How to fix Chev/GM 4WD shift problem C-0305

This is a real world diagnosis and repair of a 2000 Chev Suburban with a 4WD that will not shift back into 2WD, with a "service 4WD" message on the dash. Diagnostic code C-0305 was seen on a scan tool.
Note that in retrospect the sensor waveforms were not seen because there was an open on the sensor side of the breakout leads. For an updated accurate waveform see the followup video, • Propshaft Speed Sensor... . Also note how the observation of a 0.7 volt bias voltage seen here but not seen in the followup video says something about the internal design of the circuit.
This is an amateur demonstration, offered in good faith for free, but with no guarantee as to safety or reliability of the information. Auto repair comes with intrinsic risks, risks that are clearly related the the experience of the technician. Any misadventure you encounter if you decide to accept the risk of fixing your own vehicle is your responsibility.
Music clip at the end is
Roboskater by Audionautix, licensed under a Creative Commons Attribution license (creativecommons.org/licenses/...)
Artist: audionautix.com/

[@spelunkerd]

At 8:36, the waveform shown is flat because there is a bad connection on the sensor side of the breakout lead. The module side of the breakout tap is intact, which is why we see the bias voltage. For anybody who is interested, I posted an video update today regarding the true waveforms, here kzbin.info/www/bejne/lXWzm2t9l9OliMk. The fact that bias voltage was present when the circuit was open suggests that the module uses that voltage to screen for circuit problems like open, short to ground, high resistance, etc. These two-wire variable reluctance analog sensors are very similar to old style wheel speed sensors.

[@unclefester2]

Many thanks for sharing - Although the diagnosis was interesting, the wrap-up at the end was a great instrument for approaching these things. Please keep them coming wherever possible. Thank you once again, from the UK.

[@ZoeyTaylor1994]

Thank you for taking the time to make this, very informative and impressive editing skills. Clear and concise video.

[@dwoodog]

Nice job as usual. Very satisfying to keep older equipment running.

[@angelaracino6236]

I enjoy the diagnostic testing and logical thinking to verify the problem, followed by schematics and diagrams to further be able to follow along. Thank you for taking us along the journey. Your friends and patriots from Fort Lauderdale Florida USA

[@spelunkerd]

Thanks for stopping by, Angela!

[@bastholio]

Thank you for sharing this excellent video! I particularly appreciate your complete and thorough description of what you are doing. Good stuff!!

[@neverstoplearning382]

Nicely done troubleshooting process. Thank you for taking your time to share valuable knowledge. God bless.

[@billk5727]

Thanks for the video, very well done! Very nice equipment to have fun with also. Stay safe and take care, Bill

[@wyattoneable]

Fantastic editing with the picture in picture option. That helped move things along nicely. Too bad the expected wave pattern didn't take place. But yes, it's fun to use the tools you have...especially when you have time.

[@Georgiaguntraining]

Well done. Great explanation. My 2007 with the same xfer case did this today for the first time. Will read codes tomorrow and see if this applies. Thank you!

[@russellbean9337]

Great video and it answered the questions as to why my truck does the same thing.Thank you

[@8power0]

FIRST TIME VIEWER LOVE THE COMPLEX TROUBLESHOOTING POWERUSER VIDEOS... SO PLEASE KEEP THEM COMING THANKS FOR YOUR TIME !

[@spelunkerd]

Thank you for stopping by, it is so nice to learn that some people actually find these helpful. I did a playlist on automotive oscilloscope and electronic fundamentals if you are interested. Dave

[@andersonrowland]

Another great video! Thank you for your time.

[@markchuml1125]

Great Video for my C-0305 code. Thank you

[@stevenchan1057]

Man!.. you are awesome! I like how you think and how you go about your tests. I aspire to be like that one day. You just gained a new groupie lol

[@alkhashtee]

Love the way you present the problem, explain it and fix it, you are so smart. Subscribed now

[@spelunkerd]

What a kind remark, thank you.

[@alkhashtee]

@@spelunkerd You deserve it sir

[@Mrbumbons1]

Well done. Great video.

