NEED HELP Tranny slipping and surging. 4WD switch flashing

filthyfingers

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2002 Envoy SLE with 130k miles. As soon as I pulled out of the driveway it felt like the transmission was slipping badly. I havent used 4WD in months. Always in 2HI position. I noticed the 4LO light was on and the 2HI light flashing. I believe it’s stuck in 2HI.
No CEL. No codes. Moving the selector doesn’t do anything, no sound, no changes in the 2 lights. Shifter cable works. Fluid color and level look normal.
Any advice is appreciated. Thanks
 

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Mooseman

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Dec 4, 2011
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Has the TCCM ever been replaced? The original 02 and some 03 TCM's had an internal defect. Mine would light all the lights and then shut off with a service 4x4 light on. It could also be the encoder motor stuck between two gears.

Could also be the switch. Try cleaning the contacts by turning it back and forth through all settings with the ignition off.
 
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TJBaker57

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Aug 16, 2015
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If the service 4wd light is on, and you do not have the ability clear codes with a scantool you can (and should) reset the TCCM by removing the ATC fuse under the hood for a moment then replacing it. Be sure to have the selector switch in the 2Hi position before replacing the fuse.
 

Matt

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Dec 2, 2011
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I'm going to say that the TCCM is farked. Classic 02 behavior, but do the diag like Moose and TJ suggested first.
 

filthyfingers

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I am the original owner and keep pretty good records. Don’t see where TCC module was ever replaced. I’ll clean the switch tomorrow and start hunting a module. Thanks very much.
 

TJBaker57

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Aug 16, 2015
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My guess would be the Transfer Case Range Position Sensor or a poor connection in that circuit. The display indicating a position of 4Lo means the TCCM is getting back a sensor signal voltage of about 0.77 volts. If the transfer case is actually in 2Hi the signal voltage should be in the neighborhood of 2.75 volts. Mine had a cracked solder joint, the result of a poor solder job from the factory.
 

TJBaker57

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Aug 16, 2015
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I am the original owner and keep pretty good records. Don’t see where TCC module was ever replaced. I’ll clean the switch tomorrow and start hunting a module. Thanks very much.


The Upull yards around here have plenty and having pulled a few I can say it actually can be done without tools even! You do need to know the tricks :wink: I have 3 on the workbench right now!
 

filthyfingers

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Apr 7, 2012
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My guess would be the Transfer Case Range Position Sensor or a poor connection in that circuit. The display indicating a position of 4Lo means the TCCM is getting back a sensor signal voltage of about 0.77 volts. If the transfer case is actually in 2Hi the signal voltage should be in the neighborhood of 2.75 volts. Mine had a cracked solder joint, the result of a poor solder job from the factory.
Thanks. I’ll check it tomorrow.
 

TJBaker57

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Aug 16, 2015
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Colorado
Thanks. I’ll check it tomorrow.


Do you have a bluetooth obd2 adapter? For most of the versions of the TCCM I can tell how to read out the Range Position Sensor return signal voltage. If not by OBD2 adapter the voltage can also be read out by backprobing the proper wire(s) at the TCCM in situ through the left end dashboard access panel. I have a graphic of the pertinent wiring if you should choose to do so. Otherwsie it requires a laborious process under the truck.
 

filthyfingers

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Do you have a bluetooth obd2 adapter? For most of the versions of the TCCM I can tell how to read out the Range Position Sensor return signal voltage. If not by OBD2 adapter the voltage can also be read out by backprobing the proper wire(s) at the TCCM in situ through the left end dashboard access panel. I have a graphic of the pertinent wiring if you should choose to do so. Otherwsie it requires a laborious process under the truck.
No Bluetooth adapter but ok on the OBD2. Graphic? Backprobing?
 

TJBaker57

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Aug 16, 2015
2,907
Colorado
So a brief (maybe) rundown on the pertinent stuff. To start with the TCCM sends out a 5 volt signal voltage to the dashboard selector switch and both a 5 volt and a low reference ground to the position sensor down at the transfer case shift motor. When the dash switch is moved it sends back a lower voltage to the TCCM. This is how the TCCM know what range is desired by the position of the switch on the dash. Similarly the range position sensor inside the transfer case shift moter (encoder motor) sends back a less than 5 volt signal reporting the current position of the transfer case & shift motor. If either of these sensor/switches get worn or dirty the signals back to the TCCM don't really represent what the true state of the devices are. Failing or otherwise faulty connections produce erratic behaviours.

