My oil pressure gauge is acting up, not sure why

So I have looked around and can’t find anyone with a similar issue, here goes.

So since I got the truck back on the road with a brand new shop built motor ( known good shop ) oil pressure gauge won’t go above 20-25, on cold start to warmed up it’s all the same, I have a new oil pressure sensing unit in the truck, I also put a brand new remanned oil pressure gauge motor and nothing has fixed it ( old one did the same thing, I thought it was that ), I’m not sure what else to check, I have cleaned connections on back of gauge cluster and I have checked connections throughout the truck that I know of to no prevail, if anyone knows of what this could be, that would be amazing, also I know the ground strap that’s supposed to go to the back of the motor is not attached but the one to the cab is attached if that is important any, thank you in advance. It’s a 454 vortec

Have you attached a mechanical gauge to the engine to see what the oil pressure actually is?

movietvet said:

Have you attached a mechanical gauge to the engine to see what the oil pressure actually is?

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I have not had a chance to, but the motor is brand new, and I have driven it enough to where if the gauge was correct it would have came apart, so I don’t think it’s a mechanical fault but I will be getting a mechanical gauge and checking here shortly

Yea, you don't "think it's a mechanical fault" and that is why you need to be for sure. Any gauge that tells you the oil pressure, starts with that, the actual mechanical pressure.

1998chev454 said:

So I have looked around and can’t find anyone with a similar issue, here goes.

So since I got the truck back on the road with a brand new shop built motor ( known good shop ) oil pressure gauge won’t go above 20-25, on cold start to warmed up it’s all the same, I have a new oil pressure sensing unit in the truck, I also put a brand new remanned oil pressure gauge motor and nothing has fixed it ( old one did the same thing, I thought it was that ), I’m not sure what else to check, I have cleaned connections on back of gauge cluster and I have checked connections throughout the truck that I know of to no prevail, if anyone knows of what this could be, that would be amazing, also I know the ground strap that’s supposed to go to the back of the motor is not attached but the one to the cab is attached if that is important any, thank you in advance. It’s a 454 vortec

Click to expand...

Greetings 1998chev454,

I reviewed the electric engine oil pressure circuit, and fortunately it's pretty simple.

* 0 ohms = 0 psi indicated.
* 90 ohms = full scale (80 psi) indicated.

So what we need to figure out is if the circuit can't get above ~30 ohms resistance,
or is the sending unit/wiring harness to gauge delivering the full 0-90 ohms to the
gauge, and the gauge itself is at fault?

To help clarify all this, here's an excerpt taken from the '98 C/K FSM:



Alright. The FSM refers to a special tool to apply first 0 ohms of resistance (0 psi), followed by 90 ohms of resistance. (80 psi)

Once upon a time I would have referred you to Radio Shack, but then I would be dating myself. :0)
Instead, you could test (and even calibrate) your gauge by disconnecting from the sending unit and
putting a 90-ohm resistor like this sweet NOS piece inbetween the engine block and the sending unit wire:


NOTE: I don't know what current is pulled through the engine oil pressure gauge circuit. Could I use a 1/4 watt job? IDK. So this NOS 10-watt, 90 ohm wire-wound resistor
should outlast me as an understressed GMT400 diagnostic device. It came in under my threshold of pain, and I've been wanting to verify the accuracy of my own gauge,
so I decided to splurge just now. :0) I'll be sure to take a couple of pics and write up what I discover once I get a chance to do this on the chore truck.

So, the short answer is to get a 90 ohm resistor, connect the oil sending unit wire to that, and then use the plastic fork trick
(documented elsewhere in this forum) to align the pointer to 80 psi when this resistor is in circuit. Then (assuming a good
sending unit) you should now get an accurate reading. (Of course if the gauge is a bit jittery, then you may want to follow
other people's advice about adding some high centistoke oil to your gauge motor in order to smooth things out. (Gauge dampening thread)

And in order to know what the repaired gauge should be reading, like mentioned in previous responses
a decent mechanical oil pressure gauge takes all the guesswork out of this area. (See attached for
something that's worked well for me in the past.)

Hope the above was helpful. Let us know what you discover.

Best of luck --

movietvet said:

Yea, you don't "think it's a mechanical fault" and that is why you need to be for sure. Any gauge that tells you the oil pressure, starts with that, the actual mechanical pressure.

Click to expand...

Is there anywhere else to tap into for oil pressure, the sending unit is unable to get reached because of how the previous owner did the electronics, I will have to tear apart most of the intake for this so I am wondering if there is any other spot I can tap into or if that’s the only way

movietvet said:

Yea, you don't "think it's a mechanical fault" and that is why you need to be for sure. Any gauge that tells you the oil pressure, starts with that, the actual mechanical pressure.

Click to expand...

also if it’s not mechanical what would it be circuit wise ? Cause I have replaced both sending unit and the gauge motor in the cluster and cleaned the connections that go from the truck into the gauge cluster, so I’m not sure what else there could be

caminonut said:

Do you mean by gauge motor you’re talking about the step motor?
Also, if you replaced it, did you check the shunt resistor on the back of it for continuity?, and check it for a Ohms reading.
Also check any of your grounds they’re very important.

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Yes I replaced the stepper motor on the back

I did not know there was a shunt resistor and I will have to go back and check

I know I have a good ground to cab, not sure about filler cap to frame one, and I know the one that’s supposed to go from frame to engine is gone and I plan on replacing all of em

1998chev454 said:

Is there anywhere else to tap into for oil pressure, the sending unit is unable to get reached because of how the previous owner did the electronics, I will have to tear apart most of the intake for this so I am wondering if there is any other spot I can tap into or if that’s the only way

Click to expand...

Yes, I connected my mechanical gauge to a pressure port right above the oil filter. I have the TBI 5.7 though. I would think the Vortec V8 and V6 would be similar.