alternator and wow.... Car rides smoothly. Thank you,,, Thank you,,,, Thank you!!!

[@jonnyaykroyd]

Nice video Dan! Presumably the reason why you shouldn't do this test at the battery is because the battery acts like a big capacitor, smoothing out the peaks of the AC current. Would have been good if you had tried this with a digital multimeter, to see if the condition could have been seen without a scope in this case. I appreciate that in many cases a multimeter may not be fast enough.

[@josealers67]

ScannerDanner's EPD book is excellent. (Engine Performance Diagnostics)..His videos are excellent... Thank you again Mr.Danner.

[@ScannerDannerPremium]

Thank you Jose! Would you mind adding this comment to my comments page of my website?

[@josealers67]

No problem .

[@oskariolematon6265]

This exact issue happened to Dodge Neon, but apparently with crank position sensor. The car just did not run correctly, did not rew over 3000rpm and alternator worked only under 1100rpm and some other issues also. Changed a lot of things and nothing worked, until now, after seeing this video, unplugged the alternator wires and car runs great! THANKS!

[@ScannerDanner]

experience my friend. I have had my butt kicked by these before. Also the speedometer reading 20 mph sitting still is a red flag and I have seen this exact symptom on another Chrysler product.

[@abcdefgh9138]

Nice video, sir! I've encountered this on Caravans and the like. Good to know Chrysler is consistent across its lineup!

[@johnreid859]

I guess the best part of your video for me was how to test for a bad diode without the scope. I noticed the same problem with my speedometer. New alternator took care of it.

[@thatmechanicguy8773]

This is a seriously awesome test that I've never thought of doing. Thanks man.

[@chrisohlemacher3916]

Thanks for the video, great work ScannerDanner! I have a 2004 MINI Cooper project that I picked up for a couple hundred bucks because of a "bad transmission". I tried to reset the CVT adaptations but realized the Output Speed Sensor was reading zero...because the wires were burned up! Things I did to fix the issue...traced wiring looking for shorts, shorts to ground,etc. Replaced engine computer, engine fuse block, engine wire harness, transmission control computer, transmission valve body and of course a handful of sensors. Every time, a shorted out Output Speed Sensor shortly after starting (traveling no farther than 100 yards). This weekend, I replaced the alternator (I don't have the scan tools you have)...and this MINI is road worthy again. Something in the old alternator was causing the voltage/amperage spike in this car. I've got BMW software on my laptop and was able to watch the analog readings for the speedometer and the output speed sensor. It appears that all is good now...I was cruising the county roads yesterday for test rides!

[@ScannerDanner]

Awesome! What a nightmare that sounded like

[@TheCoatneyadkins]

Mr. Danner, my name is Cole... first of THANK YOU for your videos, I've learned so much b it's unreal.... I'm actually in tech school, I learn more watching your videos about electric/electronic circuitry than I do school.... honestly... but I am confused as to how you hooked up the leads on your scope to do the different tests, and in some of your other videos, i had a hard time keeping up because I couldn't figure out where you hooked up the leads of your equipment... other than that the info is incredible, thank you for not being stingy with this powerful knowledge!! I wish I could afford your field guide book, maybe when I get out of school i will be able to purchase that and study it. thanks again have a good one man!

[@shinola228]

Wow - you just saved me a lot time and money searching for the reason my transmission was not down shifting. I wouldn't have guessed it was the alternator in a million years- but that's exactly what it was. Thank you for making this video.

[@ScannerDanner]

You're welcome!

[@ScannerDanner]

Yes! When a scope is set to DC couple (default on most scopes) you can view both AC and DC signals.

[@ScannerDanner]

agree my friend. In this case I was simply focusing on the alternator diode test as a stand alone. Not my usual troubleshooting method for sure. It was late in the day. This car came in, I had a half hour to shoot. I needed this test to be shown. Next time I will do it from start to finish for sure.

