NEED HELP Tranny slipping and surging. 4WD switch flashing

filthyfingers

Registered Member
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

Master Blaster
Moderator
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.
 
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

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

filthyfingers

Registered Member
Thread Starter
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.
 
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.
 
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

Registered Member
Thread Starter
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.
 
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

Registered Member
Thread Starter
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?
 
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
 
Last edited:

filthyfingers

Registered Member
Thread Starter
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.
 

filthyfingers

Registered Member
Thread Starter
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.
 

Mooseman

Master Blaster
Moderator
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.
 

filthyfingers

Registered Member
Thread Starter
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

Master Blaster
Moderator
Definitely Upulls are cheaper than anywhere else.
 

filthyfingers

Registered Member
Thread Starter
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

Registered Member
Thread Starter
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!
 

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