Using the Autel Ultra lab scope (Relative Compression Test)

How to perform a relative compression test using battery voltage. For you guys without a lab scope, use your ears! Listen to the way the engine cranks.
#AutelUltra
#autel
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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.

[@ScannerDanner]

When you do an RC test using battery voltage instead of current (as I did in this video), if you simply reverse your test leads (red to neg., black to pos.) this will flip the pattern and make it easier to read. I didn't do that and I should have.

[@isalmankhan1]

You're the Master👍 Cool new tip SD👍 Thanks for sharing👍

[@juanportillo2201]

I didn't know that, you learn something new everyday. My MP408 has a way to invert a waveform, its no the best but it works. I imagine the Ultra also has a way to invert a waveform.

[@N33DS]

Sir. The "customer" is Needs and the "Old Chrysler", her name is Blew. Lol. Thanks for taking a look at her sir. Bought a parts Jeep, doing a heart transplant and building the old one up. #LetsGoPens . Thanks Danner(s).

[@juergenscholl2843]

You may want to hook up one channel to the engine block, this will show you RC with compression peaks upwards as well, without inverting leads or the image. Usally ground voltage is pretty hashfree.

[@Dogpool]

I have a autel scanner that can accept that scope. What do you think of it? I have been resisting buying it because it didn’t seem that great

[@billyyoder8171]

Thank you Danners of the house of Scanners. Have a blessed and safe week to you and your families.

[@mattbickford2640]

LOVE the intro, did not see the glasses coming out, should be entertaining if paul forgets where his glasses are lol! thanks for the share!

[@fabianthorne6831]

Thanks again Paul and Caleb,we always learn a lot of good knowledge from you and your family,God bless you and your family.

[@ThunderbirdRocket]

Excellent explanation / description of how to analyze the relative compression pattern made by hooking up the scope to the battery . Displaying voltage drops across the battery as compression (and amperage draw) rise in each cylinder . Thanks for explaining how the problem with sealing / lack of compression within a cylinder greatly affects the compression (air fill - “inhaling” ) of the NEXT cylinder. And that stands out on the display pattern too . But is not to be confused with the main culprit . Thanks for sharing such detail coaching us that it will show up as big voltage drop (large amperage draw) consequently . 🙌🏼

[@TheJunebugz20]

Wow never in my life did I know you can Diag. Low compression through a battery .incredible . Thanks bro

[@ScannerDanner]

Glad to help

[@pablogonzalez4801]

Gracias SD por ayudar y a aportar con tus impresionantes videos a la comunidad de técnicos , muchos aprendemos de ti y de tus técnicas de diagnóstico , le escribo en español para que sepas que también en la comunidad de habla hispana tus geniales e instructivos videos son vistos y nos enseñan y educan , GRACIAS POR EDUCAR Y COMPARTIR

[@eazy91]

NETA

[@8953147]

Always appreciate going over information Paul!!

[@rjeromej8149]

To Invert the measurement on the Ultra; select the Channel, select Probe Setting, and turn “Inverted” from Off to On. As far as an Amp Clamp, the Autel MSOAK Kit is the great addition with the Ultra. I use it all the time. Very informative video Paul.

[@ScannerDanner]

Thank you! Great tip

[@rainbowdiamonds2865]

Your brother is perfect man. Thanks for your Videos. All good for you and your Family.

[@Pablo_Automotive]

Thanks a lot team Danner, the Saturdays are always better when you upload a vídeo 👌🏆🔧🔩☺

[@cullenmiller8170]

Nice video. I appreciate the ones that teach basics. I saw Autel offers a kit that has high and low amp clamps and a paddle type ignition coil tester. Thanks to the three D’s.

[@steveanderson4553]

Great Video Paul !! Really enjoyed watching 😎👊

[@johntarpley1140]

So nice the techno this day and time wish I had that stuff back in 83 when I created the running compression test for GM and Cadillac great job danner

[@exit9mastertec7]

This man is the greatest teacher

[@warpath58]

Hey buddy great information. I can't believe that I have never seen this before. Thanks for sharing your endless knowledge with us.