[@stuzman52]

Great video Dave and always nice when you have time to pull a scope out to look at waveforms. I was thinking along with Eric about why you didn’t see a waveform on the good sensor. It’s possible either the front or rear sensors have a floating ground. With both channels hooked up, the floating ground would be grounded out with the other sensor through the scope’s channel grounds. Although the 4424 scope has isolated inputs, the grounds on the scope could still be shared because the grounds are using capacitors. To verify, you could only hook up one channel at a time. Have a great day Dave! 😀👍

[@spelunkerd]

Good point. After a little thought, I believe I had a bad connection at the sensor end. With good connectivity at the wire loom I was able to see the bias voltage on the module side, but the sensor was invisible. I'm going to repeat that part, and use the scan tool to verify I've got a good connection. Attaching the leads is done blind, and I was trying to get by with very small leads to avoid spreading terminals, so it is not easy.

[@nothingfree3524]

Better old school 4x4 just. Lock in and go ! Transfer case motors ? Module for engaging front axle ? Modules , sensor's computers for engaging front axle ? OVER KILL ! Having same problem with 2000 sierra once fix down the road ! Then back to old school ! Less headaches ! Informative video !

[@spelunkerd]

I know what you mean, it can apply to many systems these days. On the other hand, the original locking hubs in my Dad's 1970 F150 truck were irritating, too. We had to hop out and manually engage the hubs, which wasn't so great when you suddenly bog down in the mud. I prefer the old style levers rather than a dysfunctional encoder motor.

[@SouthMainAuto]

Interesting you couldn't see the waveform when checking with the Pico... according to service info the front drive shaft speed sensor is a permanent magnet (PM) generator. Sensor produces a pulsing AC voltage. AC voltage level and number of pulses increases as speed increases. ATC control module converts pulsating AC voltage to drive shaft RPM, which is used for calculations and to monitor differences between front and rear drive shaft speed. It is also used in AUTO mode to determine amount of slip and percent of torque to apply to front axle. Did you mess with it anymore? It is not unusual to see OE's float the AC waveform on a DC bias of a volt or so. Great now I am gonna loose sleep... THANKS! 😁

[@wyattoneable]

Do you ever sleep Eric? :)

[@spelunkerd]

It's always nice to get a visit from a widely respected professional teacher like you. Thank you Eric!

[@SouthMainAuto]

@@wyattoneable When I'm dead ;)

[@SouthMainAuto]

@@spelunkerd I dunno about any of that man... I just fix cars... period

[@timtryon1443]

Been under my GMC SIERRA 4 days and tore Dash a part. Check out 4 wheel panel. Automatic 4 lite up and 4 low lite up In neutral 2 high. Check out All ground's sanded put back got 2 high lite back on tried reverse sounds like it 's ripping a part. Banging .

[@ozzstars_cars]

Remember the old days of getting out of the vehicle locking and unlocking hubs, no lights to deal with. Haha

[@spelunkerd]

Yeah, I liked the mechanical systems where you pulled a lever. My dad had a truck with locking hubs, I remember rolling into a bog and then trying to lock hubs in retrospect. What a mess!

[@chrisace3086]

This guy tells and does it like a pro...having same issue with my 02 Tahoe. Replace the selector check all fuses and clean the ground under the drive side and still won't engage. Don't really have time to fix it myself been trying to find someone that does it straight forward like you. Gotta watch out for them fake shops that screws up your bought parts like what happen in my town figure I help out with money to them get top and bottom ball joints replace. This worker didn't grease up the joints plus put a hole in one that I check. Owner found out I was calling him out and try to big boy me on phone toldem straight yeah I said what I said and I would beat whoever work on my truck and fuck up.

[@realrich983]

Impressive man.

[@darellashford1816]

You my sr is a bad man 🙏 thank you for your information it will be passed on And greatly appreciated

[@spelunkerd]

Today I used a video made by somebody else regarding how to replace the HVAC actuators on a Passat. I left the same remark as you did on his channel, he made it much easier than it would have been without that info. The mutual sharing that happens on KZbin can be a game changer. Thanks for the feedback!

[@TheGraffiti600rr]

Im gonna try this (sensors) maybe tomorrow. I've been told it probably the 2 solenoids on the side. This is easier and cheaper.

