4WD No Longer Engages

[Graboon]

1991 Cherokee 4.0L - the front drive won't engage when I shift into 4Hi anymore.

I took it to a dealer last year (to diag the problem) and they disassembled it, said they needed a part that would take a while to get, then re-assembled and sent me on my way. On the way home, I tested the 4WD and it WORKED! It kept working for a few weeks but then stopped working again.

Any ideas?

[jeep!_r]

On these years of cherokees the four wheel drive had a vaccum switch that operated the front drive axle. When the operator shifts the transfer case to 4WD, there is a vaccum switch on the t case that opens up and applies vaccum to a vaccum controlling motor on the front axle. There there it pulls a splined hub and engages the axle. This is done, so no hubs are needed to be locked in(like on a ford truck), but still gives the advantages on the hubs. To start one must make sure there is vaccum to the switch. Also, its a good idea to make sure the vaccum hoses aren't cracks. This can be done by locating the switch on top of t case and tracing the hoses back. If they need to order a hard part, it'll probably be either the vaccum motor, or the switch. The motor can be tested by removing it from the axle, and sucking on it and seeing if it moves. Why it worked for a few weeks I couldn't tell you, but some things are fluke. The switch I have never heard of one going bad, so I've never tested one.
B

[GPN 05]

Yea, it's the Vaccum Ass. on the Front Axle.

It has a plug on it, take it off and clean it. Email me, and I will send you a website on how to fix your problem.

[gtmud]

A good way to test the vacuum motor on the axle housing is simply find you a vacuum source off the manifold. connect one end of vacuum tubing to it than the other to one of the ports on the motor(2).(suction engage & disengage) Start engine and put T-case in gear, drive forward, if no change, try other port. You 4WD light works from this motor, so you'll know when it engages. This little trick worked on my wrangler when the lines got cooked by the exhaust. Good luck. Yes, the switches on the T-case do go bad. How well does the engine run? May have a hole in the vacuum tube supplying the switch from the manifold.

[Edbrooks]

Yes, the switches do go bad. I replaced mine, as well as the vacuum harness. Not difficult to do. Also, I found that on mine (88 4 ltr) the vacuum plug that goes to the tranny switch had worked loose. It did not fall off but was far enough off the pins on the tranny that there was a vacuum leak. It had sucked up dirt, clogging the vacuum lines. Easiest way was to replace it all.

Jack up the right front wheel. Remove the vacuum motor, which is easy to do. (Engine off, transmission in Park) With the vacuum motor removed, use a screwdriver to see if you can slide the locking collar on the axle - you will have to rotate the right front wheel to be able to do this. The collar should slide easily. While the vacuum motor is off, operate the shifter fork manually to be sure it can move smoothly.

Start the engine. Leave the vehicle in Park. Shift to 4wd. The vacuum motor should operate. If it does not, you have a vacuum problem. Shift back to 2wd; the vacuum motor should return to its normal position.

Shut off the engine, reinstall the vacuum motor if it worked OK. If not, check the vacuum going to it. When you reinstall the vacuum motor be sure the shifter fork engages the collar properly.

Lower the vehicle and check the front axle lubricant. Be sure it is to the fill point.

Question: Does the 4WD light come on when you shift to 4WD? If not, the vacuum assembly, the vacuum harness, or where the harness plugs into the tranny, may be loose. Since that has a tendency to work loose, I used a piece of metal clothesline wrapped around the transmission, and through the plug on the harness, where it plugs into the transmission, to hold it in place.

Ed