4WD Inop

darceye

Original poster
Member
Jan 2, 2012
4
Migrated from the other site...thanks for the heads up.

First snow here, so last time I used 4WD was in March. Last week, switched into AWD and heard the front acutator motor shift. All seemed normal. Tried to switch to 4WD-HI and the light just blinked and never shifted. Tried to shift back to 2WD and lights blinked, but never shifted and stayed in AWD. I did not hear the front actuator try to switch back to disconnect.

Everytime I try to shift out of AWD, the light blinks to the new selection, but after about 10 seconds, the light stays solid on in AWD. If I try to swithch into 4WD after a couple of times, it will blow the number 8 fuse and the Service 4WD light comes on and the 4 indicator lights do not light. Put in a new fuse and the Service 4WD light goes out and the indicator light shows still stuck in AWD.

The AWD light is on, but does not work. Only the back wheel spin. Also cannot hear the front wheel actuator run any longer...

Kind of like the actuator motor worked the first time, then never again.

Looks for any suggustions as to next step to take.

2003 Envoy. 97000 miles. 4.2 liter

Thanks for reading.
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
Welcome! The 4WD system is sort of my specialty.

Your problem sounds like it's related to a jammed transfer case encoder motor. The front axle actuator doesn't have a large enough motor to blow fuses, but the encoder motor can. It also has internal planetary gears that can break and jam.

But especially worrisome is the transfer case itself. Many owners don't get the memo (or notice in the Owner's Manual :wink:) that the fluid has to be changed religiously every 50K miles. Are you current on fluid changes? Your differentials are also coming due at 100K, but the transfer case is a surprisingly short interval.

A dry transfer case can cause excessive force at the control shaft that the encoder motor rotates. You can remove the encoder motor from the transfer case and run the system in the air, after a fuse-pull to reset the system.

Are you a DIY mechanic or just looking for a bit of information before taking it into an independent shop?
 

darceye

Original poster
Member
Jan 2, 2012
4
Yikes...was not aware of the fluid change intervals. I did change the Automatic Transmission fluid but not transfer case or diffenentials. Funny, I had an 02 TrailBlazer that I ran 130,000 miles...never changed the fluid on it either...lots of highway miles on that one and little use of AWD. Guess I was lucky there.

Good info on the front actuator not pulling enough current to blow a fuse.

My guess now is the front actuator is engaged, thus the indicator showing AWD. I will lift the front and check to see if the drive shaft turns.

Guess the next step after that is to pull the tranfer case encoder motor. Hopefully I am not past the point of no return on repair.

Yes I am a DIY mechanic. Any advice on what to look for when pulling the encoder motor? I do have access to a car lift.
 

The_Roadie

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Member
Nov 19, 2011
9,957
Portland, OR
darceye said:
Yikes...was not aware of the fluid change intervals. I did change the Automatic Transmission fluid but not transfer case or diffenentials. Funny, I had an 02 TrailBlazer that I ran 130,000 miles...never changed the fluid on it either...lots of highway miles on that one and little use of AWD. Guess I was lucky there.
I've never pulled my encoder motor, but it's only four bolts. Then try to rotate the shaft it connects to, and run the encoder motor in midair.

We hear from a lot of second (and subsequent) owners with dry transfer cases, who bitch out the former owners for passing on a known lemon. Didn't know it could be inadvertent.
 

jamienfld

Member
Jan 4, 2012
16
Hello just joined from the other site also. Mine is doing the same thing only not blowing the fuses. After reading few sites thought might be the TCCM. Would the TCCM cause this? I can't hear the transfercase motor or the actuator. Like yours mine would Also blink on 2wd or 4wd if i turned the switch to them but then go back to the solid light on awd. Now it dont even blink anymore when I turn the switch just stays on the awd. Was going to take someplace to get scanned for trouble code guess they can do that if i got the service 4wd light on?
 

