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No 4wd - slector blink and then goes back to 2wd.

BlueZ71

Full Member
Joined
Jun 11, 2002
Messages
242
Location
Central NJ
Hi All -

I have a 2002 Avalanche 1500 Z71.  When I try to select 4hi or 4 low, or Auto 4wd, the selector light just blinks and either default back to 4wd, or just keeps blinking until I push 2wd again.  I can feel the truck go into 4 low, and the gear ratio changes, but the light never stops blinking and the front axles don't engage either. 

I can also make the transfer case go into neutral with no issues so it seems the transfer case computer is OK.  I replaced the front axle actuator with no luck, was thinking about the encoder motor next, but there is so little good info out there on these 4wd systems.

Checked all the fuses, did the reset removing the TCCM fuse, and there is no service 4wd light on - any thoughts? 

 
Not sure if the 02 has the same switch setup as the '03 and newer but it is possible the switch is bad. I would at a minimum insure the switch is firmly connected to its wire hardness. And you will want to check for codes. It may not show "service 4wd" but still throw a code.

 
When was the last time you changed the fluid in the transfer case.

Can you hear the encoder motor trying to move when you try 4hi.
 
I changed the transfer case fluid at about 100k miles, it has 179k on it now.

I can hear the motor doing something, but not quite sure what. 
 
You could try putting the tcase in neutral then take off the encoder motor and see if you can shift it manually.

Block the wheels first.
 
MS03 2500 said:
You could try putting the tcase in neutral then take off the encoder motor and see if you can shift it manually.

Block the wheels first.

That's what I'm going to do, maybe tonight. 
 
BlueZ71 said:
Hi All -

I have a 2002 Avalanche 1500 Z71.  When I try to select 4hi or 4 low, or Auto 4wd, the selector light just blinks and either default back to 4wd, or just keeps blinking until I push 2wd again.  I can feel the truck go into 4 low, and the gear ratio changes, but the light never stops blinking and the front axles don't engage either.  

I can also make the transfer case go into neutral with no issues so it seems the transfer case computer is OK.  I replaced the front axle actuator with no luck, was thinking about the encoder motor next, but there is so little good info out there on these 4wd systems.

Checked all the fuses, did the reset removing the TCCM fuse, and there is no service 4wd light on - any thoughts?  
Your 2002 Avalanche has a NP236 transfer case.  Should be very similar to the NP236 in the Blazers and I have some info for them.

https://blazerforum.com/forum/2nd-gen-s-series-1995-2005-tech-41/np8-auto-4wd-transfer-case-info-2001-blazer-89044/

Super-easy to diagnose as long as you have a Tech2 and a factory manual.  Unfortunately few of us do, so we sometimes have to punt and take educated guesses.

In my experience, it sounds like your encoder motor is indeed taking the transfer case into a different mode (the motor and TCCM works) - but the TCCM isn't getting the feedback from the encoder position sensor that detects the shift has taken place, so it reverts the system back to the 2WD position.  (I know you said defaults back to 4WD in your post but I don't think that is really what you meant).

If you can do it yourself, then simply put an encoder motor on it (position sensor is internal).

p.s pulling fuses resets nothing on the 4WD.

Good luck.
 
Well, the encoder motor didn't do it. 

The transfer case sifts fine, and it even goes into 4wd low, the only thing not working is the front axle actuator.  I checked and cleaned up the grounds near the drivers door and now my low engine oil light works again, but no front axle lock.

Only thing left is the TCCM and possibly a break in the harness which I don't see.

And yes - pulling the fuse can help, there is a TSB about the TCCM failing to acknowledge a wake up call from the BCM, pulling the fuse fixes it temporarily until you can get a reflash. 
 
Don't forget to put the tcase in neutral before disconnecting the tccm power.
 
Do you have a scanner that can read "C" or "U" codes?
 
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