[@rstar88105]

Some times I have used a battery, alternator, starter tester from Midtronics that has given an error message over two different instances before that read "excessive noise" with a car running which sounds like excessive AC voltage measured but both times the customer wanted no further testing done. It just made me think of how I wanted to further test the Alternator output as you described in this video. Thanks Paul.

[@jbroheretoday]

2004 Sebring had PO700 plus the codes for input and output sensors. Since the speedo was indicating 20 mph without moving I suspected more than the tranny controller. Glad I saw your video. Tested the alternator which showed bad diodes even though it was putting out sufficiently to keep the battery charged. A new alternator fixed everything including the whine that sounded like a torque converter. Thanks man.

[@JohnDoe-uq3mx]

Had one on my own vehicle about 2 days ago. Before the real problems started to happen, the RABS light would randomly appear then go away. Later on, the RABS would always kick into antilock mode while coming to a stop when there was no lock condition on dry pavement, which would cause the vehicle to lose more of its braking effectiveness as the vehicle slowed down. About a week later, the RABS would keep trying to unlock the rear wheels, while at a standstill at a red light. That ticking noise at a stop while holding the foot on the brake reminded me of what a grandfather clock sounds like. This was mind boggling. A few weeks later, at around 35-40 MPH, the speedo dropped erratically to zero while the transmission dropped into first gear, then the problem would clear up. This was a progressive RABS signal loss to where the computer still picked up the signal, but the RABS part of it was losing the signal. After swapping out a RABS sensor, the transmission wouldn't shift out of first, and with KOEO, the ABS dash light would stay on after the first few seconds. I must have shaken something loose in the process. So I back traced the two RABS sensor wires and I found an open. I found out that one of them 2 wires were slowly powdering up into corrosion on the inside of the original wire insulation, and kept building up more resistance as it continued to degrade. A replacement wire fixed it all. The car has about 252,000 miles on it, so something was bound to give. 2000 Ford Ranger. All of that because of 1 wire.

[@ziadin5366]

what a great diagnoser you are DANNER,,,thanx BLESS YOU

[@I88-4-88I]

Right on! 👍.... I have aslo seen a bad alternator cause EGR pentle faults, Backfiring through the exhaust, and all other sorts of strange things... Excellent job Sir.

[@ScannerDanner]

I would agree with Mr. Halderman. I just prefer to do it at the alternator BAT post. However this can be difficult to get to sometimes and the battery is your only option. Just be aware that it will be lower when measured at the battery

[@Ottonic6]

Great video. I wouldn't have thought the alternator would be the cause of this problem. But I also am not familiar on how a speed sensor works. So I will check that out. Thanks

[@idada459]

Great video Paul you are a blessing to all of us diy around the world keep it up!

[@ScannerDanner]

Thanks man!

[@tmr7990]

My town and country has the same things going on. Speedometer going crazy and the code for output sensor even after new sensors installed. Thank you so much!!

[@ScannerDanner]

Good thing you did some homework! Thanks and good providence

[@TheOvnicar]

HEY PAUL HOW R U ? ANOTHER AMAZING VIDEO MY FRIEND THIS IS WHY IM HERE ELECTRONIC DIAGNOSTIC GOD BLESS YOU FAMILY. THANK YOU VERY MUCH FOR YOUR VIDEOS

[@mctobbsi]

You are probably right! I will have to start with the regulator. Thank you very much again for the help and taking your time! Keep up the good work. I love your attention to detail and the thorough diagnostics you do!

[@smartchishaya1670]

One of the greatest man, great teacher you have greatly changed my life.well done .thank you

[@arbachesglaukus1324]

Without expensive diagnostic tools, anyone can check the charging system in 3 minutes using a $5 multimeter. Test 1 check the battery voltage , engine off, multimeter set at 20 V DC, the reading should be over 12.6 V. Test 2 check alternator charging with engine running. The reading should be between 13.5 V to 14.5 volts. Test 3 bad diode. With engine running test voltage at battery terminal for any AC voltage. Set multimeter at AC. Anything over 2 v AC indicates diode trouble. Mine was reading 30 V AC. Replaced alternator and the serpentine belt.