[@ScannerDanner]

Thanks for watching!

[@MyHomeGarage]

I appreciate your channel along with others like you. But you especially have Really made me realize my 15+ years when I was a quote on quote Honda Platinum Technician meant nothing lol. maybe it was my age at the time and where I was. But I have become a better Technician now that I am no longer in the trade and watching and learning from you and others. So much I did not know or maybe never had a chance to use in the field. Recently Diagnosed a compression issue on a vehicle I maintain. compression issue. As simple as it sounds. Sometimes the confidence it takes to make that call can be difficult when its a family members car and they count on you. Keep up the good work. I am looking to go in to teaching in the future and will be looking in to expanding my knowledge. Keep it up and good health to you and your family from me and mine up in Ontario.

[@IAMNOSLEEP]

Thanks for sharing Paul... Cheers.

[@mral168]

I benefited a lot from you thank you

[@amr-50]

Thanks danner thats a very informative video as usual you are the leader

[@benhurworld]

Scanner danner single handedely puting us on to whole different level of skills .this man is great

[@ScannerDanner]

thanks my friend

[@grad0n]

I just saw Car Wizard show the relative compression test with the Autel last week. I remember when you did it years ago with an amp probe. Very cool! P.S. I still haven’t found the culprit of the P0705 on my lady’s 2000 Jeep Cherokee XJ. . .

[@stevenkietzke5582]

Once again an amazing video Thanks and God bless

[@tracydiller9378]

Another great video from the Danner Brothers and Son.I was thinking about getting the new Autel scanner but, i think i will stick with the Verus Edge not saying the Autel is a bad tool.God bless and stay safe.

[@steveguest8028]

Class Paul that’s worth knowing.... Thank you 👍

[@johnbonifas6040]

Nice Job Paul I use the low Amp prob and I ac it with the Amp clamp works great

[@mobileautorepairbypacheco1806]

I just want to tell you thank you for not being selfish and sharing your knowledge. Once studying a book look what I said teach the wise how to have more wisdom and Be wiser. And I say the same thing to you for giving us your wisdom. GOD will reward you with more wisdom

[@johnturner8829]

Caleb must be learning a lot from you. Maybe we could see him do some diag and you run the camera...?? Maybe a nice video.

[@stephen6945]

We should agree that if John Turners comment above gets 150 likes, Caleb does a diagnostic video.

[@isalmankhan1]

Bravo Ingenious👍 Nobody (except you) could teach such a creative test with basic scope accessories to Tech's like myself who have limited resources to work with👍 Thanks for educating us SD, Stay Safe & Blessed Bro You're Awesome😎 As usual Thanks to James Danner & Caleb Danner😉

[@MarioDallaRiva]

Spanks, boys! 👏🏻👍🏻 Great info.

[@kingtracker184]

excellent video brother

[@cowboy41231]

Awesome. I've always wondered if this could work without an amp clamp, but couldn't find DIRECT parameters setting... I'm sure it's not different but would love to see this on my Uscope! Love your diagnostics book btw danner! Been subbed for years! I also like this "shorter" format, to the point!!

[@markbrzezinski8889]

Two gems! "When there is low compression the next one reads high." Then it always drops but is still high for the next cylinders."

[@waynenocton]

Pretty damned cool! After top dead center is reached the compressed air actually helps spin the engine, that’s why the high peak is next to the low reading. This video should really get the views, I already shared it with several people, super cool one.

