Oil pressure guage readings - What are yours? (1 Viewer)

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In researching whether to use the factory sender or the LS sender in the LS swap I'm doing, I tested the factory LS oil pressure sensor and got a resistance vs pressure chart. I then tested the 3FE sensor, and it acts as a switch. No pressure, open circuit. Anything about 5psi, it has 43 ohms. Doesn't matter how much pressure I throw at it, it's either open, or 43 ohms anything above 5psi.

I thought maybe it was bad, so I ordered a new OEM one to test. Same exact operation, except 63 ohms instead of 43 ohms. The pressure "sender" is acting like a dummy light switch, yet it's connected to the pressure gauge. As this is not my LC, I have no idea what the pressure gauge does during engine operation. Based on the testing, the gauge should read low when engine is off, and something when engine is running, and pretty much stay at the same spot regardless of temp/rpm (it may raise a little with RPM due to higher voltage output from the alternator. I don't know if the gauge has voltage regulation)

Is this what you guys are seeing? This oil pressure switch is the same from 1985 to 1996.
 
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Zjohnsonua

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It definitely doesn't act as a switch. Put the sender on a test rig and increment the applied pressure and the gauge will move accordingly. As best as I can tell, the gauge electronics are just very sensitive to very small resistance changes.

I've seen several LS swaps that use an adapter to get the OE sender used in the LS oil pressure port.
 
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It definitely doesn't act as a switch. Put the sender on a test rig and increment the applied pressure and the gauge will move accordingly. As best as I can tell, the gauge electronics are just very sensitive to very small resistance changes.

I've seen several LS swaps that use an adapter to get the OE sender used in the LS oil pressure port.

I'm stumped as to how the gauge changes if the resistance doesn't change on the sender. It doesn't even budge .1 ohms, and I tested from 5psi to 100psi.
 
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Well, I set up a test rig, and I'm still stumped. I pulled the cluster and fed it power and ran it through the original 3FE sensor. I applied 10 psi, and the gauge barely moved up. The more pressure I applied, the higher the gauge went, though it only swept about the lower 1/3 of the gauge. I checked resistance across the sensor at each interval, and it was always 43 ohms. I hooked up the new sensor. It acted the same, however, the gauge worked through about the first half of the sweep, so the gauge responded better to the new sensor. Every time I checked resistance across the new sensor, it was 63 ohms.

So I hooked up a variable 100 ohm resistor instead of the the sensor. At 43 ohms the gauge is above the H mark. At 63 ohms its just below the H. At 100 ohms it's just below 3/4. Apparently, however this circuit works, it's not measurable with a multimeter.

Anyone more versed than me want to explain what's going on to this confused individual? Either way I'm using the stock sender with adapters.
 
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Zjohnsonua

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You've done what I did, and are left with the same questions I have.

My theory is the gage is set up as a qtr bridge circuit, which can be highly sensitive to milliohm variations - which most DVMs won't pick up.
 
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You've done what I did, and are left with the same questions I have.

My theory is the gage is set up as a qtr bridge circuit, which can be highly sensitive to milliohm variations - which most DVMs won't pick up.

That would be a good theory. However, the fact I can actuate the gauge with a potentiometer, which changes resistance my entire ohms, and when I apply the resistance that I read on each sensor to the gauge I get a completely different gauge position than I do when plugging in the sensors, makes me think something else is going on. What it is, I do not know.
 

Zjohnsonua

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I totally whiffed on the pot test in post #4. Back to the drawing board.

My pressure gage is wonky. Been meaning to replace it. I'll tear into this one to see what's happening on the board.
 
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