[@ScannerDanner]

without looking at the diagram I don't know if the voltage regulator supplies power or ground to the field windings of the alternator so I cannot answer that. you most likely need to replace the regulator. One that is overcharging is usually not a wiring problem, one that is not charging at all is where more detailed wiring tests are needed

[@psychofish21]

Great video! Thats why I love being a tech, always new interesting problems to keep you on your toes.

[@tigeroftheeye7121]

You can use the cheap Harbor Freight voltmeter and test ac voltage directly at the alternator post. Anything over .5 (1/2) volts AC will cause major Havoc with your electrical system. Just tested a Cadillac CTS with a ton of fault codes. Red 14.2 DC but 46 volts AC No expensive scope needed. Old diagnostic trick from the 90s in the car stereo business. Great video though. Most people Overlook the charging systems. Double check all of your grounds too

[@neilcoates5418]

Paul what setting do you set your volt meter at to read the AC? I'm dealing with something where when I take off from a start, all the gauges drop to 0 then come back and speedometer is erratic. Also some shift issues and things Im reading leads me to alternator.

[@mman454]

Wow I didn't even realize that was the alternator humming away until you unplugged it! I thought that was just someone running a shopvac in the background. Lol!

[@ScannerDanner]

Thank you! I just wish I would have shown the average AC voltage reading on this one. I forgot to do it.

[@ScannerDanner]

:-) I don't like getting my hands dirty. Had to in this case to get to the output speed sensor connector

[@dalix1951]

Awesome! Thank you Paul for sharing this. Best video & explanation I've ever seen on a bad diode.

[@ScannerDanner]

It is the Snap-On Vantage Pro (awesome tool!)

[@sleezybiker]

You do such a great job explaining things.

[@michelpiddy3587]

REALLY GOOD AND IM IMPRESSED WITH THE TESTING MACHINE USED ON THIS DEMONSTRATION

[@fredautos]

Great, very helpful! Thanks Mr Paul

[@elrober707]

Bad alternator on a 2012 Jeep 3.6l, caused codes poo19 & p000c. Erroneous crank sensor reading. Cheers! Mr Danner.

[@ScannerDanner]

+roberto MArtinez nice job dude

[@nickayivor8432]

Good Afternoon ScannerDanner have a great day 👍 Take care ScannerDanner 👍 🌟 🌟 🌟 🌟 🌟 Great tutorial thank you 👍 ScannerDanner From Nick Ayivor from London England UK 🇬🇧

[@660at7800]

Picoscope is by far the best scope out there. Any cheap scope that runs through a smart phone or iPad will work for some applications, but you will not be able to view most network communication including CAN, A/F sensors, 2-wire speed sensors, and a number of other circuits. The Snap-On Vantage Pro would be my second choice, and the Verus or Verdict would be 3rd. I use all three on a weekly basis.

[@FinalStopMobileAutoTech]

++Applause++ Another awesome video by Mr Paul Danner!! Excellent presentation!!!

[@22lambo]

Can a bad diode in an alternator permanently damage electrical components?

[@averyalexander2303]

Great question! Never saw that happen but I would think it could, especially over time. Batteries do not like AC current, it heats them up and doesn't charge them effectively. Also, DC capacitors such as the ones used in computer modules don't like excessive AC ripple and could suffer an early failure if subjected to an excessively high amount for an extended period of time. Some modules may have some type of built in protection built in, but I wouldn't count on it. Hopefully someone with more electrical engineering knowledge can chime in as I have wondered about this too.

[@MightyMao]

Damn!!! thanks for the video, I am having this issue since yesterday on my Pacifica 2006, was thinking on buying both input and output sensors, but now I see the light....just the alternator, great video!