[@bradhilson3236]

try the same test with WOT

[@raycastillo3168]

I could always tell when Danner is approaching the camera you can hear his shoes scraping lol you the man Danner I love your comments. I am pretty new to scoping I just purchased recently the Autel Ultra and I’ve been playing around with the settings you should make more videos with the Autel I have noticed that that is starting to be your to go to scanner lol there is absolutely no one out there with better KZbin videos the scanner Danner

[@ScannerDanner]

🤣 me too! He doesn't tie them tight and they flow around. For sure I'll be doing more Autel Ultra videos! Thanks my friend

[@jeffreywilsonrubikscubeonb2428]

"Just accept it!" Hahaha so good. Awesome edits...love you guys 😎

[@ScannerDanner]

Thank you!! 😁

[@gertjan0089]

Love Autel Ultra video!!

[@kyletuttle9064]

Thanks for sharing

[@jhondixx7119]

Dan scan awesome content

[@krazykoolkarl1970]

I love doing this

[@henriqueazulao]

Awesome, I'm Brazilian,thanks

[@heartlandmechanic]

That's a great video ., Thanks

[@labradormcgraw]

I agree with Caleb - braver for wearing the glasses than for showing the mistakes.

[@ScannerDanner]

🤣🤣

[@Nabeelco]

Hey, I legit think those are cool! But then again, I'm not exactly a spring chicken.

[@mahdiscope2545]

If you don't know there is an option in Autel to invert waveforms (math-channel option) ,,, and I don't know why people disliking the video ! Damn Love you danner family , and I would like to say that that the new camera makes your videos much more better and professional 😍

[@ScannerDanner]

It's not just the camera. It's the editor! (My son) Thanks for noticing!

[@tigerksewsankar7667]

Thank you sir.

[@mobileautoserviceanddiagno2997]

Good video

[@alexgonz5550]

Always learning 👍👍👍👍👍👍👍👍👍👍☕️☕️☕️

[@prmayner]

I have 2 different Launch scanners an Autel a Pegisys and a few others. Snap on is by far the best scanner out there, in my opinion. They have the best software. carry on Scanner.

[@zoticus1]

You can use as attenuator, which decreases noise significantly.

[@mayowaodus6310]

Hi scanner danner. I have an autel mp408 and each time I try to do the RC using voltage I get a noisy scope reading. I was told it’s a common problem from the autel leads. Have just gotten a current amp clamp and same noise persists. Is 200us/div the norm to use while doing the RC. Thanks for this video

[@kevinfrye1822]

I got a question regarding the RC test . What if you have a amp clamp that has A/C and D/C function scale like the esi electronics clamp ? Do you use the A/C scale if your using Ac coupling on the snap on labscope? I just bought the clamp so haven't played with it yet and curious .

[@JoitaConstantin]

Hi Paul. How come you don’t use the relative compresion tool from Ditex? Is stand alone, and there are no problems by filtering and so on.

[@randy1ization]

I wonder if you could use a timing light and your ear.. if the light flashes at the same time as the engine "dips" you got the weak cylinder?

[@booboothefirst3]

Where did you get those glasses?

[@PedroGomez-bd9ro]

The starter liked this video

[@throttlebottle5906]

use a makeshift amp clamp(current sensor) just a single piece of wire tight wrapped around the battery cable going to starter around 6 turns should give good voltage for the scope, simply hook the scope leads to each end of the wrapped wire ends. good enough for relative testing graph :)

[@ScannerDanner]

That sounds like work 😃

[@fascistpedant758]

That was my first amp clamp ~40 yrs ago. the beauty of it is that it's naturally "AC coupled". The problem with it is that it's naturally "AC coupled" and doesn't capture constant current.

[@bhavdeepsingh3549]

Can you please give some details about hybrid cars

[@112879jcarlos]

Scannerdanner, CCD connector it's just a stupid (jumper cable) it's been used on newer Chrysler models 17 and Up I believe. Autel only. For transmission and Abs codes only.

[@edwinlomonaco6754]

I thought I saw an injector one dtc. No? Maybe that injector is washing out the cylinder? Thank you for this video. I often do rc tests on the battery due to the continued complexity of battery cables in newer vehicles. But I wasn't aware of AC coupling. Gonna try that soon.