[@jasonblanyar4381]

Thanks buddy what you do to get it out of 4wd helped me a lot thank you 🙏🏻

[@kdlittlehawk]

I've searched everywhere and can not find just the ring sensor for my 2002 Chevy Avalanche. It's my understanding that they are different than the 2003 and ups which you can easily purchase. The only option for me is to buy the entire encoder motor assembly...which I don't want. But I did see in this video that you were working on a 2000 Suburban which is basically the same chassis as the Avs. And yes, I know a scan tool would help but I have no access to one.

[@zx8401ztv]

Well done :-D When you said there were two sensors i did wonder if they were kind and made them the same type, swapping was in my mind too :-D. You have the dead one to hack apart if you are nosey like me ha ha :-D I watch quite a few pro repairers and the pico pops up quite often. South main auto and pine hollow diagnostics come to mind. 2 million ohms lol, as good as open circuit nearly.

[@spelunkerd]

Normally I don't like scopes that rely on a laptop, but in this case the automotive add-ons with the PICO are outstanding. I'm sure the scopes you use in electronics have better sample rates and more versatility. For automotives it is hard to beat the PICO. That said, there are tiny hand held scopes sold for a fraction of the price that might give most of what a practical tech will need.

[@zx8401ztv]

@@spelunkerd Yes i have the traditional oscilliscopes, but most of the time i use them for basic checks on a waveform spec. It's like using a cheap multimeter to do a Go - No Go check, the accuracy is not too important, just having the ability to say 'That's close enough.' will do :-D. Most things are not too critical.

[@justineasterling6034]

Great video! Are the front and rear prop shaft sensors for auto 4x4 only? So they are on the NP8 but not the NP1?

[@spelunkerd]

I don't know.

[@SVImpavidus]

A really good diagnostic video. Just wondering if the ABS light while on the stands could have been a slightly dragging disc brake or a bearing thats seen better days? Just a thought... Whats the hardware/software your using for a PC/Mac oscilloscope. We had to sell ours before we left on the big journey and I have to say we could have used it to help out a few folks, but space is an issue aboard. We have a boat a few berths away that has inverter issues on a pure sine wave inverter. I think the sine wave is clipped or modified not pure as its giving the microwave convolutions and has destroyed two toothbrush chargers. Thats almost always a sign (no pun intended) that the AC waveform is not right. So I want to check it..... Sail Safe guys. Ant, Cid and the pooch crew.

[@spelunkerd]

Thanks for stopping by, as you know I follow your sailing videos! This scope is a Picoscope, but it isn't practical because of size in a boat while cruising. Also this scope is aimed at 12 volt automotives, and is not designed for 230V coming off an inverter. Yes, you're exactly right, we had an inverter that did exactly that, weird AC waveforms. My computer wouldn't charge with that old inverter, we tossed it and bought another. If you are interested, you can buy compact hand held scopes for about $70 that would work for 12-24 volt circuits on a boat. One place scopes are useful for little marine diesel engines is to do a relative compression test, looking for variance in compression between cylinders. For that you need an amp clamp, capable of reading up to ~150A for the starter motor draw.

[@spelunkerd]

You got me thinking about a dragging caliper, ha ha. In this case I don't think so, but I'll check disc temp after a drive and I'll check that caliper the next time I get up on stands. The front diff on these trucks is on the left side, and there is extra hardware on the right, including the shift lock mechanism. So with more inertia and gears on the right it wouldn't be uncommon for slightly more drag that would be noticeable under a zero resistance situation. The diff compensates for even tiny differences in drag.

[@bgregg55]

I have scan tool envy.

[@spelunkerd]

Ha ha, I know what you mean. Scanner and scope envy can be like a dog chasing cars, you never get enough toys. Once you have all the tools, you realize (with humility) that the most valuable tool is between your ears. Many better techs do what I do with basic tools, I admire their efficient and practical workflow.

[@stevenwebb7949]

What kind of scan tool?

[@rogerw2101]

Thank you for this post . My 2000 Tahoe is doing this. It would shift in to 2 hi once in a while but now it don't shift in at all today . When I turn the key on it go s in to 4hi and won't unlock . The only way I could get 2 hi was to go to Auto 4wd and go back and forth a few times then all the lights go out and it goes to 2hi . So I need to change the front shaft speed censor ? And I can't get it in 4 lo at all either. The service 4 wd light stays on all the time too .

