99 Sub. stuck in 4lo

I have a 99 Sub. with a 6.5TD and auto 4WD. On vacation the service 4WD light came on intermitantly. About 30 miles from home the check engine light came on and the cruise control cut out. We stopped at a store near home to get some bread and when I restarted the truck, it was in 4 wheel drive low. I never touched any of the 4WD buttons. All the lights on the 4WD swithches were out and it would not shift out of 4WD low. The one TDC that was set that I was able to retreive with a simple code reader was P1875. The diagnostice procedure for this code points to either bad wiring on ciruit 1694, a bad transfer case control module or a bad PCM. However, without a tech 2 or similar scanner I cannot confirm any of these (except the wiring on circuit 1694 seems OK, no shorts or opens). I cleared code 1875 and it did not reset. But the Service 4WD light is still on and its stuck in 4lo. I cannot retrieve any transfer case codes. My question is, has anyone seen this problem before, or does anyone have a tech 2 near Springfield, IL that wouldn't mind helping me diagnose this.

Thanks
Bill Riley

I had some unpleasant transfer case issues last fall with my 04 1/2 ton. One of which was some faulty speed sensors. I did my diagnostics between the dealer and throwing some parts at it. Long boring story.

-Try disconnecting the batteries and resetting the system.
-Start the truck. If the lights come on at the switch, try shifting the case.
-If the lights blink, but no shift, its most likely the shift motor
-If no lights its most likely the dash switch. These were common failures on the later model trucks, but could have failed on yours too. The switch is the cheapest part (about $50) so if you start throwing parts at it, this would be the only one I'd replace without any further scan info.
-On occansion the transfer case control CPU has been known to be a problem, but not as much as the switch or the motor.

Crash, thanks for the thoughts. I am still struggling with this and based on the fact that it shifted into 4lo by itself, I figured the control module was bad. Bought new one, no dice. I did find a test for the dash switch so I'm going to try that. I don't think I'm going to try throwing more parts at it until I can figure out what's wrong. I have the complete shop manual for my truck, but diagnosis requires a scanner that can interact and command the various components. So, if anyone has a tech2 or similar scanner near Springfield, IL., I'd be happy to pay for your time and/or provide some adult beverage (after the work's done).

Bill

A friend of mine experienced the exact problem with his 2004 Sierra. At that time, it was under warranty, so he took it to the dealership. The mechanic found the problem to be a couple of crushed wires in the 4WD circuit which shorted out, and kicked the transfer case into 4LO.

I replaced the encoder motor. All is well.

Bill

I hate to say it but, they are much less prone to failure when they have the little Johnson Bar on the floor.

Glad you got it fixed though.

best

Robyn