my 89 and 90 4 liter 5 speed comanches oil pressure gauges don't work, they point to the far right. My 87 auto 4 liter Cherokee oil pressure gauge works. What usually goes wrong with these? wiring, ground, the gauge itself?
Thanks!

 

You need to make sure the Commanches have the proper sending unit for the gauge.

Somebody may have put one on there that is supposed to be for the light as they are cheaper, but wrong.

 

If the wire from the sending unit to the gauge is grounded somewhere, the gauge will read as you describe.

How do they read with the wire disconnected from the sender and the key on?

 

I disconnected the wire on the sending unit on my 85 2.5L and when you turn the key on it pegs it to the right. If the wiring is the same it could be a bad sending unit or broken wire if it is still connected I would guess.

 

That ain't a sending unit issue if it pegs the gauge when unplugged. It's a wiring issue. The wire you took off the sender is grounded out somewhere, pegging the gauge.

 

If you can't see a chafed wire pretty easily,it might be easier just to run a new direct wire from the sender to the gauge cluster.

 

I recently swapped over my 96 2.5l to a full instrument cluster and tested the tach before finishing everything up. The oil pressure went all the to the right as described in this thread. Completed the upgrade by changing the temp and oil sending units and everything worked perfectly. So its possible the OP's problem is the sending unit.

 

It works fine when it is plugged in, but pegs out when unplugged so I figure the gauge is reading 0 with low resistance and higher with high resistance from the sending unit, when the circuit is broker it maxes out.