1048575
1989 YJ 4.2 automatic.
To understand how the system works might help to repair it.
On the dash is a light that tells you when you are in 4wd.
Under the battery (Passenger side) is a vacuum operated switch for the 4wd light (One wire to ground the other to the dash 4wd light).
The shift motor is where all of the vacuum hoses attach on the front axle.
From there to the drivers wheel (USA) is the long axle and to the passenger wheel is the short axle.
Inside the shift motor is a coupler that when engaged connects them both to be one axle.
On the shift motor there are three connections.
One vacuum line engages the coupler, and at the same time another sends vacuum to the switch under the battery to ground the 4wd dash light.
The third is to disengage the coupler and at that time the vacuum is cut off the ground to the dash light switch, which turns off the light.
The diagram is pretty simple, and so is the system. If it's hooked up correctly, it usually works (Unless their is another problem)
Things that can be bad are.
1. Inside the shift motor, the coupler is broken or it isn't installed correctly)(I find installation is usually the problem)
2.The vacuum lines are incorrectly routed or broken (Note: the colored plastic lines are very brittle with age. One leak and it won't work)(The Jeep will also run ruff)
3.The vacuum/light switch under the battery isn't grounded to the firewall correctly or it's broken (Usually it's loose and not grounded)
4. The dash light is not working (Bad bulb or wiring) (Usually it's the bulb)
Note: The wiring is simple. When vacuum is applied to the switch it connects the two wires inside and allows ground to the 4wd light (It lights up)
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