[@ScannerDanner]

If you did, it would have been from unplugging the injector

[@jhondixx7119]

Dan what was the connec connector you hooked up in the beginning of the video was that one of the filters that comes with the 919 Ultra I'm old school and just updating my tooling currently doing classes but there were two filter amplitude or something or other that came with the scope if you could explain to me what do those do and in what scenarios do those come into play I have a class tomorrow night with Worldpac might explain it

[@ScannerDanner]

I dont remember. What time frame? I'm sorry

[@YouCanFixIt]

Why did you not used the obd2 port as an input of voltage and select to use the battery

[@PeterSinBorisa]

На осциллографах autoscope4 , diamag2 есть специальный тест по относительной компрессии. Можно провести его методом css, можно так же токовыми клещами и датчиком синхронизации. Как мне кажется там это удобнее и быстрее чем этим осциллографом

[@snubbelbuff1471]

@10:07 "Just accept it" ...That's what I hear from my mechanic too...

[@N33DS]

Favorite part of the video. He used to do it all the time in class when his brain would trail off into something we had yet to cover, it'd be "just accept it".

[@irvinmclaughlin7294]

hey great vidieos i have a question i have a 2006 1500 ram has skim . lost key .is there a way to by pass or trick controller to start? has ngc or do i have to send controller off to be deleated ?

[@ScannerDanner]

I think you can disable the SKIM, I have on an 04 model, not sure about 06 and you need the right tool to do it

[@williamcook2499]

So, of your were to buy a snap on vs autel? Or both? I honestly have used both and I see I have to have both. Some times one can where the other is a slab of.... attempt the other is fast and quicker by millions of ways. I honestly see the need for both. I believe a full lad scope is great but the wisdoms and the combustion chamber is soon to be more than less a dark day in this world. So if I have to diagnose electric cars vs. Combustion. I want to stay tuned in on that too.

[@bartscave]

Honestly, I find using ac coupled for RC tests almost exactly the same as using the amp clamp.

[@taneesescu]

i have a hyun-die accent 2016. Replaced all 4 suspension and control arms plus RF wheel bearing. Then I added 2 new front tires from shop and they balanced, and finally big-o alignment. Afterwards play in steering after 40mph. Using a full scanner from YouCanic for DTC I got the C1211, C1208 and C1207 which are wheel speed sensors. No ABS or traction control light. reset didn't remove them. Saw a TSB regarding SAS after wheel alignment. Big o states in small print they don't do the ESC in basic alignment stuff. didn't know that until yesterday. I took to Hyundai and told them this and they told me they found nothing wrong and everything looks tight but I need fender liner and should change the brake fluid and bulb in 3rd window break light IMMEDIATELY. Im fighting them to get the SAS reset but I'm at a loss on what to do. Not even sure it is the SAS honestly. Everything else checked out on their inspection and torque. Any ideas please and I mean PLEASE! I got $100 right now for anyone who can help me over the phone.

[@woog74]

Hey Danner would a vacuum gauge not have shown this with engine running? Great video for me as I just got a MS919, (the little brother to what you are using) and have not done RC cranking with it yet. Very helpful tips on set up.

[@ScannerDanner]

possibly depending on the failure, but many, many compression loss faults will show a good steady needle on a vacuum gauge

[@scientist100]

If valve related, then yes if on the intake side only.

[@woog74]

@@scientist100You are saying that a leaking exhaust valve can not change how a cylinder seals?

[@scientist100]

@@woog74 it will using an oscilloscope in cylinder but wont be visible with a vacuum gage at the intake manifold.

[@woog74]

@@scientist100 ​ @scientist100 To state that it is absolutely not possible to see a leaking exhaust valve through a vacuum test is incorrect. Here's a link to a video, showing that a vacuum gauge will fluctuate with an exhaust valve not seating. Fast forwards to 3:52. kzbin.info/www/bejne/rJmWi2B8a7B6oas

[@doogeerugged89]

where do i go to get hands on training for lab scopes , in general ? Can you go to an hands on ASE training class?