[@wave3151]

didn't the spikes make 4v peak to peak ? seems to be 2v peak to peak without the spikes got your book and now i am interested in your lecture video badly. Your born to be a teacher Appreciate sharing your knowledge

[@rickbauza]

paul the technology on todays automobile electronic system has gone through the roof. techs coming out of school these days better be prepared to tackle these problems cause if they are working for a dealer flat rate system theyll be eaten alive.ive seen on some vehicles having a fuse for the charging system,and ive also seen the p.c.m in control of the charging system. great video my friend great job thanks rick.

[@nleonard86]

Paul's getting his hands dirty, 0:58. Not a common sight lol. Great video btw, very helpful info.

[@HouseCallAutoRepair]

This is SOOOO old school, and probably hard as hell for the newbies to comprehend... But this probably could have been solved with the AM radio!!!

[@ScannerDanner]

ahh I understand now. I'm not sure I can answer that. Didn't even pay attention to it until now. That number you were would have been a different number had I set my meter on an AC setting. Which is different than ac coupling a scope. I believe an actual AC setting takes the peak to peak voltage and multiplies it by .637 or something like that to get an AC average reading. On a scope if it is +2 and -2 your average is 0

[@johnwrench4speed]

According to James Halderman ac voltage checks can be done with a digital multimeter set to ac voltage and connected to the battery with engine running @2000 rpm. Good diodes would read less than 400 mV ac, defective diodes reading greater than 500 mV ac.

[@ScannerDanner]

yes and for eBook/book owners I have special offers on these videos. Contact me through my website for more info. Thanks!

[@dustinnist7548]

plus those snap on vantage pro scanners are good too!

[@ScannerDanner]

No, that will just show you the amperage. You can just use the graphing multimeter and set it to AC voltage. This will be a graphed average ac voltage. Just make sure you are less than 300mv doing it this way.

[@a.a7648]

Very nice Just replaced speed sensors nothing changes Well alternator

[@geoffrwelsh]

Darn. I had a 91 Regal years back with digital speedo that would read 0 with KOEO but like: 3, 4, 2, 4, 3 with KOER and then act totally normal once I started driving past 5 mph. never thought to try unhooking the alternator. Brilliant! GW

[@abdul-kabiralegbe5660]

I remember this weird flickering of the backlights on the combination meter on a mid 2000s Hyundai Elantra when interior lights are ON. The flickering correlated with engine rpm. At the time I suspected the RF Noise Filter capacitor on the ignition circuit. It just might have been a bad AC ripple from the alternator. Thanks.

[@sonofsteve566]

this is next test, I believe alternator diode is causing ac wave to CPS (Camshaft position sensor). no scope but 👍.

[@kalindambozi2356]

Great video. Never seen such a problem before but I have been equipped in advance

[@randymiller9219]

The fuses that were missing are the #6 fuse 10amp OCM/VIST Fan/wastegate, #7 fuse 15amp reverse lockout Sol Batt SRT10 , #10 fuse 5amp trx-off road pkg sen (gas engine only), #13 fuse 25amp rwal-pwr batt abs module feed, #24 block fuse 20amp subwoofer amplifier SRT10, and the #42 block fuse 30amp diesel pcm. I installed the missing fuses and replaced the negative battery terminal because it was a little loose and now it runs great.

[@ScannerDanner]

Most of those fuses are probably not used on your truck. A way to tell is to look into the slots, you will see a missing terminal on one side.

