Expert: Carl Brandt - 10/11/2007

Question
QUESTION: The 4WD indicator light in my instrument panel on my '88 Jeep YJ won't come on. It still goes into 4WD. It has a NP231 t-case on it. Any ideas on what could be causing this? I have already checked the bulb in the panel, it checks out...

ANSWER: Garrett,

Are you sure that it is actually going into 4-wheel drive.  On the passenger side of the front axle there is a vacuum motor that moves a shifting fork locking the front axles up on that side.  This is controlled by a pair of vacuum lines that comes from a vacuum switch on the transfer case.  When the transfer case is engaged into 4-wheel drive the shift fork, if operating properly will hit an electrical switch that turns on the light when the front axle engages.

It is quite common for the vacuum lines to break, or come loose from the vacuum motor.  The electrical lines from the switch can break also.

If the front end is working, then you may have a bad switch going to the light, or a broken wire.

Carl

---------- FOLLOW-UP ----------

QUESTION: Yes it does go into 4WD, that was the first thing I checked.

I checked the 4 vacuum lines coming from the t-case(NP231), there are four multi-colored lines, red green yellow and blue. the red one goes towards the instrument panel, the yellow and green goes to the front axle, and the 4th is cut, maybe this is it? Any clue where this one should lead to?

Answer
Garrett,

Your model is slightly different than mine according to the manual.  My manual doesn't show the color code, but it appears that your year Jeep's 4wd indicator switch is controlled by a vacuum line where mine is controlled by an electric switch.

Your vacuum lines should be as follows:
1 from the the transfer case switch to the front of the vacuum motor, 1 from the transfer case switch to a tee on the top of the vacuum motor which tees into a line from the back of the vacuum motor and to a 4wd indicator light and switch.  There are 2 more lines going from the transfer case switch-one going to a check valve, and the other to an air vent filter.

I would suggest that you purchase a Chilton's manual for your Jeep, as they show a diagram/picture of the vacuum line routing for this unit.  They cost $20 to $25 at most auto parts stores, and are invaluable for trouble shooting, or repair.

Carl