Envoy transfer case encoder motor stuck in neutral?

[jmac219]

I have a 02 Envoy SLT. The other day I was driving and stopped at stop sign. I went to press the gas, and the truck would not move in ANY gear. When I put it in park, I heard a loud ratcheting noise coming from underneath the truck. The transmission fluid was fine. Then I tried to put it in 4hi, low and behold the truck regained all of its gears. When I put it back in 2hi, it worked fine again. I replaced the 4x4 selector switch. Well... this issue happened again. So I hooked up my Autel MD802 scanner on it to read the automatic transfer case to read the encoder motor position on the scanner itself. I noticed that sometimes when I shift the 4x4 selector on 4wd, auto, and 4wd. The transfer case encoder motor will still show neutral on the scanner, even though the 4wd selector switch is either on 2hi, auto, or 4wd. And the truck will not move when my scanner shows that the transfer case encoder motor position is on neutral.. This does not always happen. Sometimes it will go into 2hi & 4wd. Also, the neutral light does not illuminate on the 4wd selector switch.

SO could the transfer case encoder motor be the culprit here? And causing the transfer case to be stuck into the neutral position at times? Or could it be the TCCM? I'm just looking to see if anyone else has had this problem before. Also, I am shifting from 2hi to 4hi or lo when my gear shifter is placed into neutral.

Thanks!

 

[IllogicTC]

I would expect the motor to be, yes. I'm surprised the Service 4x4 lamp isn't coming up since the motor is obviously not achieving the desired position. It might be a flaky TCCM though, too. Not entirely sure, but leaning toward the motor.

It's good that you have a scanner that could identify that, my first thought was a scrapped planetary gearset in the head of the case, like another guy had. The grinding was the parking pawl attempting to stop the still-spinning transmission components. While you're lining your ducks up, I would suggest when shifting to Park to first shift to neutral for maybe 10-15 seconds, then briskly (not break-neck, but don't let it hover over the Reverse selection) shift to park. Hopefully by then the transmission will have lost its inertia and the pawl can engage. But as an extra precaution, set the parking brake. Obviously if the TC is in neutral the parking pawl is just a glorified transmission staller.

I'm sorry nobody got to this one earlier, usually The_Roadie hops on the 4x4 threads quite quickly

 

[The_Roadie]

Welcome! Yah - got busy...Sorry. Could be a lot of broken bits inside the transfer case as well.

How many miles? Have you and/or a previous owner been religious about changing the transfer case fluid every 50K? What's theage, level and nature of the TC fluid now?

2002 TCCMs were NOTORIOUS for being flaky and not communicating, and then the PCM will set the "Service 4WD" lamp. Only cure for that is swapping to the updated 2003+ TCCM.

But there's an interlock, so you have to be in transmission NEUTRAL to get into 4LO, and I thought the transfer case NEUTRAL has the same requirement. I didn't think any sort of control switch fault except for an open harness, could cause a command to go into TC neutral.

Do all your other mode transitions, such as 2HI->A4WD, and A4WD->4HI, make the expected noises from the TC encoder motor and the front axle disconnect actuator?

 

[jmac219]

Ok, so I uploaded a video of the issue. Hopefully it explains it a little better. Sorry the video is hard to watch because it is sideways... But hopefully it can explain it better.

Updated Video
https://www.youtube.com/watch?v=eP_uXlkTHOA&feature=youtu.be
Sorry if I'm hard to understand. I'm coming off of a nasty cold & in the process of moving, so I'm pretty tired.
To answer your question. The transfer case fluid was changed 30K ago. Sorry if the video doesn't show much. When it is in the 4hi or 4lo mode, it DOES work. I confirmed this by lifting all four tires off the ground.

 

[IllogicTC]

Interesting that in one case you switched the selector one way, and the encoder seems to either just jump to a random spot in that direction. The encoder's voltages are measured via the TCCM, it would appear at least that part of the circuitry is functioning but why it's not throwing up a service 4x4 I have no idea. Does the service 4x4 lamp even work? (should light up during starting) You are very fortunate to have a detailed scan tool like that, too!

Just past the 2:30 mark, you go to shift to 2HI and the voltage jumps down and then back up in the 3.xx range, showing 4Auto again. Also toward the beginning you switch to 4HI, and it stays in 4Auto even though you say "OK" the commanded position readout clearly changes, but the encoder does not move at all, the reference voltage stays the exact same.

 

[Mooseman]

I would get a TCCM from a junker (2003+) and swap that first. If you go to a pick-a-part (if you have one), should be fairly cheap. If it still acts the same way, then on to the encoder motor.

That IS one nice scanner

 

[jmac219]

Ok, so check my video post for a updated video. I remade it so it could be viewed better. I did check to see if the 4wd service light even illuminates when I turn the key on the on position. It does work, however there are no DTC's showing. Thanks Mooseman. It sure was a upgrade from my Innova 3160 scanner. I can finally read all of the modules! Which is very handy sometimes. Hopefully this video is much easier to view than the first one!

