Oil pressure gauge troubleshooting?

[jms]

The factory oil pressure gauge in my '85 Jimmy didn't take too kindly to my last Moab trip. After a few wacko displays (including the dial going all the way around), things have settled to this: the dial centers on '0' with engine off, IGN on. With the engine running, the dial jumps beyond the scale and stays there, no matter whether engine is cold or warmed up.

I finally had the gauge out yesterday (this is really fun with the roll cage in the way...), and measured voltage at the connector: I presume the bottom connector is ground, top left had 13V with IGN on (so I assume it's power), couldn't measure anything at the top right connector, neither relative to ground nor relative to power. Shouldn't I get a signal at that connector? I only measured at idle, should I get a signal when revving it up (i.e. change in pressure)?

I can't really rule out the sending unit as being defective; I'm having a hard time getting to it due to dist./wires/cruise being in the way, so I only wiggled the (brown) wire so far. Can anybody point me in the right direction on how to diagnose where the problem may be? I should say I have oil pressure, the engine runs fine.

I'd rather not go with an aftermarket gauge, I don't have anymore space under the dash (due to four other gauges, tranny/tach/water temp/cyl. head), and I dont want to redo the whole dash with aftermarket anyways. Thanks for the input, michael

 

[miniwally]

Michael I would replace the sending unit as it shuld be fairly cheap before investing to much labor and thought into what it may be.
I do not know a lot about the gauges but I would think the sending unit simply gives the gauge varying amounts of voltage to show you oil pressure on the gauge.
Hope this helps.

 

[jms]

Thanks Brandon, I was afraid someone might suggest just that. I'll probably have to get distributor and cruise out of the way before I can even touch the sender. I think I'll try and measure voltage at or close to the sender to rule out a connection problem on the way to the printed dash circuit.

 

[rodzzilla]

My experiences w/bad senders has always been where it would show reduced/no pressure. My 85 Burb had was still reading low Pressure w/a new sender. I hooked a mech. guage in for a temp. check to ensure my engine had pressure. Was going to buy a new factory guage to replace it. Wonder if there is a short in the circuit somewhere?

 

[jms]

Thanks for the input, no, I can't rule out a short somewhere. That's why I'm thinking I should measure somewhere close to the sender.

 

[pauly383]

Sounds Crazy but patronize me here, I had exact same problem with a gauge :

I can verify by experimentation with other gauges that sometimes it is that ceramic resistor behind gauge that fails. I have had a gauge that would read past 260 all the time , just out of curiosity I removed the ceramic resistor and swapped another on it and my gauge worked again, the other gauge with my resistor read past 260 all the time. There are thousands of those sitting in yards all over the place. /forums/images/graemlins/k5.gif

 

[jms]

Just for the record: it looks like it was the sending unit. Put a new one in this morning, seems to work fine. Thanks to everyone for their input, michael.