Truck: 2000 Silverado V8 Pushbutton 4WD with about 130,000 miles on it, driven as a daily commuter with no “off road” stressing.

Short version: Truck ran perfectly for years, got in it one morning, fired it up, dropped it into gear and realized that it was in low-range 4WD. Working the transfer case push buttons produced a noise like a big ratchet working back and forth under the truck. Immediately the sound took on a life of its own and the noise repeated itself without any further input from the push-buttons.

I soon realized that this was the repeated operation of the servo mechanism trying to make the transfer case go into and out of 4WD without further control from the dashboard. Truck is now completely unusable as it cannot be driven on dry pavement while in 4WD.

Long version: Symptom started as above. It has now been over 12 months with the truck having been looked at by several transmission specialists and the local GM dealer with no repair success.

What I know…The servo motor assembly has been exchanged at least three times with new units…The transfer case has been “gone through” on the vehicle and no faults were found…Every switch and relay that anyone here knows of has been substituted…The electrical grounds are supposed to have been traced and cleaned…The symptom remains the same as it was to begin with.

The independent transmission shop, a two man shop with lots of experience and a great reputation, called in a “transmission guru” friend from the GM dealer over in Johnson City. He replaced “two relays/black boxes”, what ever they were, with no success. The shop has gone over their work repeatedly with no success.

The dealer said that they would diagnose the problem for $38 and give me an estimate. Their transmission man came back with “it is a bad transfer case” and an estimate for $2200.00 to replace it. I couldn’t afford that so I had the truck transported back to the independent shop. The “bad transfer case” option was discussed but the shop is very leery of it being that failure and so am I.

I am not able to take a chance by having them install a known good transfer case just on my say so, so I am coming here on bended knee asking if anyone is familiar with this situation.

Any suggestions gladly received. Thanks, Jack

 

First things first, does it have the auto 4x4 option on it? If it does it could have a bad wheel sensor. If it doesnt then its time to dig a little deeper and do some research and find out what sends signal to that servo and motor. And Im sure there are only a few options, I will pull out my tranny book later and look and post what I find. As long as the dash switch has been replaced then its something that the pcm would rely on for information. Does the tranny shop have a snap on scanner? can they watch "real time" data? this would provide some help if they know what they are looking at.

 

Thanks for the reply Chris !

"First things first, does it have the auto 4x4 option on it?" If you mean a button at the bottom of the panel that says something long those lines, the answer is yes.

If it does it could have a bad wheel sensor. Do you suspect that although the "automatic" option has not been selected there would still be a signal given to the system that would affect the electronic selection? What sort of "wheel sensor" would you mean? Are you thinking of an ABS rotation sensor?

If it doesn't then its time to dig a little deeper and do some research and find out what sends signal to that servo and motor. I do not have access to any sort of logic tree so what ever you can find will be appreciated.

And I'm sure there are only a few options, I will pull out my tranny book later and look and post what I find. As long as the dash switch has been replaced then its something that the pcm would rely on for information.

Does the tranny shop have a snap on scanner? can they watch "real time" data? this would provide some help if they know what they are looking at. I am not certain, but I will ask in the next day or so. Are you speaking of an OBD scanner?

As I said in the initial post, I am just uncomfortable depending on the dealer assessment of it being a transfer case internal mechanical problem. I am with you on the idea that there must be an overlooked electrical controller issue. I have been a mechanic for over forty years and what does get to me is that after all of my inquiries no one has popped up saying that "Hey, that is a relatively common problem, and the fix is this and that and such and such.

Jack

 

Since this does have auto 4x4 I am willing to bet the issue is with that part of the system.
As far as a sensor goes Im not sure if they are at the wheels or if it uses output shaft speed vs vehicle speed to determine if the wheels are slipping. A Snap On scanner has the ability to read data in real time and yes it is a OBDII scanner but it does alot more (with the right module).
As far as this being a common issue....this truck is just 10yrs old and I think that this problem will creap up more and more as these trucks get older.
I will look into it a little more and see what I can find.