Help me troubleshoot my oil pressure guage

[72farmer]

I have a 87 Silverado with a LS swap engine and trans.My electric oil pressure guage has never worked.I tore in to it today and I need some help.
The guage needle does not go back to zero when you turn it a little and let it go.It will go all the way around without stopping anywhere.There is resistance when turning the needle.How does it work on voltage or resistance?Do you think I need a guage or a sending unit on back of motor?I read 12VDC on one of the three terminals in dash.

[ray_mcavoy]

These gauges use what is called an "air core" meter movement. There are 2 electromagnetic coils inside that act upon a small piece of iron attached to the needle shaft. Current flowing through one of the coils pulls the needle toward the zero / low side while current flowing through the other coil pulls the needle toward the high side. So when there is no power applied to the gauge, the needle position is meaningless.

The sending unit is a variable resistance type that varies the current flowing through the coils which in turn moves the needle. And (in my experience anyway), it is more likely for the sending unit to be bad rather than the gauge itself. Poor connections in the wiring (especially where the cluster plug & gauge clips make contact with the printed circuit on the back of the cluster) are also somewhat common as well.

If you have a mulitmeter, you can test the oil pressure sending unit on the engine by measuring it's resistance. Do that by unplugging the wire from the sending unit and connecting the meter between the terminal on the sending unit and a good ground on the engine. With the engine stopped (zero oil pressure) you should get a reading on the meter close to 0Ω. And when you start the engine, the resistance reading should go up. The exact value will depend on how much oil pressure your engine has, but with a 60PSI sending unit, you should see around 45Ω at 30PSI and close to 90Ω at 60PSI.

[72farmer]

If I need a sensor do I get one for a 87 350 or a 02 5.3 engine?Is the ohm test the same values on newer sensor?

[ray_mcavoy]

The resistance range of the sending unit has to match the gauge. I'm not sure what the newer sensors have for a resistance range but there is a good chance it's different. So if you do need a new sensor, you'd want to get one meant for the '87 350.

I think the threads for the oil pressure sender are metric on the LS engines while the older small blocks had 1/8" NPT threads. So a thread adapter will be necessary to use the '87 sending unit on the LS engine. There might already be one of those adapters on your engine if an '87 style oil sending unit was fitted to the LS engine during the swap.