1999 GMC Jimmy - 4WD problem

Hello,

First of all, I have read the 4WD System operation file, very informative.

Here is the problem I am having. When I press the button for 4 wheel Hi, the light usually flashes then returns back to 2 WD. If I continue to press, on occasion, I can get it into 4 WD. (I know not good, but could not make it up a hill). Likewise, to come out of 4 WD, same thing occurs.

Other notable is that very seldom this may occur, I press 4 WD and start to hear gears grinding coming from the back end of the vehicle.

I've never worked on the 4WD or vacuum system. Can anyone direct me to what could be the root cause?

NOTE: I do not have autotraction and I do not get any problem lights on the dash.

Thanks in advance,

LESTER

Any advice greatfully appreciated.

Do a search on this board. A lot of people are having the same problem and one resent post with your same symptoms.

The first thing to do would be to figure out if the problem is the transfer case or the front axle.

With the engine running and the wheels blocked, the truck in 2wd reach underneath and turn the front driveshaft. It should turn freely.

Then put the truck in 4wd, reach underneath and try to turn the front driveshaft again. It should be locked, if it is then your transfer case is probably working ok. Jack up one front wheel and spin it. If it spins but the driveshaft doesn't turn (it should still be locked) then the problem is with the front actuator or possibly the vacuum switch.

Post back with the results.

James