99 Jimmy "Service 4WD" & not shifting

[wetback1]

I have a 99 Jimmy with a 4 button transfer case (Autotrac) that won't shift out of 4 hi. Had it to the dealer and they pulled codes C0324 (transfer case lock circuit high), C0387 (unable to complete shift).

I've done a ton of reading and following threads and although this is not an isolated incident with these trucks, no one ever seems to get to the bones of the matter and say 'replace this'...

Dealer wants to replace TCM ($1000) and then encoder motor (another $1000). I have removed the TCM, actually removed the cover to see if anything looks fried (sounds low tech but it's helped me before) - everything looks good. All connections are corrosion free and tight.

Next I replaced the encoder motor ($267 - no wonder GM is in trouble) and managed to shift into and out of 4WD 3 or 4 times before the light came on again and the unit stayed in 4Hi. Currently have the battery disconnected waiting for the codes to clear.

Front diff is locked in 4 and unlocked in 2, so that portion is working as it's supposed to. I can hear the relays clicking and the encoder motor running but it tries to switch twice and then reverts to 4Hi. I could shift the case manually when I had the encoder removed.

So - what's next? I can get a 'body control module' from Rock Auto but it supposedly has to be programmed before installation. I read somewhere that 75% of TCM's returned for warranty had nothing wrong with them. I'm wondering if there's another sensor somewhere that's not working properly. Can't see 2 encoders having the same problem.

Any suggestions would be appreciated - I don't have much hair left to pull out

Thanks!!!

[MT-2500]

Originally Posted by wetback1

I have a 99 Jimmy with a 4 button transfer case (Autotrac) that won't shift out of 4 hi. Had it to the dealer and they pulled codes C0324 (transfer case lock circuit high), C0387 (unable to complete shift).

I've done a ton of reading and following threads and although this is not an isolated incident with these trucks, no one ever seems to get to the bones of the matter and say 'replace this'...

Dealer wants to replace TCM ($1000) and then encoder motor (another $1000). I have removed the TCM, actually removed the cover to see if anything looks fried (sounds low tech but it's helped me before) - everything looks good. All connections are corrosion free and tight.

Next I replaced the encoder motor ($267 - no wonder GM is in trouble) and managed to shift into and out of 4WD 3 or 4 times before the light came on again and the unit stayed in 4Hi. Currently have the battery disconnected waiting for the codes to clear.

Front diff is locked in 4 and unlocked in 2, so that portion is working as it's supposed to. I can hear the relays clicking and the encoder motor running but it tries to switch twice and then reverts to 4Hi. I could shift the case manually when I had the encoder removed.

So - what's next? I can get a 'body control module' from Rock Auto but it supposedly has to be programmed before installation. I read somewhere that 75% of TCM's returned for warranty had nothing wrong with them. I'm wondering if there's another sensor somewhere that's not working properly. Can't see 2 encoders having the same problem.

Any suggestions would be appreciated - I don't have much hair left to pull out

Thanks!!!

Did the dealer test things out or just pull codes and want to start throwing parts at it?
Did he go threw the code 324 repair flow chart in the repair manual?

Proper testing before throwing parts at it.

Code 324 is a transfer case brake control solenoid or coil or circuit problem.

And can be caused by a bad decoder motor.

You need to get a repair flow chart for code 324 and go threw the complet repair flow chart.

Repair flow charts can be found in the factory repair manual or a all data online sub.
http://www.alldata.com/products/diy/index.html

[wetback1]

I wanted to repost for the benefit of others who may (will) run into this issue, since I think the main benefit of these posts is helping others with the same problem.

Firstly, Alldata is a great resource, but is geared more towards shops than the guy in his driveway who is trying to save a few bucks. I have no access to a GM scan tool or tool # J-getreal. That being said, I did use Alldata's flowcharts and schematics to solve the problem.

When I tried to shift into or out of 4X I would get a buzzing from the encoder motor and the solenoids in the TCCM would click. Same thing after the encoder was replaced. Finally I grounded pin E1 (top left tan wire on big orange plug) and presto - away it went. Shifted flawlessly without a buzz. Seems there is a clutch solenoid that holds the shift (encoder) motor in place at all times EXCEPT during a 4X shift. The TCCM must release this clutch before shifting (by grounding the tan wire) and this is what wasn't happening. I'm fairly certain that one of the solenoids in the TCCM is faulty, but without trying to replace it I'm not sure. In any case I found a new one on Ebay for $200, plugged it in and it works fine.

So in this case aside from my time and aggravation;
Rebuilt encoder motor $270 (wasn't really required)
Ebay TCCM module $220
Alldata subscription $ 26
TOTAL: $516

GM shop quote: $2500
Saved: $1984
What I learned about GM shift on the fly 4X's --- priceless!!

And for those that aren't sure (because I wasn't) - the transfer case control module does NOT require programming. The body control module which works the lighting, keyless entry, security, etc., DOES need to be programmed if it's replaced.

Hope this helps someone else out who is as lost as I was...

[MT-2500]

Thanks for posting back how it went.
All Data is one of the best the best repair info for a DIY or professional.
I know it is had to navagate the first time using it.
But All Data is same as factory repair info and is owned by Auto zone one of the main DIY auto parts stores.
The All Data DIY online sub is set up for the DIY repair person.
http://www.alldata.com/products/diy/index.html