The_Roadie

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Member
Nov 19, 2011
9,957
Portland, OR
jamienfld said:
I can't hear the transfercase motor or the actuator.
That could be bad TCCM or wiring, or a double-failure of the actuator and encoder motor. A shop (not necessarily a dealer) with a high end scan tool can talk to the TCCM and command it to go through its paces.

Are you a DIY'er with some experience and a meter? Want to trace the schematics and see what you can find?

It could even be the switch, but a meter is the best way to troubleshoot that as well. Do the lights all flash when you first turn the ignition on, which is called the "lamp test" process?
 

silverfox

Member
Jan 4, 2012
2
Good day I also am having some problems with the transfercase and was not aware of the fluid situation and it sounds like it might be my problem.When in AWLD now it feels like it is loading up and snaps free so imedeitly put into 2WD and drives fine.I then thought I would see how it was in 4WLH and went forward very slow and it seemed alright but after about 25 yrds it loaded up aswell. I have drove this truck in ALWD for 3 winters straight and leave it in ALWD from Nov to the end of Mar because I live in Northwestern Ontario Canada and have not had a moments trouble.I was also informed about the bearing size of the shaft that holds the gears for automatic engagement when rear wheels slip then the ALWD kicks in was to small and after awhile this bearing would pack up.There is now a updated transfercase that solved this.Has anyone else heard this and if so what year does the updated transfercase come in from factory.
 

jamienfld

Member
Jan 4, 2012
16
when I turn the key the lights don't blink they are all on but don't blink then after few secs they go off and the awd light stays on. when i turn key ahead without starting and everyting off I could hear a small whining noise. Then turn key off and i could hear it again only last like 2 sec's. could that be the encoder motor?
 

silverfox

Member
Jan 4, 2012
2
That whining noise is most likely the fuel pump pressuring up the fuel system.I could be wrong but I don't think your going to hear the motor engage or disengage when you cycle the key.You should get a bulb test when cycling key.
 

RayVoy

Member
Nov 20, 2011
939
jamienfld said:
Also corroded wires thanks for the help.
Electrical current, copper wire, water and salt do not mix.

A large percentage of our 4wd problems can be traced to water intrusion.
 

darceye

Original poster
Member
Jan 2, 2012
4
Good news for you Jami. I too have ordered an encoder motor. Due in Friday. Hopeful this is the issue here too. Where was the corrosion on the wires you mentioned? At the encoder motor connector? Also, I did change the transfer case oil. It had 97000 miles on it. Only recovered about 1.25 qts from drain hole. Even with all those miles, it was still blue in color. Darker blue than new, but I was expecting it to be black. I put 2 qts in of the AutoTrakII. $8.95 / qt at local Chevy dealer
 

Fire06

Member
Dec 18, 2011
7,223
That 50,000 mile transfer case oil change is very important. If you do it the benefits will be seen.
 

jamienfld

Member
Jan 4, 2012
16
Not sure what wires had the corrosion on them. On the bill says fixed wires going to transfercase that were corrored. ya hope that new encoder motor works out for you.
 

jamienfld

Member
Jan 4, 2012
16
So went out to truck today and the light is back on same way was befor. So took it back now waiting to hear back from him to see what the problem is.
 

darceye

Original poster
Member
Jan 2, 2012
4
Installed a new encoder motor in the transfer case. All is working correctly again. Thanks to all for helping me diagnose the problem. While under there, changed the fluids in the front and rear differential.
 

jamienfld

Member
Jan 4, 2012
16
Ya forgot to post mine is working now also. The encoder motor they bought was faulty he took it back and got another one now it's working again. thx for the help
 

bakersfour

Member
Jan 22, 2012
16
my 02 tb ext 4x4 not working. no selector lights and no 4x4 service light. Did the fuse pull but no diff. Installed new switch , still nothing. Is the next step to replace tccm?
 

