Hello All,

I read past threads about 4x4 shifting issues and I have had a loud thumping noise from the front end when coming out of 4-hi for the past month or so. Now it will not go into 4-hi...switch panel just blinks several times and goes back to 2 wheel. But, if I go to neutral and engage for 4-low, it goes in. It would seem that if I had a linkage, or vacuum problem that it would affect 4hi and 4low. Anybody been down this road before? If so, please help.

Thanks

The noise when disengaging is a clue. The left front axle is always engaged with the front differential. When the encoder motor engages 4WD HI, a vacuum switch inside the transfer case is activated which supplies the actuator with vacuum. The actuator is located under the battery. The actuator pulls a cable that is attached to a shift fork inside the right axle tube. The shift fork engages the clutch gear with the clutch shaft and the axle becomes live. When the shift fork moves, it operates a switch that tells the TCCM that the axle is engaged. The TCCM has no way of monitoring if the axle is in fact engaged, only that the shift fork moved to the proper position. If something in the shift mechanism is damaged, quite possibly from the thump you've been hearing, the switch may not be getting activated. If the switch is not satisfied, the TCCM will revert back to 2WD HI. Here is a link to an image of the front axle, it shows the clutch gear etc etc.
http://img413.imageshack.us/my.php?image=frontdifferentialxw4.jpg You can check if the axle is engaging very easily. Engine idling in PARK, select 2WD, jack up right front wheel only, the wheel should spin by hand. Select 4WD and the wheel should not spin. Post your results.

The noise when disengaging is a clue. The left front axle is always engaged with the front differential. When the encoder motor engages 4WD HI, a vacuum switch inside the transfer case is activated which supplies the actuator with vacuum. The actuator is located under the battery. The actuator pulls a cable that is attached to a shift fork inside the right axle tube. The shift fork engages the clutch gear with the clutch shaft and the axle becomes live. When the shift fork moves, it operates a switch that tells the TCCM that the axle is engaged. The TCCM has no way of monitoring if the axle is in fact engaged, only that the shift fork moved to the proper position. If something in the shift mechanism is damaged, quite possibly from the thump you've been hearing, the switch may not be getting activated. If the switch is not satisfied, the TCCM will revert back to 2WD HI. Here is a link to an image of the front axle, it shows the clutch gear etc etc.
http://img413.imageshack.us/my.php?image=frontdifferentialxw4.jpg You can check if the axle is engaging very easily. Engine idling in PARK, select 2WD, jack up right front wheel only, the wheel should spin by hand. Select 4WD and the wheel should not spin. Post your results.
Would this test of engagement of front axle also apply to a 94 jimmy with electronic shift 4 whl?

Would this test of engagement of front axle also apply to a 94 jimmy with electronic shift 4 whl?


Yes, it will be the same procedure.

AWD and Smartrac front axles are engaged to the differential all the time.