Oil pressure gauge reads low

[terrawombat]

A while ago I bought a full gauge cluster for my '88MJ Pioneer and swapped in a brand new oil pressure and coolant temp sender. At first, the only gauges that worked were the tach and the volt meter, but after some research on here and some tinkering, I think I've got the coolant temperature gauge to work. I also cleaned up the ground behind the left rear taillight and the fuel gauge seems to be working decently - doesn't seem to quite get to the 'Full' mark, but it also doesn't leave me stranded without fuel so I can't complain too much. Still up in the air is my oil pressure gauge. Here are some things it does:

 

On a cold startup, when the oil is nice and thick, I get a reading on the gauge around 15-20 psi that seems to hold pretty solid regardless of RPMs. Once the engine is nice and hot, the gauge drops to 0 psi with no change in pressure via RPMs. I've plugged a mechanical gauge right into where the sender goes and at a hot idle, I was getting around 15 psi and around 30 psi when I revved the engine. It's obvious the engine doesn't have 0 psi oil pressure since it doesn't knock and I can see the oil pooling up on top of the rockers when I pull the oil fill cap on the valve cover. I do know the engine has less-than-desirable oil pressure (especially for only having 78K miles on the odometer) and I intend to fix that in the near future, but in the meantime, I'd like to get my oil pressure gauge fixed for the peace of mind.

 

Does anyone have any suggestions or recommendations? when I pull the plug to the sender, the oil pressure gauge pegs to the right. When I ground the plug, the gauge pegs to the left. I'm not sure of any other tests I can perform, though...

[JeepcoMJ]

did you put the right sending units in it for coolant temp and oil pressure? those MUST be swapped to get full gauges to work properly.

[terrawombat]

Yes, the enormous oil pressure sending unit is brand new and in place. I bought this one: http://www.rockauto.com/catalog/moreinfo.php?pk=933381

 

I also swapped the coolant temperature sender to a brand new one intended for gauges, not idiot lights. I also have a spare set from my other MJ that I originally used, but they produced the same results...was hoping new ones would fix the problem.

[JeepcoMJ]

gauge cluster from junkyard vehicle that allowed water on the dash is my only real likely bet as to why they don't work.

 

 

just replace the cluster.

[terrawombat]

gauge cluster from junkyard vehicle that allowed water on the dash is my only real likely bet as to why they don't work.

 

 

just replace the cluster.

 

Yea, I was leaning in that direction...I'll probably pick up another cluster when one comes up.

[jimoshel]

Swapping is OK if you know your swapping in a good part. Swap in a bad part and you just may wipe out a good part in the circuit.Now your going nuts trying to figure out why a previously working circuit isn't working any longer. I prefer to test the individual components and and find out for sure what is defective before replacing parts. Remove the wire lead from the OP sending unit. With the motor shut off take an resistance reading from the terminal to ground. It should be 1 ohm, give or take a couple..With the motor running it should read 88 ohms for a OP of 80psi. Because the gauge moves when you turn the key on and off kinda indicates it's OK. A better check would be to get a 100 ohm potentiometer. Hook it in series with ground and the wire you removed from the OP sending unit. The OP gauge needle should move in direct proportion to the Pot being turned.

Same procedure for testing the temp circuit.

[terrawombat]

About 0.5 ohm with the engine off.

 

Cold idle:50 ohms

Cold rev to 3K RPM: 55 ohms

 

Hot idle: 30 ohms

Hot rev to 3K RPM: 50 ohms

 

I'll keep looking for a cluster :D

 

Thanks guys!