[@1fnjo790]

Same issue with speedometer with an 03 Town and Country, but alternator isn't making abnormally loud whining noise to tip me off to suspect alternator first. It also has TCM code P-1790, (Fault immediately after shift) and P-0720 (Output speed sensor error) I replaced both the output and input speed sensors which didn't fix issue. The audio system in the car sounded clear and fine without induced whine or static a bad alternator typically causes, and the MaxiSys displayed the VCI voltage at DLC as normal charging voltage. I did notice correlation of less of an electrical loads on the alternator the less the speedometer needle would jump and fluctuate from zero in drive at a stop. I disconnected the field connector and the speedometer would stay at zero and not jump around. I cleaned the alternator BAT stud and cable eye , and ground connections. I Checked the TCM connector and tested the grounds for TCM and loaded tested the grounds for TCM as fine. I provided extra support with Tie straps for the wiring loom at side of trans. I decided to call it a day and Google the problem and came across a video of Paul's brother, and then this one. I think I'll hook up my ole Counselor 2 and see what the alternator pattern looks like and take a few pictures if the Midtronics tester at Advance Auto doesn't print out and condemn the Alternator ripple, so I can get The Turd warranteed if Autozone tester says there's nothing wrong with it, LOL

[@ScannerDanner]

no but I've seen it on a Ford, setting a crank sensor code and it was a no start. no spark. It wasn't a bad alternator, it was a shorted battery and the AC voltage that was interfering with everything came from the battery charger I had hooked up to the car. after looking like a bad PCM, I started checking powers and grounds and that is when I caught it. I removed the battery charger and used a jump pack and the car fired right up. shorted battery is all it was. (Section 1 pg.26 in my eBook)

[@albertocervantes4432]

Thanks Paul Danner!!!! Very good video!!!! Is it possible to do this Test with a voltohmmeter?! Thank you again!!!

[@cobbysparks9891]

Excellent video as always. My question is, how did you know to go straight to the alternator? Would starting off with a wiring diagram help at all? Or is it down to experience?

[@tuloko16]

Id say more of “knowing” how things work in the system. An alternator produces AC voltage by nature which is then turned into DC by the diodes. The speedometer system works off from an ac signal produced by the sensor ( magnets and theeth on a gear and some wizardry) produce ac. Just like an alternator would. A lot of this stuff you can piece together with general knowledge of how systems work, and they are pretty standard from vehicle to vehicle. That is why Paul always stresses learning basics, then you apply them to different systems.

[@spierotti14]

Cool video! About 3 months I had an 05 f150 5.4l with a cam phaser knock and cel. Codes were for cam sensor 1 circut fault on bank 2 and the sometimes bank 1 also and had sometimes the trans would act like its in neutral and was very dangerous and would throw incorrect gear ratio fault codes. Solution=bad alternator diodes...crazy...dmm on alt post and neg ac volt 2.2v ac

[@ScannerDanner]

I believe it is section 1 part 6. i know it is toward the end. I'll get an exact time for you in a minute

[@PaulC1980]

Always learning when I watch scanner danner:),thanks for sharing with the world

[@ozzstars_cars]

Noisy alternators annoy me. Great video

[@PaulCTownsend]

We've seen this problem numerous times as well as far as the alternator making the speedo read. But the last time we checked was on a Chrysler minivan unhooking the control wires the pattern stayed the same and the speedo showed MPH at a stand still. We had to unhook the battery positive cable at the alternator before it flat lined on the Vantage( that's odd.)???

[@hankthebugman]

The subject of scopes and scanners is a common thread through your videos. I would find it very interesting to have you profile some scanners that are available to Joe Public. My interest is enough to buy one but what to buy? Additionally, there are more and more apps becoming available ( iPad apps) that are not that expensive and I would find it really interesting if you would critique some of those. Most apps require one piece of WIFI enabled hardware to allow the car(OBD) to talk to the iPad.

[@ScannerDanner]

not sure about that reading? remember there can be multiple charging system failures and symptoms. what you are describing with a 20v output is not a diode problem but a voltage regulator problem. Is this an external regulator on this year?

[@meanodustino9563]

Strange friend of mine had a Mitsubishi Lancer and had all kinds of lights on the dash and bulbs doing weird things. Did a voltage test and it was dumping 17.3v at idle 🙄 he lost all his headlights but 1 fog light by the time he got home about 3mi away.

