83 C10 oil pressure gauge not working

EB83C10

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Looking for some input on my oil pressure gauge. I've found a ton of information on the sight here and used it for trouble shooting. But I still can't get it working.

I have a 83 C10 that I acquired in pieces. Original SM 465 Transmission. Swapped in a crate 383 and all new Painless wiring harness. All new ground wires engine, frame, cab, etc...

Gauge reads Approximately 15 psi at all times when the engine is running.
I originally had the old sending unit installed and I replaced with a new one. Same reading. No thread sealant or tape on threads to interfere with ground signal.
Connector for instrument cluster has been checked and double checked for correct pin out. Factory circuit board traced and everything looks good. Grounds to bus bar clean. All new wiring and connectors. All other gauges and back lights working.
When I unplug the wire at the sender the gauge pegs. Ground it and it drops to zero.
Ceramic resistor on back of gauge reads 85 ohms. I've had trouble finding information about whether this is the correct reading for the oil pressure gauge resistor? If it's not, what is?
I checked the pressure with a mechanical gauge and it reads 50 psi cold and approximately 37 hot with fluctuation when revved.

What's next? Is the gauge itself bad? If so, what's the best place to buy a reliable new one? Or where can I get a good used one?

Attached picture showing gauge with needle where it sits with no change whatsoever with the engine running.

Thanks!
 

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wlwarnke

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Does gauge move at all? Key off, key on engine off, etc?

These sensors are 0-60ohms = 0-60psi

That’s ohms to ground.

Lift sensor wire, should peg gauge high.

Short sensor wire to ground, should read zero.

If no deflection, remove gauge from cluster and bench test. I can’t recall if the gauges have 2 or 3 terminals. Seem to recall 3. That would be +, -, and sensor (0-60ohm).
 

EB83C10

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Yes. Gauge goes to zero with the engine off. Stays steady at 15 while running.
And yes I did that. When I unplug the wire at the sender the gauge pegs. Ground it and it drops to zero.
Yes it has 3 wires. +, -, and sensor
 

wlwarnke

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Have resistors, or a potentiometer, or a decade box? Check the calibration of the gauge. Connect the sensor to you compressor regulator (1/8” npt) and confirm both gauge and sensor are accurate.

Every “good” sensor I’ve tested has been pretty damn accurate.

Gauges however are never linear. Usually pretty close at 0 and 60 but usually off a bit at mid scale.
 

EB83C10

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I only have some 91 ohm resistors. I do not have a potentiometer or a decade box.

I'll have to try checking sensor and gauge by applying air. I have not done that yet.
What about the no fluctuation when under a load or different rpms?
 

wlwarnke

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It’s hard to say. Since lifting wire pegs gauge, you probably don’t have a grounded wire (atleast not a direct short). Since grounding wire reads zero it’s def not open wire.

If you have a way to simulate pressure (compressor, regulator, good gauge) remove sensor from truck, connect to regulator, and run a length of wire from sensor to the factory wire. Key on, engine off, and see what she does.
 

wlwarnke

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Two 91ohm resistors in parallel would be 45.5ohm

That’s 45.5psi

3 would be 91/3, etc.

30.3 psi
 

EB83C10

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I found my problem! RustyPile chimed in on a private thread and helped me out.

I was testing my ceramic resistor on the back of the gauge with it installed. I did not realize I should be testing it off the gauge. My ohm reading of the ceramic resistor was actually the reading the resistance of the gauge.

I pulled the resistor and tested and got zero. I added a 91 ohm resistor and installed it back in the dash and started the engine and it now matches what the mechanical gauge was showing +/- a couple psi.

Thanks for all the help!
 

wlwarnke

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Good to hear.
 

84c10Tony

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Good info, cool beans, thanks guys...
 

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