 

[The_Roadie]

That scanner is to die for.

Anyway, the TCCM is clearly out to lunch because when the encoder motor fails to go to the commanded position, the TCCM should flash the mode switch light on the desired mode, and yours just goes happily along, lying to you that it's int he right mode. If the encoder motor potentiometer wiper was dirty, that could explain the bad voltage readback, but not the TCCM behavior.

Wish you had done one more thing in the video, though (hope I didn't miss it) - try on purpose to go into NEUTRAL. You have to hold it a number of seconds to satisfy the TCCM interlock. Would be interested what the readback voltages are.

To make it driveable - I'd get it into 2WD then yank front fuse 8 to freeze the encoder motor in place.

 

[IllogicTC]

It should be pointed out that the actual NEUTRAL selection is a "secret" selection not clearly identified on the fascia of the switch. You'll see a little red N light up as part of the POST, that's where you need to crank it. If I remember right it's spring-loaded kinda like the DRL Defeat selection.

 

[jmac219]

Ok so I followed the instructions in the owners manual on how to manually put the transfer case into neutral. I took a short video of my failed attempt. At this point I'm thinking it's a bad TCCM. I also went through some of the data on the transfer case on my scanner in the video. Sorry if the video is hard to see again. My phone camera is not the greatest.

Also I would like to add. I know you can't hear it in the video. Most of the time when I am shifting the 4wd knob. I can only hear the front actuator engaging. I do not always hear the normal transfer case clunk from underneath. It only does it randomly when I switch through the selector modes.

Thanks for everyone's replies! https://www.youtube.com/watch?v=31ymN0IMS4A&feature=youtu.be

 

[IllogicTC]

A pull-a-part TCCM from 03+ seems like a viable solution. Getting one new is always an option, but not only is it more expensive, but you'll have an added cost of getting it programmed on top of that. Used units are already set up.

 

[The_Roadie]

I'm thinking the encoder motor potentiometer behavior is also suspect, and it's relatively easy to pull it off and check it out.

 

[The_Roadie]

YOu did exactly the right thing, and the TCCM even saw the commanded mode was Neutral. Either the TCCM failed to command the encoder motor to move, or it it misread (or the encoder motor mis-sent) the position feedback voltage.

Something's odd about the display on your scan tool, and I'd guess it's a software bug - at one point it says the encoder motor is in a "2 LO" mode. We don't have a 2 LO mode, although a few of us have speculated over the years aboutfiddling with the logic and making it happen. It involves commanding the front axle disconnect to release, while the transfer case is in low range. Could be useful on trails where you need low range for torque, but you're going to be turning the front wheels and don't want to stress the CV shafts. People have broken stuff (mostly the disconnect) on trails under these conditions, but it usually involves stupidly high throttle application also.

 

[IllogicTC]

I noticed that, too. I'm curious though if it's a mis-statement in the reader or in the TCCM though, unless he's got some GM-specific programming loaded up in the scanner it's just accessing what it can and repeating what it's told.

 

[jmac219]

Ok so I'm looking on Ebay for a used TCCM. I would be doing the job myself. I know people keep saying to use a TCCM from 2003+. However most of the one's on Ebay just say 02-05. How do I know it's not from a 2002? Here is the one I currently have on my Envoy.

 

[The_Roadie]

That's the bad version. Look for a 12590220.

 

[jmac219]

I just ordered a used #12590220 off Ebay. I will let you all know how it works when I hook it up. Thanks again!

 

[IllogicTC]

I just ordered a used #12590220 off Ebay. I will let you all know how it works when I hook it up. Thanks again!

I wonder if it will come programmed though, or if you'll have to take it to the stealership...

 

[The_Roadie]

Used usually means "removed from a vehicle that was running", so it won't need to be subject to the dealer's cold, clutching, mitts.

 

[IllogicTC]

I didn't see the "used" part, I went straight from ordered a, to the part number.

 

[jmac219]

Update:I just put the used TCCM on. Still the same results. So I'm thinking the encoder motor itself needs to be replaced... I'll check back in when I get it on. Is dorman a decent brand for a new encoder motor?

 

[The_Roadie]

Awwww, too bad. Dorman can be hit or miss on some items like fan clutches. Not sure I've heard anything flaky about their encoder motors, though. If you were closer I'd loan you one of my spares to try.

 

[Mooseman]

I've read about one Dorman encoder that failed and exchanged under warranty. YMMV

 

[IllogicTC]

Update:I just put the used TCCM on. Still the same results.

It's okay, it was still a good investment as the 2002 TCCMs aren't the best anyway. You've done a proactive retrofit!