I find checking the switch and range position sensor return voltages via a bluetooth OBD2 adapter and the Torque Pro Android app the easiest way to see what's going on with the inputs to the TCCM. It requires some custom setup as it does not come with these capabilities.

Second easiest way I know is to use a multimeter and backprobing the connections at the TCCM. This requires knowing which wire(s) to check and having something to probe with. An actual backprobe set is nice but it can be done with other things like a piece of small gauge wire or a needle with alligator clip jumpers.

So for the encoder (range position sensor) return one would read the voltage between C1, B6 and C1, A7. You can substitute a known good ground for C1, A7. For the dashboard switch read the voltage at C1, A6 and ground.

IMG_20200422_175022.jpg

The voltages should be close to these values...

Screenshot_20210728-163645.png
 
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filthyfingers

Original poster
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Apr 7, 2012
103
So a brief (maybe) rundown on the pertinent stuff. To start with the TCCM sends out a 5 volt signal voltage to the dashboard selector switch and both a 5 volt and a low reference ground to the position sensor down at the transfer case shift motor. When the dash switch is moved it sends back a lower voltage to the TCCM. This is how the TCCM know what range is desired by the position of the switch on the dash. Similarly the range position sensor inside the transfer case shift moter (encoder motor) sends back a less than 5 volt signal reporting the current position of the transfer case & shift motor. If either of these sensor/switches get worn or dirty the signals back to the TCCM don't really represent what the true state of the devices are. Failing or otherwise faulty connections produce erratic behaviours.

I find checking the switch and range position sensor return voltages via a bluetooth OBD2 adapter and the Torque Pro Android app the easiest way to see what's going on with the inputs to the TCCM. It requires some custom setup as it does not come with these capabilities.

Second easiest way I know is to use a multimeter and backprobing the connections at the TCCM. This requires knowing which wire(s) to check and having something to probe with. An actual backprobe set is nice but it can be done with other things like a piece of small gauge wire or a needle with alligator clip jumpers.

So for the encoder (range position sensor) return one would read the voltage between C1, B6 and C1, A7. You can substitute a known good ground for C1, A7. For the dashboard switch read the voltage at C1, A6 and ground.

View attachment 101351

The voltages should be close to these values...

View attachment 101352
A lot to digest. Thanks. I’m on it.
 
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filthyfingers

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Today removed the side panel and saw TCCM was not mounted, just laying there. Now I’m sure I did replace it 10 or more years ago. I took her for a drive and although the switch said 2HI I could feel that 4WD was engaged.
I switched to A4WD - no change. I switched to 4HI and the light changed to 4HI and stayed there. Back in the driveway with the key out, I rotated the switch selector several cycles and left it in 2HI.
Started up and heard the 4WD disengage. Now in 2HI runs like a champ! I’m hesitant to mess with it now. But I would like to buy a spare module. Does it matter which part number? New or used?
Y’all are great. Thanks again.
 
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Mooseman

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Dec 4, 2011
25,487
Ottawa, ON
Sounds like it was the switch so it's all good.

For a spare TCCM, look for any used one from 04+. A new one would require programming and is too expensive. Looks like you replaced it when the original one failed. Haven't heard of any of the later ones failing but it could happen I suppose.
 
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filthyfingers

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Sounds like it was the switch so it's all good.

For a spare TCCM, look for any used one from 04+. A new one would require programming and is too expensive. Looks like you replaced it when the original one failed. Haven't heard of any of the later ones failing but it could happen I suppose.
Looking on eBay. Several available.
 