[@thecowfy]

had a porsche 944 2.5 that would not run normally.checked ac voltage in the water system if i remember clearly(more then ten years ago) and it was too high.3V+.just changed the alternator.FIXED.

[@thecowfy]

unplugged the alt, now i remember and the engine smoothed out.

[@ScannerDanner]

.02? It was -2.8 and positive 2.4 or something like that. There was a 4 volt peak to peak AC sign wave! Not sure where you saw .02. Help me with that. Maybe I missed something.

[@richardcranium5839]

depending on the alternator wiring(internally) you can compare usually 3 waves. most alternators are delta or wye wound making them 3 phase. all 3 should be smooth and identical,smooth and symmetrical. also (which is more difficult to see) is to test the ground side.in order for an alternator to work there has to be positine and negative diodes. a bad negative can give the same problems. it may show in the positive side but not as pronounced. good he showed the speed signal as this confirms it is probably the same noise. these cars no likey noise!!! and there is lots of ways to make noise

[@hamidizadi]

Great video. What was that alternator noise due to?? The bearing or something else?? I have the same noise coming from my car.

[@lobizoli]

Thanks for this awesome video! I have a Mercedes where the ac voltage read 30.5v at the cigarette lighter!? I'm using a regular multimeter. How can that be? Thanks again, I'm addicted to your videos.

[@motherfuckermike7529]

Bro fucking 8 years later, thank you

[@ScannerDanner]

Did this help you then? Can't believe this was 8 years ago!

[@ScannerDanner]

no it is not. these spikes are normal and look different on every car. I have a nice case study on this in my section 1 video lectures. If you are interested in it send me a pm

[@mussaramadan]

You are a life saver I had po442 po700_0344_0339 o340_0725_0562.….. and unfortunately I changed 3 of them cranck sensor camshaft sensor and pcv valve untill i found your video . Thanks so much

[@ScannerDanner]

Awesome, sounds like you had this problem

[@brionadderley1643]

@@ScannerDanner so a bad alternator can simulate transmission problems such as shifting/slipping errors ?

[@ScannerDanner]

@@brionadderley1643 correct, because the output speed sensor would be giving incorrect readings

[@brionadderley1643]

@@ScannerDanner thanks for the reply sir; Hey i have another question if you dont mind; i recently done some swapping of parts in my honda 98 accord one of which was the IACV(Idle air control valve) and after doing so i recognize that the car now has excessive fuel consumption (accelerates automayically) and when changing the IACV should the TPS be adjusted cause the old one(factory) which seemed to be defective had my rpm really low like the car almost sounded off when running 650rpm u know ? Thank you alot !! God bless you sir ~

[@ScannerDanner]

@@brionadderley1643 no, the TPS should not need to be adjusted after an IAC valve replacement. At this point, we need to move this to my forum www.scannerdanner.com/forum/post-your-repair-questions-here.html it is free to join and I have paid moderators who are there to help me help you guys. I can't do back and forth messages here, it is too difficult to keep up. Things to look at for you in the mean time: 1. a vacuum leak will make it idle high 2. incorrect minimum idle speed (IAC unplugged, what is your RPM at idle?) 3. an incorrect IAC valve 4. how did it run before the IAC was replaced? Why was it replace? All this stuff needs to be answered on the forum. Thanks! Look forward to seeing you there.

[@IM_REDEEMED_777]

Thanks so much!!!! I have been using my 2001 grand caravan for 2 years with this problem.... Im going to replace the altenator before I go to the mechanic.... I have thr same symptoms 20mph on parking anf neutral.... And its stays on 2nf gear wen the ac is on... And it miss fire... For lack of mechanical words...

[@ScannerDanner]

2 years! That's crazy. If you disconnect the smaller wire connector, then start the engine, the alternator will now not charge. See if your symptoms are gone. If they are, then you should replace the alternator.