[@spelunkerd]

So many things could do this, I wouldn't want to condemn the sensor without proving it on live data. The encoder motor does the shifting and it may be going bad in your case. I would check the fluid levels and find a friend with a quality scan tool to get the codes and show live data. With the light on there is information you need to retrieve from the computer. This generation of transfer case was vulnerable to a problem called 'pump rub', where the pump wears a hole in the case causing levels to fall. Hopefully yours is OK, it would be obvious on inspection, with oil leaking from the case and low levels.

[@rogerw2101]

@@spelunkerd thank you for your reply. It was shifting but took a bunch of do n . Now it won't unless I go to 4 auto mess around then it will go in. I know I have to get it plugged in to see what I have going on . I would like to get a Tech II. Reader . I found a GM reader on Amazon for around 120.00 bucks . I didn't think that was bad

[@d46512]

370km on the original engine? If I recall you replaced the TC a few years ago, didn't you? Nice to see you keep the old gal going. More money for new tools!

[@spelunkerd]

Yeah, same truck, still going strong. Thanks for stopping by, Kevin! Dave

[@danielb7660]

What scanner tool do you use?

[@spelunkerd]

Presently I use the Autel 906 BT, but it doesn't offer a bus communication diagram, what they call 'topology'. That's critical for high level scan tools. The big problem with higher level scan tools is how expensive the updates are, so most just buy another scan tool every few years. That gets expensive if you don't use them every day! Mid level scan tools that show live data and allow some ability for bidirectional control are what most of us want, and prices for knockoffs are less than pro versions. I haven't yet found a scan tool that I don't learn to hate, but the Autel has been pretty good for the price.

[@Dave-zb7ss]

What's the make of the scope used in the video?

[@spelunkerd]

Oh, the scope is a Pico 4 channel 4425. Works great, software is aimed at auto mechanics, you need a windows laptop computer, it is an older version. Not as powerful or functional as a freestanding scope, but the software makes all the difference. New models have added features that are useful, but if you find an older version at a discount I'd give it a look.

[@robrussell609]

Was there drivability issues such as resistance(slowing) while turning? I’m having the same issue but isn’t drivable in 4wd have to switch out with the procedure.

[@spelunkerd]

Yes, it is normal for these vehicles to experience difficulty turning on asphalt when 4WD is engaged, especially in 4WD low. It is also hard on universal joints and tires. They design it this way to try and distribute power to all four wheels, without completely spinning out when one wheel has no traction.

[@robrussell609]

@@spelunkerd id figured it was shifting on its own into 4lo now I can breathe a lil easier

[@rickykojdecki6422]

Did you ever fix The transmission fluid leak? Or does anyone know where that might be coming from? Its a pretty heavy leak on an 03 avalanche and it seems like I would have to drop the trans to even figure out where it's coming from.

[@spelunkerd]

Yeah, I simply tightened the bolts holding the diff to the trans. I had neglected to put locktite on those bolts when I had the diff off, years ago. Problem solved just by retorqueing. I did a video, years ago, of the diff removal in case you're interested. In this vehicle the diff fluid and the trans fluid are different colors, so it is fairly easy to tell the two apart by looking at the color.

[@rickykojdecki6422]

@@spelunkerd man that's awesome you replied so quick. I have another question about a much bigger problem with the transmission. My truck when in drive stays in one gear doesn't shift up or down. I changed both shift solenoids, filter and speed sensor flushed the fluid still have the problem. I really want to say it's electrical but I don't know what would cause the vehicle to being one single gear. I can get to about 45 mph maxing out at 6,000 RPMs and won't get any faster

[@rickykojdecki6422]

My rear passenger brake (emergency brake) also locked up on my girl when she was driving and the caliper obliterated the mud flap and the bottom of my truck leaving shrapnel. So I just plugged the line after bleeding and replacing fluid. So all the lines and connections to that brake are shot. That wouldn't possibly be a reason it stopped shifting would it?

[@spelunkerd]

@@rickykojdecki6422 It would be easy enough to fix the brake caliper on that side, but no guarantees on the trans problem. A high end scan tool might show more clues, but unfortunately I think you're headed toward an opinion from a transmission shop. It is far beyond my humble knowledge base.

[@stevenwebb7949]

What kind of scan tool are you using?

[@spelunkerd]

Autel 906BT. Updates are expensive but for now it is still running despite the fact I didn't pay for continuous upgrades. Any tool with live data should be able to show what you need for this kind of repair, you don't need the most expensive scan tool.