[@ScannerDanner]

hands on classes are offered at my school but it would be part of a 16 month program www.rosedaletech.org I can train you online via my website but the hands on part you'd have to practice yourself www.scannerdanner.com/join-scannerdanner-premium.html Hope to see you there!

[@henryrobinson9837]

great video Scanner, Could you tell me more about the LPF capabilities of the Autel scope. i'm interested in buying one.

[@ScannerDanner]

LPF ??

[@henryrobinson9837]

@@ScannerDanner sorry , low pass filter. is what i was asking about, whats the minimum range

[@ScannerDanner]

@@henryrobinson9837 not sure offhand sorry

[@weshawkins7165]

Can any amp clamp be used or does it have to be an Autel one. I’m looking at purchasing a scope but can’t afford a Pico and already have the Autel ms906bt. I’m looking at the mo406 Autel scope.

[@ScannerDanner]

Any amp clamp my friend. Check out the ones my friends at AESwave sell. www.scannerdanner.com/tools.html

[@RicardoRfiles]

would it be possible to measure the voltage drop in the cable that goes from the battery to the starter instead of measuring the battery voltage directly?

[@ScannerDanner]

You'd be working harder for those connections. Maybe it wouldn't be too bad on this Jeep but if you did get a pattern it would be much less amplitude than the one I showed. So no real advantage with that.

[@christopherenger1602]

After this would it be wise to do a leak down on cylinder #1?

[@ScannerDanner]

Yes sir, if you were going to do the repair, absolutely!

[@lovetolearn5253]

I agree the snap on list and pid set up is way more friendly user. I went to scan a VW with my autel today because my snap on was on a avalanche. I was try to tackle 2 cars at once. I gave up on the autel it wanted to know all this sh#t to identify the car. I just went in to generic mode and grab the codes. I cant stand that autel does that. I dont want to spend 15 mins looking up what the car has and dont. I completely understand if your programming but all I wanted to do is see the misfire counter and codes. For the price autel is a awesome scanner put that little stuff drives me nut.

[@ScannerDanner]

100% agree

[@fixitman8028]

Paul. Thanks for the video. You mentioned negative fuel trim is indicative of low compression. Is it possible to see long term fuel trim at negative 30 due to poor compression? The vehicle has 235,000 miles and With EGR, evap, disconnected from intake fuel trim doesn’t change. Fuel pressure is normal, new air filter and oil change still negative 30 LT fuel trim. I am down to worn timing chain or compression as reason. Not sure if compression could be that big of a factor. Vehicle run great other than rich. 2005 caravan. Thanks for any thoughts. God Bless you for what you do!

[@ScannerDanner]

Not always. That is just one cause. There are many possibilities for a rich condition Injector flow should be verified, purge valve stuck open, o2 sensor function should be verified, head gasket issue can skew the O2 and cause this etc etc

[@fixitman8028]

@@ScannerDanner Thanks for the response. You are a blessing to so many of us!

[@ScannerDanner]

@fixitman8028 if you need additional help, post to my forum! It is free to join and we can absolutely help you www.scannerdanner.com

[@jamesbarratt593]

Went to a job today with my AL539B and after replacing Harmonic balancer crank pulley on 7 series BMW the customer had a flat batt. 8v when checked with Fluke multimeter. No chance to turn engine. just click. SO the battery was fitted down into the hole in the boot where it lived and on the top was no markings like CCA etc, and no figures either to put into this battery tester. Not without removing battery which was a long one I could see. So with my jump leads out of my van to battery I let it slowly get charge back, only not before having tested the battery. no jump leads connected for the test. I guessed the battery. Told the AL539B the CCA was 700 and ran the test. It came back saying battery only produced 143CCA so battery needed changing. 10 minutes later with my heavy duty jump leads from a lorry juicing up the battery the customer went for engine start. The BMW 3.0 started. I disconnected the leads. ten mins later he switched engine off and tried to restart. It cranked perfect and restarted from just the battery which the Autel AL539B had said was rubbish. Were my guessed figures the real reason here, or was I close enough with my guess but the tool is not accurate? Finished that job and left. unfortunately will never know the battery figures. So have to simply watch out on other cars. Verify if the tool is good or poop.