[@314Tazo]

An open or shorted diode or diodes will cause excessive AC ripple. A multimeter, set on, AC volts, will indicate if the ripple voltage is more than 2 volts AC. The capacitor, also, helps in reducing the ripple voltage. No one talks about the capacitor failing and causing ripple voltage.

[@lilameona]

best of the best diagnosis

[@donniejohnson7499]

Thanks for the video paul .

[@gerardlavallee7510]

That was very useful I had this problem, now it fix, thank so much !

[@arfk1986]

amazing Boss. your are helping the humanity to get better. Thanks :)

[@ScannerDanner]

thank you Rene

[@hflores37]

si yo tenia ese mismo problema y la transmission estando en D haciendo un stop se cambiaba a segunda y arrancaba en segunda...y el gasto de combustible era mas alto de lo normal......y la aguja del spidometro se movia sin estar manejando...y habia un cambio estando detenido......cambie el alternador y se compuso todo....todo se arreglo....ahora no tengo ningun problema......cambie los sensores de velocidad entrada ..salida....y no habia cambio......difinitivamente el alternador estaba mandando una senal y eso producia el problema............problema arreglado....

[@steve197232]

Thanks Paul.Keep up the good work.

[@GRUSSNewton]

Great video Paul, I'll do some experimenting with my 'little handheld Verus'' ;-) I wish it was a Verus!

[@FubarGRblog]

The peak to peak voltage in the yellow trace is misleadingly high because the scope takes into account the sharp spikes. The green trace doesn't have these spikes so at first glance it looks like it has less ripple. When you take a measurement at the battery there's this big piece of cable that comes from the alternator to the battery positive. This has enough inductance to filter out all those high frequency spikes and makes the waveform look less rippled. In essence, the two waveforms are identical

[@ScannerDanner]

So it is the battery cable itself that "filters" the waveform, not the battery? For inductance to take place in this cable there must be current flow, so could we say it is more than just the cable?

[@FubarGRblog]

ScannerDanner First of all, I’m a big fan and love your videos! Keep up the good work! Inductance is a fundamental property of all conductors (like resistance) and it doesn't depend on current. Any piece of wire will have some inductance and it doesn't even need to be coiled, even a straight wire is an inductor. The longer the cable, the more inductance it has. What you have there is basically an LC lowpass filter like this: upload.wikimedia.org/wikipedia/commons/e/eb/Lowpass_Filter_LC.svg ,where the inductor is the piece of cable and the capacitor is the car battery. The alternator outputs a low frequency voltage (around 1KHz at idle) with some high frequency noise added, I am guessing at some hundreds of KHz, probably caused by the failing rectifier. At the end of the cable, the high frequency noise is attenuated while the low frequency voltage is passing through. I guess that if you could pierce the insulation and probe the cable at various points along its length, you’d see the noise gradually decreasing as you probe closer to the battery. This is because the inductance increases with cable length and the filtering effect gets stronger. If you disconnect the battery, the noise will pass through. You need both the inductive and capacitive elements to get the filtering effect.

[@ScannerDannerPremium]

fubar.gr I'm struggling with this a little. Help me with this statement. "If you disconnect the battery, the noise will pass through. You need both the inductive and capacitive elements to get the filtering effect." This is what I meant by current flow. If I disconnect the battery and measured the cable end at the battery, it would be identical to the alt. BAT post. I think I understand what you are saying. I guess I think of inductance as counter electromotive force or is this something different? So for example when I teach my students about coil current ramping we talk in great detail about why the ramp is there and CEMF is key in understanding this. Or when we talk about self induction and mutual induction of an ignition coil (same subject really). It takes current flow through the conductor to create the magnetic field, and as this field expands and moves we create the counter voltage. (is this term inductance different than what I am describing? Thanks so much!