Mooseman

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Definitely Upulls are cheaper than anywhere else.
 

filthyfingers

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Okay I’m calling it SOLVED. Replaced the TCCM snagged from eBay and a Dorman selector switch. With a little care, the switch was easily replaced from the front. That was weeks ago. ALL GOOD.
Thanks again. YOU GUYS ROCK!
 

filthyfingers

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Today removed the side panel and saw TCCM was not mounted, just laying there. Now I’m sure I did replace it 10 or more years ago. I took her for a drive and although the switch said 2HI I could feel that 4WD was engaged.
I switched to A4WD - no change. I switched to 4HI and the light changed to 4HI and stayed there. Back in the driveway with the key out, I rotated the switch selector several cycles and left it in 2HI.
Started up and heard the 4WD disengage. Now in 2HI runs like a champ! I’m hesitant to mess with it now. But I would like to buy a spare module. Does it matter which part number? New or used?
Y’all are great. Thanks again.
Late response… for the sake of continuity. The 4WD failed again a few days later (July). Still at this point, solved!
 

filthyfingers

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Now, 2 1/2 years later, same problem returned. TCCM seems to have failed. Stuck in 4WDLow. Selector switch is ok. I have a spare and swapped it out- no help. I was hoping that the system might default to 2WD, not happening so far. Even with the TCCM removed, still stuck in 4WDLo.
Any way to force 2WD?
 

Mooseman

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Dec 4, 2011
25,487
Ottawa, ON
You can remove the encoder motor and turn the shaft with some pliers.

So you replaced the TCCM with a known good one I would assume. If that's the case, I would assume the encoder motor has failed. Any flashing lights or a 4WD error light on dash?
 

filthyfingers

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Used TCCM is on order, be about a week away. 4WD error light on the dash. Light on the switch stuck on 4Lo. This is the third time. Same symptoms. Priors were fixed by replacing the TCCM. I’ll check out the encoder motor. Thanks!
 

Mooseman

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Dec 4, 2011
25,487
Ottawa, ON
The TCCM is probably good if the light is staying on 4lo. It's reporting it's stuck there. My money is on the encoder motor. You can remove it from the TC connected and turn the selector switch. If it doesn't move, it's toast.
 

mrrsm

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And for the sake of covering all the bases... This 2006 GM TSB makes some additional Diagnostic Observations and suggests the need for updating the PCM-BCM Calibration via a Bi-Directional Scan Tool capable of Reading any CXXXX Codes and Writing the Calibrations as PASS-THRU Devices following the SPS Procedures via "The Good Old TIS2000 Software" and (Tech 2, Autel MaxiDAS DS708, VXDIAG NANO, etc.):
 

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filthyfingers

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The TCCM is probably good if the light is staying on 4lo. It's reporting it's stuck there. My money is on the encoder motor. You can remove it from the TC connected and turn the selector switch. If it doesn't move, it's toast.
Removed old encoder motor. New one doesn’t line up. Connected it unmounted and turned the switch. Switch lights performed normally for a short time. Not sure if the motor turned. Still unable to mount the new one. Its position still doesn’t line up.
Now nothing works. The switch is dark, the new motor doesn’t turn, and the transmission won’t go into gear. And no codes except for the light on the instrument cluster. Got another encoder motor on order.
I was unable to turn the shaft with pliers. Don’t want to damage it. I suppose there is a spline tool. Why wouldnt the transmission engage?
 

mrrsm

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And this Bulletin covers the post install set up information:

Bulletin No.: 02-04-21-006E

Date: July 20, 2006

TECHNICAL
Subject:
Inoperative 4WD/AWD Lamps, Inoperative 4WD/AWD System, DTC C0550 Set, No Communication with TCCM (Reprogram Transfer Case Control Module)

Models:
2004 Buick Rainier
2002-2004 Chevrolet TrailBlazer, TrailBlazer EXT
2003-2004 Chevrolet Avalanche, Silverado, Suburban, Tahoe
2002-2004 GMC Envoy, Envoy XL
2003-2004 GMC Sierra, Yukon, Yukon XL
2004 GMC Envoy XUV
2002-2004 Oldsmobile Bravada

with Electronic Shift or Active Transfer Case (RPOs NP1, NP4 or NP8)

Supercede:

This bulletin is being revised to add Subject information and change the labor operation. This bulletin cancels and supersedes Bulletin Number 03-04-21-006. Please discard Corporate Bulletin Numbers 02-04-21-006D and 03-04-21-006 (Section 04 - Driveline/Axle).