[@seanisthebomb117]

just wondering, could you hook up the scope leads backwards to invert the signal (out phase so speak) or does the A/C coupling essentially filtered so it cant be inverted by lead hookup?

[@ScannerDanner]

Sure, that will invert the pattern, but AC coupling is blocking all DC which allows me to zoom more on the voltage waveform

[@seanisthebomb117]

@ScannerDanner thanks Paul. Keep up the great work! In my tech school, we were big on using OScopes. I ended up in heavy diesel/heavy industrial, and I feel like bringing the OScope back into this world is going to save me time and bring in accuracy. Thanks for all the refreshers!

[@TheJunebugz20]

👏🏾

[@AnomclovisRkelly]

Hello I which to know if you can accept apprenticeship in your workplace or you can only do online training , and how much can it cost me please

[@ScannerDanner]

We only offer online training via my website www.scannerdanner.com/join-scannerdanner-premium.html It is $11 a month giving you access to over 550 videos and counting

[@rrmech11]

👍👍

[@fascistpedant758]

Starter motors, like most motors, draw more current when they're turning slower, not because they know they're "forcing" more. I think the current spike on the cylinder following a cylinder with low compression is due to lower engine speed due to the failed expansion event of the bad cylinder. How do you keep the women off you when you're wearing those sexy glasses? Thanks for that "flipping the screen" trick. It'll sure beat standing on my fricken head every time. Have a great weekend!

[@gearstil]

Hello, did you unplugged the injector fuse? why is the car not starting when you crank it?

[@ScannerDanner]

Gas pedal to the floor cuts injection pulse on most cars during cranking

[@marktirabassi7160]

Hi scanner Danner. I need your advise ,. There's a scanner called the aidtool. Sold on Alibaba from a company in schezine china. It sells for 1500$ comes with adaptors and a blue tooth vci box. I'm trying to program a 2002 Volkswagen key. No clue what I'm doing. ? Any thoughts

[@ScannerDanner]

Unfortunately no but we may be able to help you in my forum. It is free to join www.scannerdanner.com hope to see you there

[@turbobuick33]

Does the Autel Ultra come with all the OBD 1 Asian connectors?

[@ScannerDanner]

No sir

[@northernpatriot9078]

Don't feel bad I need bifocals too now I just don't use them yet too uncool 😆

[@strikeryachts]

I watched a lot of your clips, why are you guys not using a battery maintainer/charger on your diag work. I connect on every car before I start any work and make sure the battery is fully charged before releasing the car. Mainly doing German car work.

[@primetimemidwest1543]

Hey Dan I need ur help my 01 f150 was running good then while running it started running rough five minutes after I started it and now it has 12 codes from two codes of Bank 1 and 2 leak now those r gone after doing intake gasket

[@ScannerDanner]

Check the harness that runs down to the O2 sensor, that you didn't pinch it in the intake manifold. You probably have some blown O2 heater fuses too is my guess

[@martyisabeliever]

Amp Clamp and COP probe are ibn a seperate kit...

[@LynxStarAuto]

That's how I invert my signals! Flip the screen 🤣 Autel scopes are a pain

[@JPhilBradley]

So the intro to this video gets its own like.

[@ScannerDanner]

I'm blind close up these days lol

[@JPhilBradley]

@@ScannerDanner literally laughed out loud.

[@dariusjonsson2358]

Very cool technique. However, you could have just seen that the engine says 4.0 and assumed that the head gasket is blown out.

[@nikovandh]

Can anyone give me recommendations on a first lab scope to purchase for diagnostics? I'm getting much better over time with electrical knowledge and want to gain some personal experience with lab scoping through my own vehicles / friends/family cars. Would like to spend less than $1000 for a good setup with everything I need. Lemme know!

