QuestionQUESTION: I am having trouble with my GMC's 4WD system. This morning (sorta cold, 11 deg f) I went to put it into 4WD, it went in, then it seemed to pop out. The lights went out on all of the selection lights (A4WD, 2HI, etc), and the Service 4WD message came up. I checked to see what exactly is going on, the transfer case is engaged in 4WD and turns both front and rear drive shafts, however, the front axle doesn't turn. I know in this system the transfer case motor must engage first, then a signal is sent to engage the front differential motor to engage the differential. I think the problem is one of two items, either the front differential engagement motor is in-op or the transfer case motor is malfunctioning and sending false signals to the VCM (vehicle control module). I tried looking for a diagnosis procedure, but I can't find one in my Hanyes book. Do you have any suggestions? I would like to fix this now if I can figure out a definite cause of the problem. Unfortunately I don't have an OBD-II scanner or a diagnosis procedure. Also, if I take the motor out and manually turn the transfer case back to 2HI, will this cause problems with the orientation of the motor? If I can't fix the problem here, I would like to switch the transfer case into 2HI so when I have to drive home (1400 miles) at the end of the month, I'm not in 4WD all the time. Thanks.
ANSWER: Hi
From what you tell me and from my experience I would have to condem the Transfer Case Encoder Motor. They are pretty famous or infamous for creating the problems you describe. On the 2000 up K trucks I have NEVER replaced the front differential actuator. That being said I guess anything is possible. Scanning it would probally reveal a C0327 along with a few other codes that according to GM diagnosis would have you replacing the TCCM which I do not think is your problem.
As far as the Encoder motor, if you do take it out and manually move it the tranfer case to 2wd, leave the encoder out do not reinstall it until your home or until you have a new encoder. When installing a new encoder, make sure to swap from the old one to the new one , a small spacer washer located on the upper rear bolt. Also do not turn the Encoder, turn the transfer case. Best advice - put in 4wd nuetral (if you can) and then change the encoder. The new one comes in nuetral. If not just turn the case o nuetral. After you rehook battery to truck, put in 2wd, then go through the other 4wd gears.
I Hope it helps.
If any thing else I can do, please do not hesitate to write.
Good Luck
Paul
---------- FOLLOW-UP ----------
QUESTION: I have an update, I've been driving around for a few days and the 4WD service light is gone but the 4WD selector panel still doesn't light up. Also, I need to check again to see if its in 4WD, because I hear the clutches in the transfer case are pretty stiff, especially when its cold. So the front drive shaft might be turning even if the t-case is in 2WD. Any advice on the new condition?
AnswerHi
If you could or can SAFELY jack or lift vehicle and SAFELY SECURE it and run it with all 4 wheels off ground at same time that will let you know if it is in 2wd or 4wd. Front driveshaft turns at all times anyways. The Actuator on front differential locks in the front axles. The transfer case just transfer power to front wheels in either low or high gear ratio settings when selection is made. The Encoder Motor recieves this input from TCCM, makes the proper motor output selection and sends a signal back to the TCCM of the status of the shift. Somehere in here I THINK is where your problem lies. It is PROBABALLY a bad voltage getting back to the TCCM from the Encoder. COULD be wiring. Could be a faulty ground. So the front differential, driveshaft and internal transfer case components are "freewheeling" in 2wd. Agian, I have to stick with my original diagnosis from experience and without seeing vehicle is the Transfer Case Encoder Motor.
Again, If possible Get it scanned and have the codes checked out before hand.
Thanks again
Paul