Condition

Some customers may comment that the 4WD/AWD indicator lights on the selector switch do not work, or the 4WD/AWD system may be inoperative and/or the service 4WD light is illuminated. The condition is typically intermittent and always occurs at key-up. Upon investigation, the technician may find DTC C0550 set.

Cause

The Transfer Case Control Module (TCCM) has not received the wake-up call during key-up and has remained in the "sleep mode".

When the TCCM is in the "sleep mode", it will not communicate with the class II bus or the Tech 2®.

Correction
1. Try to communicate with the TCCM. If no communication, go to step 2. If communication is possible, go to step 5.

2. Remove the underhood (ATCM or TREC) TCCM fuse.

3. Wait 30 seconds.

4. Reinstall the fuse. The module should wake up and communicate with the Tech 2®.

5. Check for codes in the TCCM. If a C0550 is found current or in history with or without other codes, replace the module and go to step 6.

6. Reprogram the TCCM with the latest TIS software using normal SPS procedures.

7. Verify operation and that no codes are present.
 

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Mooseman

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Dec 4, 2011
25,487
Ottawa, ON
If the tranny isn't engaging, it's quite possible that the transfer case is in neutral so be careful with the truck parked on its wheels as it may roll away. Use the parking brake and/or wheel chocks.

You could slip the encoder motor onto the TC shaft and turn it using the motor so the bolt holes will line up. It shouldn't be that hard to turn. Likewise using pliers.

Still a mystery while the lights don't come on. Try the reset as mentioned by @mrrsm . The TCCM should try to move the encoder motor to the selected mode on the switch.
 
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filthyfingers

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Apr 7, 2012
103


And this Bulletin covers the post install set up information:

Bulletin No.: 02-04-21-006E

Date: July 20, 2006

TECHNICAL
Subject:
Inoperative 4WD/AWD Lamps, Inoperative 4WD/AWD System, DTC C0550 Set, No Communication with TCCM (Reprogram Transfer Case Control Module)

Models:
2004 Buick Rainier
2002-2004 Chevrolet TrailBlazer, TrailBlazer EXT
2003-2004 Chevrolet Avalanche, Silverado, Suburban, Tahoe
2002-2004 GMC Envoy, Envoy XL
2003-2004 GMC Sierra, Yukon, Yukon XL
2004 GMC Envoy XUV
2002-2004 Oldsmobile Bravada

with Electronic Shift or Active Transfer Case (RPOs NP1, NP4 or NP8)

Supercede:

This bulletin is being revised to add Subject information and change the labor operation. This bulletin cancels and supersedes Bulletin Number 03-04-21-006. Please discard Corporate Bulletin Numbers 02-04-21-006D and 03-04-21-006 (Section 04 - Driveline/Axle).

Condition

Some customers may comment that the 4WD/AWD indicator lights on the selector switch do not work, or the 4WD/AWD system may be inoperative and/or the service 4WD light is illuminated. The condition is typically intermittent and always occurs at key-up. Upon investigation, the technician may find DTC C0550 set.

Cause

The Transfer Case Control Module (TCCM) has not received the wake-up call during key-up and has remained in the "sleep mode".

When the TCCM is in the "sleep mode", it will not communicate with the class II bus or the Tech 2®.

Correction
1. Try to communicate with the TCCM. If no communication, go to step 2. If communication is possible, go to step 5.

2. Remove the underhood (ATCM or TREC) TCCM fuse.

3. Wait 30 seconds.

4. Reinstall the fuse. The module should wake up and communicate with the Tech 2®.

5. Check for codes in the TCCM. If a C0550 is found current or in history with or without other codes, replace the module and go to step 6.

6. Reprogram the TCCM with the latest TIS software using normal SPS procedures.

7. Verify operation and that no codes are present.
Thanks! I’m on it! disconnectEd and reconnectEd the battery . TCCM might be asleep or worse. TCCM and another encoder motor should be here tomorrow. Can’t shift out. Could the TC be stuck in neutral? I’m up on front ramps with EBrake pulled.
 