[@williamcook2499]

All in all that was an old trick that is easy to do in most all lab scopes.

[@robertbacklund4438]

I am a retired aircraft technician. I have not found a shop where I live that I trust. I have always done most of the required matinance on my own cars. I am considering buying a good scanner and possibly a scope. I have been looking at the Autel Maxisys Elite II, Autel Maxisys Pro Mk908P, not sure spending the extra $2500 on buying the Ultra would be worth it for me as I have no plans on opening my own repair shop. Would it be better to purchase a separate scope like a Picoscope and if so how many channels do you need? Also does anyone know of any online training courses that would get me more up to speed on the newer computer controlled vehicles? Also any advice on which Autel scanner I should buy or even if I should even buy an Autel would be appreciated.

[@ScannerDanner]

Look into the Topdon Phoenix (video released two weeks ago, we premiered it) Link provided in that video as well. It is around $900

[@christopherkassner8894]

I’ve tried using a BOB at the DLC using the same voltage method. I couldn’t get a clear enough pattern. So I just ended up using my amp clamp on the starter wire. I definitely need to mess around with my settings and save it. It would be a good parking lot time saver doing it through the DLC

[@ScannerDanner]

did you AC couple it and use a filter?

[@christopherkassner8894]

I did AC couple it. I don’t think I filtered it. Also I was using my versus edge scope. So I probably needed to adjust my timebase. I was pressed for time. So I resorted to my tried and true method. I’m going to try it next week if I get a chance. It’s definitely worth the effort. It’s a quick and easy test from the drivers seat.

[@codo7]

The drop in voltage is the voltage drop due to increased amp draw due to compression: the battery must power the piston thru that. When the battery encounters the weak cylinder it actually speeds up the cranking ...only until it arrives at the next compression stroke that is normal, in this case the very next cylinder and the battery now encounters a compression that slows it down more than the previous; the amperage draw increases. Running compression is less than static because ..., well, start at the beginning....air compresses, right? Therefore the converse must also be true - it stretches; otherwise air you compress would remain compressed. When drawing air in it is being stretched and you could measure that in PSI...air at14.7 at atmospheric drawn in has a vacuum effect (our absolute manifold measurement) - less pressure; the air has been stretched...the higher the rpm, the less efficient will be the intake of air. It wont reflect C.I. displacement of the static design.

[@fascistpedant758]

Running compression is higher at WOT.

[@codo7]

@@fascistpedant758 Vacuum is lowest at WOT, more air is taken in due to less throttle restriction. The difference between running compression test at idle and a non running test is speed of cranking, throttle remains closed.

[@fascistpedant758]

@@codo7 The static compression test is supposed to be done at WOT.

[@richardcranium5839]

one thing you've managed to do is confuse me on one issue- how do you know if its a bad winding or a low compression cylinder. on a cranking test the time factor would be difficult to see i would think. if the starter pinion/ring gear ratio is even number it would be at the same place in the trace. duh i may have just answered my own question.

[@ScannerDanner]

Also, it wouldn't be timed with a specific cylinder if there was a flywheel issue. In particular, as in the case, timed exactly with the cylinder that is misfiring

[@mbs5328]

Why doesn't James' shop do engine work? Couldn't this car just need a valve adjustment on cylinder one?

[@ScannerDanner]

not with 20 cars in the parking lot waiting on repairs. He needs another tech

[@ForeignAutoworks]

Misfiring and it's a Jeep? I'm amazed a vacuum gauge wasn't the first tool drawn from the box LOL also would've been the easiest first step. Would suspect broken valve spring.

[@dws2313]

I don't understand "...when amperage/current rises, voltage drops ..." If Ohm's Law is the equation V=I x R, then increasing one side of the equation must increase the other. So, what am I missing?

[@ScannerDanner]

AC coupling a scope is what you're missing. When this is done, the scope is only indicating changes in voltage, not what the actual value of the voltage is.