1991 PZJ77 - 4WD engagement problem

[Abde]

I have a problem with the H4 shift control switch, which is as follows:

When I turn the H4 switch, the 4WD indicator on the instrument panel doesn't light up, but when I shift the gear lever from high to low, the 4WD indicator lights up even if I didn't turn the H4 switch on . Also, when I shift the gear lever from low to high and the H4 switch is on, the 4WD indicator doesn't turn off unless the H4 switch is off.

 

[Abde]

 

[TreadingLight]

Have you got the FSM for your vehicle? I'd grab that and check the troubleshooting guide and the wiring diagram and proceed from there.

I believe it works like this:

• With the tcase in High, the H4 switch would trigger the 4wd actuator and you'd be in 4 high.
• Regardless of the H4 switch, if you shift the tcase to low, it activates a pressure switch in the tcase that triggers the 4wd actuator and you are in 4 low.

Since I don't have the manual in front of me, here's a few ideas off the top of my head.

Is 4wd actually engaged when you shift the tcase into 4 lo?

1. Have someone watch the front driveshaft while you move forward slowly and see if it is turning. If so you know the 4wd actuator on the tcase is working.
2. If you then shift back to High, repeat the test and see if you are still in 4wd.
3. Then I'd turn the H4 switch off, repeat the test and confirm you are in 2wd.

I suspect these tests would all be successful as the 4wd indicator is driven by another switch on the tcase and only comes on when you're truly in 4wd.

Next, with the tcase shifter in high, hit the H4 switch and repeat the test. I suspect you're still in 2wd at this point, failing the test.

Then the question would become, why is the 4wd actuator correctly triggered by the switch in the tcase but not by the H4 button? This is where the wiring diagram in the FSM would help you diagnose.

Personally I think the solution is an h55 transmission and a manual split case with no 34 year old electronics in the mix. And while you're at it you could swap in a 1hz engine and get that missing cylinder added back in. But that would be a project.