[@stevenwebb7949]

Thank you! Your video helped me. Mine wouldn't shift out of 4wd and after watching the video, I looked under truck and found the front propshaft sensor wires were pulled out of the connector. I pushed them back in and it now works correctly. Thanks again!!

[@spelunkerd]

@@stevenwebb7949 If it happens again, you might check to be sure the connector isn't broken. There is a technical service bulletin about this problem is some of these vehicles. The solution is a slightly different connector. Thanks for the feedback!

[@stevenwebb7949]

@@spelunkerd Thanks for the heads up. I had just recently had the tranny rebuilt and I'm guessing they were responsible for the the wires being pulled out. Simple enough fix though. Thanks again!

[@kdlittlehawk]

would this be the same for a 2002 Chevy Av?

[@spelunkerd]

Probably similar. These old style variable reluctance sensors were cheap and effective, though at low speeds the data was not completely accurate. Newer vehicles have switched to a more expensive Hall effect sensor with more wires and better accuracy, but I don't know when that transition occurred. If the sensor has only two wires, odds are it is the old VR style.

[@kdlittlehawk]

@@spelunkerd thanks, today I need to take a good look and see where these sensor/s are...and test them. I have no access to a scan tool so I'll be methodical in troubleshooting it old skool'way. I already ruled out the front diff actuator, that works fine. Everyone is telling me its the encoder motor but I can hear that running hen I press the dash 4x4 buttons. And yes, it could be just the encoder ring itself..which I'll probably be testing that as well if these sensors show up as ok. It appears there are 3 electrical tabs coming off the encoder ring itself. Are you aware what reading I should get through them?

[@kdlittlehawk]

@@spelunkerd I pulled both sensors and tested them with an ohmeter, both within the specs, chk'd connectors too and re-installed. I'd like to to test the 3 pin encoder ring without removing entire assy. Do you happen to know what wires to probe and what the normal readings should be?

[@spelunkerd]

@@kdlittlehawk I've never tested the encoder motor so I can't help with that. The dealers service manual is good for details like that. Alldatadiy is pretty good for dealers manual access, although their prices recently were raised. Even so, you can get a few days of access for pretty cheap. If you could get your hands on a scan tool that reads live data you might find clues that offer more direction.

[@kdlittlehawk]

@@spelunkerd my actual "motor" is good as I can hear it run. I am talking about testing the actual ring sensor..without a scan tool (which I have no access too) Sorry for the confusion.

[@andrewheniger8748]

Makes noise when I try to get 2wheel in ,will not shift out 4 wheel. Any one know ?

[@spelunkerd]

Hard to say. The encoder motor does the shifting and it can go bad, and even bad wiring can disable the motor. Or the encoder motor may be unable to shift because of mechanical problems inside the transfer case. I would start with basics, make sure the fluid level inside the case is correct, make sure there is no leak of transfer case fluid out. A scan tool will help with diagnostics. You may find, similar to what I found, that shifting back to 2WD can happen after cycling the ignition key. I would try to avoid driving while that grinding sound is happening. This kind of repair is beyond the scope of most backyard mechanics, and even the physical challenge of removing the transfer case is much more difficult without a lift.

[@davewidmer7641]

I have a 1995 3500 when I put it in 4 by4 The blower motor turns off

[@spelunkerd]

That's interesting. When two separately fed circuits are tied together like that, think of a bad ground connection, somewhere. Voltage searching for a ground feeds back through other devices to find a way home to the battery. It's a fun exercise, made easier with a wiring diagram. Most often it's a bad chassis ground, from one of the two, so you may want to look there, first. Or, if you have aftermarket devices under your dash, they can mess things up, too.

[@davewidmer7641]

@@spelunkerd thank you for your help.

[@maciejolejniczak151]

What happened to a dodge ??

[@spelunkerd]

Still have it, working on another Dodge Ram video tonight, ha ha. Love these old cars. Thanks for stopping by!

[@anthony.mervin6129]

Let’s talk about the suv having 180 on the dash

[@spelunkerd]

She rolled past 400 km last summer, still going strong. Those old LS engines, without VVI or other complexities, last far longer than the body.

[@AlexGarcia-jw7ef]

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