Check the TORQUE ON-DEMAND relay. It is used to control the auto 4x4 mode. Basicly it engages the front axle momenterily by pulsing an electronic clutch that is mounted inside the transfer case. The faster the pusles the more torque provided to the front axle. If this relay is dead then it can not send the pulses. The relay is mounted directly above the left edge of the passengers side glove box. It is mounted on a sheet metal part for heat disapation. When you select 4HI or 4LO this clutch is engaged (held closed) all the time, not pulsed. You may have to go to the dealer and at least pay them to diagnose the problem for you. It should only take an hour or so to pin-point the problem.

 

Still having problems? If so, have a mechanic or dealer check for DTCs relating to the transfer case speed sensors. The t-case has two speed sensors (front and rear), that feeedback to the GEM controller. They are active sensors (powered), so they need a voltage source to verify funtion. If you have 4 hi and 4 low, it is definetely not the clutch pack. 4WD Auto is controlled by the t-case speed sensors only, not any other vehicle speed sensors. In 4WD Auto, the GEM (generic electronic module) supplies a 50 Hz PWM (pulse width modulated) signal to an SSR (solid state relay), which in turn fires the clutch coil using battery voltage. This PWM signal can vary anywhere form 2% to 98% duty cycle, depending on wheel slipage, throttle position, etc. For whatever reason, the GEM is not producing a high enough duty cycle to activate the clutch in the transfer case. This is why it is best to troubleshoot by looking at the GEM diagnostic trouble codes. It should tell you if there is a bad t-case speed sensor. By the way, your t-case has no mechanical lock up as suggested earlier. If you lose your clutch coil, you lose ALL 4WD options. (Although you could get "2WD Low Range", which is what you would get shifting to 4WD Low) Good Luck

 

Thanks for all the replies!! I have fixed the auto 4wd!! My problem was either the hall sensor/speed sensors on the transfer case or the swithch on the dash was not in all the way. I replaced them and everything is operational. Thanks again!!!!!!

 

Good news!. It's too bad there are not as many diagnostics available for 4WD and ABS systems like there is for engine management.

 

Glad to hear it! Your problem was not the dash switch though. You verified that by confirming that the other positions worked ok. The dash switch does not control auto 4wd aside from the fact that it tells the GEM that your in A4WD and its time to monitor those signals relavent to the PWM coil signal. If it happens again, just replace the speed sensors. Happy 4 wheelin!

 

Good point Uncle!