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filthyfingers

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If the tranny isn't engaging, it's quite possible that the transfer case is in neutral so be careful with the truck parked on its wheels as it may roll away. Use the parking brake and/or wheel chocks.

You could slip the encoder motor onto the TC shaft and turn it using the motor so the bolt holes will line up. It shouldn't be that hard to turn. Likewise using pliers.

Still a mystery while the lights don't come on. Try the reset as mentioned by @mrrsm . The TCCM should try to move the encoder motor to the selected mode on the switch.
Thanks ! Didn’t know the TC could hang in neutral, never thought of that.
Cant tease it on bolt ears in the way.
Disconnected the battery and reset fuse #8.
Another motor and TCM expected late Tuesday.
I am having a conniption!
 

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filthyfingers

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Hanging my hopes on the incoming TCCM. Will try harder with the pliers or vice grips on the shaft tomorrow.
 

Mooseman

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Dec 4, 2011
25,487
Ottawa, ON
If it's in 4LO, it might be stuck there because the truck has to be moving 1-2 MPH to shift it. You might have to actuate the encoder motor to 4LO before installing it or try each position until you find the one that the transfer case is in and you can slip the encoder motor onto the shaft. What you might also try is have the four wheels off the ground so you can rotate them and then shift the TC shaft with pliers.
 

filthyfingers

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UPDATE
After trial and error I managed to move the TC shaft out of 4Lo. Then the shaft turned with pliers 1 or 2 positions only. The new motor was way out of position since I turned it using the switch while it was not mounted. Did the same thing with the old motor. By that time everything had shut down. Switch was dark and the TC was stuck in neutral.
Luckily the old motor had moved to the 2Hi position!
l remounted the old motor now in the 2Hi position and Bingo! Everything worked.
She started up, dropped into gear, and the selector switch read 2Hi. Drove around awhile, all okay. Im too chicken to exercise the selector switch but I do intend to return and replace the motor. Now that we’re in 2Hi.

Bottom line, How can you get back into 2Hi if the truck faults into 4Lo? Operating the motor while not mounted seemed to work for a while then everything shut down. Plus if working alone, you can’t see what’s going on.
 

Mooseman

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Dec 4, 2011
25,487
Ottawa, ON
It's weird that with the motor unmounted it works a while and then goes dead. When it goes dead, does the service 4x4 light come on? If that happens, you need a scanner to get the error code from the TCCM before it resets. @TJBaker57 could help with getting those codes using a Bluetooth Elm327 adapter and an Android device.

Maybe using it too many times screws it up, especially if it's not mounted? Maybe the switch is flaky?

As far as not being able to shift it out of 4lo, it's usually not an issue unless something is not working right, like now. It shouldn't be a normal affair to remove the motor to shift it. 4lo is also not something that used very often unless you off road on a regular basis.
 

filthyfingers

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Apr 7, 2012
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I never use 4Lo ever. I’m the original owner of this baby and have survived 3 4WD failures. I always use 2Hi, and each failure defaulted to 4Lo. Is that significant?
 

Mooseman

Moderator
Dec 4, 2011
25,487
Ottawa, ON
Very strange. It shouldn't shift to anything if you don't ask for it. Or again maybe the switch gets corroded and then defaults to 4lo because of the resistance value the TCCM reads? That's just a guess. It would be good to exercise the switch while the truck is OFF through all its settings and put it back to 2wd once in a while. Or if you never use anything but 2wd, you could just make sure it's in 2wd and unplug the motor. You'll get the service 4x4 light but you can ignore it. It should not move from that position as the motor is very low geared and the TC wouldn't be able to move the motor gears and it has a brake.

BTW, just in case you didn't know, you have to replace the TC fluid every 50k miles regardless if you use 4x4 or not.
 

filthyfingers

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Apr 7, 2012
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Separated the connector, taped up the ends. Now stuck in 2Hi. HORRAY!
Maybe I’ll replace the TC motor, maybe not. I have months to think about it. If the failure mode historically always defaulted to 4Lo it will likely happen again. No thanks.
Meanwhile, thanks again! You guys rock!
DONATION TO THE ’NATION!
 
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