1998 Jimmy 4-Wh problem

98 Jimmy 180k, Shift to 4-WH and can hear actuator shift. 4W-L works also. Yet the front hubs do not lock. Button lights show no problem. Checked all fuses. None blown that I could find. Can not hear any vacuum leak. ???

You may be hearing the actuator working in the transfer case but the actuator for the front differential may not be engaging. (If it has one).

If so, check to see if you have power going to it. Some actuator switches incorporate a gas cylinder that expands under heat from a power source.

A quick check at the auto part store indicates the your actuator is vacuum operated. Check for vacuum at the unit and test it with a vacuum pump tester.

Sorry about the previous post.

If the 4X4 lights don't blink, indicating a problem, then the actuator is working and activating the switch in the front differential. The left front axle is always engaged with the differential, if the right front axle shaft does not engage, (4WD will not work until it does), the problem is with the clutch shaft or linkage in the front axle that engages the right axle to the differential.

98 Jimmy 180k, Shift to 4-WH and can hear actuator shift. 4W-L works also. Yet the front hubs do not lock. Button lights show no problem. Checked all fuses. None blown that I could find. Can not hear any vacuum leak. ???
This could be more complicated than you think. I was through this last summer with my 2001 Jimmy. I would push 4WD and it would blink but not change into 4WD. If it's trying to change, you will hear it trying as you drive. At that point, it may not be vacuum related. Sometimes it would change to 4WD but eventually it stayed in 2WD over time. At that point, it could be vacuum related. Everybody above is right. You must check all vacuums incl. the actuator under the battery and measure voltages to the Transfer Case Control Module (TCCM) located against the wall under the glove compartment. You must remove the plastic Trim Panel.

What I found on my Jimmy after a very long diagostic process was that I had some water dripping on the wiring harness into the Transfer Case Control Module. It must of shorted it because a couple of pins were very corroded. Harness plug had to be rewired. It still didn't work. Finally I broke down with the replacement of the TCCM which is $500.

So, to be clear, with the front wheels off the ground, if I turn one wheel and the other wheel also turns (opposite direction, of course), the front differential IS engaged? I ask because I'm having intermittent problems with my front end DISengaging...(I think the cable might need to be replaced)

Thanx!

The Transfer Case Control Module (TCCM) located against the wall under the glove compartment. I would start there because it is easier than any further diagnosis which gets very involved if your doing it yourself. What you are looking for is corrosion on the wires going into the wiring harness. If you have that then it will involve (going to GM dealer - not off the shelf stuff) getting a new wiring connector ($20), the F-connectors ($32), soldering skills, and some patience. After that, it will cost $500 for a new TCCM (or go to a junk yard and get one at a steal). It all worked for me 100% (knock on wood). This might explain the intermittent 4-Wheel disengagement. No guarantee. Sounds similar to my situation. Good luck.

So, to be clear, with the front wheels off the ground, if I turn one wheel and the other wheel also turns (opposite direction, of course), the front differential IS engaged? I ask because I'm having intermittent problems with my front end DISengaging...(I think the cable might need to be replaced)

Thanx!

That is correct. There could be a problem with the actuator under the battery tray, the actuator cable, the clutch mechanism at the passenger side front axle shaft or vacuum going to the actuator. The actuator should have manifold vacuum any time the engine is running and 4 wheel is selected.

That is correct. There could be a problem with the actuator under the battery tray, the actuator cable, the clutch mechanism at the passenger side front axle shaft or vacuum going to the actuator. The actuator should have manifold vacuum any time the engine is running and 4 wheel is selected.

I'm suspecting perhaps BOTH the vacuum switch on the transfer case and the actuator cable to the front axle...(haven't checked for vacuum in both 2hi and 4wd yet...I know it should only be there in 4wd...vacuum in 2hi suggests vacuum switch on transfer case)

Clutch mechanism on the passenger side front axle shaft is a new one on me... :dunno:

Clutch mechanism on the passenger side front axle shaft is a new one on me... :dunno:

When vacuum is applied to the actuator under the battery, it pulls a cable. The other end of the cable is connected to a clutch mechanism that engages the passenger side axle shaft to the front differential. The driver side axle is engaged with the differential all the time, when the passenger side axle is engaged, the differential becomes "live". At the clutch mechanism, both inner and outer axle shafts rotate at the same speed allowing 4 wheel "shift on the fly". When you select 4LO, the gear ratio is changed in the transfer case, that's why you need to be almost standing still to shift in or out of 4LO. Similar to a manual shift non syncromesh transmission.

Items 9 thru 23 show the cable, clutch and shift mechanism:

http://i561.photobucket.com/albums/ss58/cwhook/4X4Frontdifferential-Copy.jpg