Oil pressure gauge sits at 30, does not move. How to test?

[MountainGoat1966]

Like the title says, my oil pressure gauge (84K10 with 383 stroker) suddenly decided to stop working.
Normally, sits at 60 when first started up, and at highway speeds.
Idles around 45-50 psi.
Now it just sits at the 30 mark, never moves.
I tested the resistance at the sender, engine off and running.
4 ohms running and around 90 engine off.
I think I got those in the right order.
Looks like I need to test the gauge now to verify it's failed.
Any hints?

[1970pelle]

I know the service manual has a test for trouble shooting the gauges. Do you have access to GM service manuals?

Are you needing to know if the engine is making the correct oil pressure? Hooking up a manual oil pressure gauge, is the easiest way to find out. I prefer a manual over a electric gauge myself.

Ken

[hatzie]

The service manuals are available... Look in my signature.
The oil pressure gauge is the same ohmage as the fuel gauge. 0=L 90=H

Grounding the sender lead with the ignition switched on should drive the needle to the bottom of its sweep 0.
Unplugging the sender should drive the needle to the top of its sweep (usually 60).

If it stays at 30 then you have a damaged sender wire, bad power connection to the gauge, or fouled up gauge. Probably a fouled up gauge but I'd still check the power connections.

[MountainGoat1966]

Thanks, guys ........
I know the engine has oil pressure because the electric fuel pump still works.
I started the engine with the tan wire unplugged when I tested the ohms at the sender unit.
Gauge still did not move.
It was a sudden failure, as I had driven to the big city that day (120 miles round trip) and the gauge was at its normal 60 psi on cold startup in the morning.
All the other gauges work and the fuses are good.
I do have a brand new mechanical gauge I could install, but I sorta like the factory OEM look in my trucks.
Guess I am going to put in some repair time tracking it down and delay my winter preps..

[hatzie]

If the gauge doesn't move with the sender unplugged it's likely the gauge itself.

I'd still check the connections in the instrument cluster bucket with the oil gauge removed... Use a Good Digital Multimeter. Test DC volts between the IGN and GND terminals with the ignition on. Test OHMs of resistance between the sender and GND terminal with the sender plugged in and engine running.

[MountainGoat1966]

I have decided either it's something electrical, or the space aliens are messing with me again.
Let the truck sit for a week unused.
I finally found my spare set of gauges in the shed so hopefully I would not need to buy any parts.
Started it this morning to move it to the carport to tear into the dash, and the oil pressure gauge is working normal again.
It must know I was going to fix it.
I have seen many cars over the years with issues that went away when the car was about to go in for repair.
I am suspecting the printed circuit board may have issues since the gas gauge suddenly decided to stop working correctly a while back.
I will try another printed circuit and can replace a couple burned out bulbs at the same time.
There's no way the horrible rough roads where I live could have any effect on my trucks ....

[hatzie]

There's an old trick I've used to rejuvenate card edge connectors in old computers and electronics test equipment.
If you are very very very careful you can use a soft Papermate Pink Pearl pencil eraser to erase corrosion from the printed circuit.
The cluster connector terminals are heavier. You can clean those with a pink pearl pencil eraser too and you don't need to be as careful just don't bend em out of shape. Once they are clean you should spray on some electronics protectant. I use plastic safe Caig De-Oxit Shield SN5 to keep freshly cleaned copper and brass terminals from instantly corroding in places like this.

The other thing to check is the connection between the studs on the gauge and the ceramic bias resistor.

[MountainGoat1966]

Thanks, Hatzie.
That gives me a good direction to go.
Might get another 30 years of service from the dash and gauges.
Do you have any suggestions for getting rid of pesky space aliens?