[@FubarGRblog]

ScannerDanner Premium Sorry for the long answer! The ignition coil back EMF is a subset of a broader phenomenon. It boils down to the fact that inductors don’t like changes in current. When there’s no current flowing and you try to push current through an inductor, it will generate a voltage in such way that opposes the current. So it takes some time for current to reach its max value. But when current is flowing steadily and you try to stop it, the inductor wants it to keep going. When you have current flowing through the primary coil and do something as extreme as abruptly switching it off, it is like introducing a very big resistance in the circuit. If it is an older points system, then it is the resistance of the open contacts air gap, if it is a newer electronic system then it is the resistance of the turned off transistor. In both cases the equivalent resistance ranges from hundreds of Mohms to Gohms. But the coil wants to keep the current flowing. So how do you push the same current through a much much bigger resistance? By generating an opposite, very high voltage. Of course the voltage generated cannot be arbitrarily high. The coil has a set amount of energy stored in its core. That’s where the secondary comes into play, it amplifies the high primary coil voltage into an even higher one. I guess it would be possible to generate sparks with just a single big coil. But then you would need special, expensive high voltage transistors. In a points ignition system there would be a lot of sparking in the distributor as well, that would damage it after a while. So the two coil system works well. The coils are galvanically isolated so there’s no high voltage going back to the low voltage side. Ok, back to the filter application of inductors. In DC, inductors have practically zero resistance. But in AC their resistance increases with frequency. High frequency means rapidly changing currents and inductors don’t like that. Capacitors are the opposite. They have infinite resistance at DC but it gradually decreases as frequency increases. Now this is a typical resistive voltage divider: tuxgraphics.org/toolbox/voltage-divider-w.gif Here there are two extreme cases 1) When R2 is much bigger than R1 the output voltage is almost equal to the input voltage. 2) When R1 is much bigger than R2 the output is almost zero. In the car we have instead of R1 the battery cable, which is an inductor. A lousy one, since it is not coiled and has no core, but an inductor nonetheless. According to this calculator chemandy.com/calculators/round-wire-inductance-calculator.htm a 4mm thick, 50cm long copper wire has an inductance of around 500nH. And instead of R2 we have the car battery, which has similar properties to a capacitor. Low frequencies “see” a low R1 and high R2, so the output voltage is almost the same as the input. High frequencies see high R1 and low R2 and the output is zero. So when you have a mix of frequencies as input, only the low ones pass. I did a simulation in LTspice. This is the equivalent circuit: i62.tinypic.com/2ew2urd.png V2 is the voltage source at 1Khz and V1 generates noise at 50KHz. I used five 100nH inductors, since inductance is distributed along the cable. The noisy output of the two voltage sources looks like this: i59.tinypic.com/e6rbf4.png This is the waveform you’d get by probing directly on the alternator post. This is the waveform between L3 and L4: i59.tinypic.com/vxcl6p.png This would be like piercing the insulation and probing near the middle of the cable. The noise is still there but attenuated. And finally, this is right after L5 and before C1. Like probing directly on the battery positive. i58.tinypic.com/2j35tti.png The noise is almost completely filtered out. Generally speaking, looking for noise in a system with an oscilloscope is really tricky business. Sometimes you’ll see noise that isn’t really there, it is simply coupling into your oscilloscope probes from some other part of the circuit. Other times you might have tons of noise, but your test gear set-up might be filtering it out.

[@danpettis1675]

ScannerDanner would this effect transmission shift points. My car is not shifting properly I'm at 3k rpms and I'm only in 2nd gear. I'm getting a p0562 low voltage. Any advice thank you sir

[@30YearOldBunta]

2002 infiniti q45, bad alternator causing false crank signal resulting in engine timing advance to slowly go to 0, car would randomly throw codes for things on the 5v system (power steering pressure sensor, vvt, ecm codes etc)

[@johnjohannemann1220]

Great info 👍🏻