The_Roadie

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Member
Nov 19, 2011
9,957
Portland, OR
bakersfour said:
my 02 tb ext 4x4 not working. no selector lights and no 4x4 service light. Did the fuse pull but no diff. Installed new switch , still nothing. Is the next step to replace tccm?
Possibly. Why did you change the switch? The 2002 TCCMs are notorious for a high failure rate, but they almost always light up the "Service 4WD" lamp when they quit communicating.

Do the lights on the switch all flash on when you turn the ignition on during the lamp test period, or are they permanently dark? Do you have a meter and can you check for power at the TCCM if I can talk you through it?
 

Wyle

Member
Dec 4, 2011
200
the roadie said:
Here's the schematic.

Not to totally steal the OP's thread, but school the ME (who owns and operates a meter) for a minute.
I see
  • a 5V reference source from the TCCM that powers the 4WD switch in the dash. Switch setting changes resistance which results in different voltage signal back to TCCM.
  • the logic circuits of the TCCM convert that into a command signal to the front actuator motor
  • I'm guessing the TCCM circuits would start flashing the lamp in the switch for the new drive mode by playing with its ground connection
  • the front actuator gets its 12V power from a RUN circuit (rear seat fuse #48) which also goes through an underhood fuse.
  • the actuator moves according to the axle actuator control signal from the TCCM and returns a position feedback signal
  • presumably, when the TCCM gets the appropriate axle switch signal returned, it finishes playing with the lamps in the switch.
I'm ignoring the propshaft sensors for the moment as they're probably there for A4WD engagement and to limit 4LO (& 4HI??) engagement when doing 80 mph

What I don't see is (1) 12V input to the TCCM or (2) anything to do with the encoder motor on the transfer case. It doesn't make sense that the TCCM would get power through whichever LED in the switch was lit and the TCCM should be controlling the encoder motor since it (the TCCM) is what knows which position the dash switch is in.

Am I understanding this thing right? :undecided:
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
Wyle said:
Not to totally steal the OP's thread, but school the ME (who owns and operates a meter) for a minute.
Oh, crap. Sorry! There are two pages of schematics. The second one has the circuits that you KNEW must be present. I enjoy having an engineering discussion with folks who can navigate a system and not get tangled up.


View attachment 18168

I see
a 5V reference source from the TCCM that powers the 4WD switch in the dash. Switch setting changes resistance which results in different voltage signal back to TCCM.
YES. The designers saved a few wires and expense by using an A/D converter in the various control modules like the TCCM and HVAC to read back from switches this way. The drawback is that dirty contacts get misinterpreted, and in the case of the TCCM, they chose very poorly and had a high resistance be the signal for 4LO or N position, instead of the much safer default position of 2HI. They should have guarded against an open much more than a short because opens happen more as the vehicle ages.
the logic circuits of the TCCM convert that into a command signal to the front actuator motor
Yes.
I'm guessing the TCCM circuits would start flashing the lamp in the switch for the new drive mode by playing with its ground connection
Yes. The TCCM also flashes all lamps at ignition-on time as a lamp test. If they don't flash then, it's evidence of a TCCM with no power, or one that doesn't wake up and run its self-diagnostic.
the front actuator gets its 12V power from a RUN circuit (rear seat fuse #48) which also goes through an underhood fuse.
Not quite. That schematic portion is a wired connection THROUGH the front fuse block, but without another fuse in series. It's only a way the designer got the wire from the rear fuse block to the chassis harness that goes to the actuator. They use that trick a few dozen times in the vehicle.
the actuator moves according to the axle actuator control signal from the TCCM and returns a position feedback signal
Yes. In the case of the front axle actuator, the position feedback is a simple high/low success signal sent by a sliding contact finger on the stripe with the red circle. When the gear train is at the fully-extended (activated) end of its travel, the finger slides over the gap and the signal changes state. The transfer case encoder motor, on the other hand, uses a potentiometer as the sensor, so what gets sent back to the TCCM is an analog signal (from the wiper of the pot) that has to be measured. They use 8-bit ADCs for 255-position detection. Same for HVAC airflow and temperature actuators.

