At some point in time, the AWD system on my 99 expy stopped working. This model has one round switch on the dash to control the 4x4 mode. The settings are: AWD, 4WH 4WL. Can anyone tell me a logical place to start diagnosing this problem? Never had any problems except last couple times I used it, seemed like it took longer than usual to disengage the front drive train(diff & xfer case).

 

There is a solenoid on the front axle that may be sticking on you. It will be easy to spot on the front axle with a shield around it.