Stock Oil Gauge testing / accuracy

[brrian]

1981 Grand Prix, converted from idiot lights to gauges. I installed an oil pressure sender for the oil gauge, and I have a temporary mechanical oil gauge. The electric gauge is nowhere close to the mechanical gauge, and I went down the GM gauge rabbit hole figuring out why. What I know or have learned:

My factory oil gauge is 0-80 PSI. I believe it's supposed to be 0-90 ohm, but as of this writing I'm not sure about that - I'm measuring between 65-70 ohms at 80PSI.​

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I think the 'raw' gauge was measured at the factory, and a resistor was chosen that brought the measurement into spec. Resistors are color-coded by value (blue, pink, orange, yellow, green), and I can adjust (calibrate?) my gauge by changing resistors.​

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Buying senders is a total crapshoot because most sellers don't publish pressure or ohm ranges, and the specs are hard to get if they can be found at all. Even if found, they're only loosely accurate (for my WVE sender, their engineer gave me a 70-105 ohm range at 80PSI. What?). The only way to know for sure is to test.​

How I test:

For the gauges, I built a janky little test board & used a linear potentiometer to act as the sender & adjust resistance. I took measurements at 10psi increments (by eyeballing the gauge marks, which is the best that can be done).​

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For the sender, I made an adapter to attach a sender to a small pancake compressor with a built-in gauge. I attached a second gauge to it first, to verify accurate pressure readings (they're reasonably close), and now I can pressurize and test each sender I get.​

My plan:

Buy and test senders until I find one that gets me close to my gauge ohm range.​

Test different resistors to fine-tune the gauge to get reasonably accurate pressure readings.​

Why all of this?:

I like originality, I like figuring out how things work, and I need reasonably accurate oil pressure readings. I also hope it's helpful to somebody else.​

Ideas if none of this works:

1 - Buy a new 78-81 Corvette gauge (newer tech, built-in resistor) and sender combo, then swap the gauge mechanicals onto my gauge face, or modify the Corvette gauge to mount in my cluster.​

2 - Have a new, stock-looking gauge face made (decal) with adjusted gauge marks.​

3 - Put a mechanical gauge in the interior somewhere (yuck).​

Anyway, hope this helps somebody someday, and I'll update this post w/ progress.

 

[brrian]

I left this for a while & am getting back to it. I bought a NOS gauge on eBay & tested it, and it's in the same ohm range as the two other stock ones I have. I now think that the stock oil gauge is supposed to be 70 ohms (not 90 ohms) at 80PSI. My 1981 service manual doesn't list the ohm range for the gauges. Does anybody have some factory document that says what the ohm range should be in 1981? I have something newer (unsure of the year) that shows 90 ohms @ 80PSI but maybe it changed before then.