View attachment 18169

presumably, when the TCCM gets the appropriate axle switch signal returned, it finishes playing with the lamps in the switch.
Exactly. If the TCCM is happy, the lamp quits flashing on the new selected mode. If anything fails (and the front actuator can also fail for sluggishness as when it's cold), then the lamp flashes and you know the selection didn't succeed.
I'm ignoring the propshaft sensors for the moment as they're probably there for A4WD engagement and to limit 4LO (& 4HI??) engagement when doing 80 mph
Again, you got it. There is a torque bias and engagement aggressiveness profile baked into the TCCM that needs to know the ground speed. And for the 4LO interlock. There is no interlock for 4HI, but it's potentially damaging to the splined disconnect to be engaged at high speed because of the way the spider gears are turning the intermediate shaft BACKWARDS.
What I don't see is (1) 12V input to the TCCM or (2) anything to do with the encoder motor on the transfer case. It doesn't make sense that the TCCM would get power through whichever LED in the switch was lit and the TCCM should be controlling the encoder motor since it (the TCCM) is what knows which position the dash switch is in.

Am I understanding this thing right? :undecided:
You got it right. I only posted half the schematics. Sorry again.
 

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bakersfour

Member
Jan 22, 2012
16
OK i am no engineer. So what am i checking and in what order. I have checked fuzes. I have no lights on switch at all. No service 4x4 light. No noises of any kind when moving switch. Any help would be appreciated.
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
bakersfour said:
OK i am no engineer. So what am i checking and in what order. I have checked fuzes. I have no lights on switch at all. No service 4x4 light. No noises of any kind when moving switch. Any help would be appreciated.
Please fill out your profile so I don't have to remember you have a 2002. Thanks. Do you have a meter? The lights on the switch never, ever flash, not even when you first turn on the ignition? Do you have a local buddy you can swap TCCMs with? How do you check fuses without a meter? Visually? Not always enough. Or do you have a meter?
 

bakersfour

Member
Jan 22, 2012
16
just checked fuzes visualy, will check with meter in morning. no where do i start checking with meterone to swap tccm with . found 2 salvage yards to get one if i have to. they want number from my unit to cross check.
 

bakersfour

Member
Jan 22, 2012
16
Ok checked #8 fuse and put in new one just to make sure. Have power to orange wire at tccm. if i have power there does that mean tccm bad. also like i said no lights ever on switch and no 4x4 service light. what to check next, thanks foe your help
 

bakersfour

Member
Jan 22, 2012
16
bakersfour said:
will try to check tomorrow. am i checking the brown wire at the tccm with it plugged in to tccm?

i didnt have time today to check wiring. looking at the schematic it looks like the brown wire comes from rear fuse panel, should that still be ground. or am i looking at something wrong
 

bakersfour

Member
Jan 22, 2012
16
the roadie said:
Possibly. Why did you change the switch? The 2002 TCCMs are notorious for a high failure rate, but they almost always light up the "Service 4WD" lamp when they quit communicating.

Do the lights on the switch all flash on when you turn the ignition on during the lamp test period, or are they permanently dark? Do you have a meter and can you check for power at the TCCM if I can talk you through it?

Picked up a tccm from salvage yard yesterday from 2004 tb. do i need to disconect battery before changing tccm?
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
No. Pulling its fuse would be a safe tactic. Disconnecting the battery has so many other side effects it should be a last resort. Such as unlearning the throttle body behavior in case yours isn't cleaned. And reducing driveability for a while until the PCM relearns the throttle body behavior. And recalibrating all the HVAC actuators which sometimes kills off marginal ones.
 

bakersfour

Member
Jan 22, 2012
16
got the 4x4 working today. installed 2004 tccm, everything works great. love this site thanks for all the info.
 

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