Oil Pressure Gauge Diagnostics

Hey! New to posting on here but been somewhat of a lurker in recent months reading posts. I see that diagnosing gauges is a relatively common thing for these GM squarebody trucks, and was wondering if someone could assist me. I have had issues with my oil pressure gauge for a while, I have replaced both the gauge, and the sending unit. Additionally my truck has a brand new painless wiring kit, every single component of the system is brand new. Gauge, wiring, cluster circuit board, sending unit. Yet I am still having issues with the gauge.

The gauge works, but is not accurate. Around 30PSI and below it reads accurate, above 30PSI it is increasingly inaccurate the higher my pressure is. The sending unit is rated for 95ohm resistance @ 60PSI and above, and this has been bench tested. Results of the bench test are as follows;

60PSI - 95ohm
45PSI - 80ohm
30PSI - 56ohm
15PSI - 27ohm
10PSI - 21ohm
5PSI - 3ohm
0PSI - 3ohm

My old oil pressure gauge utilized an external resistor, and my new gauge uses an internal resistor. Old gauge had the white block between terminals on the back of the guge with a small printed circuit for the electrical path, new one does not its just a piece of plastic to fill space. Tested the new gauge, and it reads 70ohm resistance. I have not tested the old gauge resistor yet. However, the old gauge was working good at one point, both before and after I installed the painless wiring kit. Grounding shouldnt be an issue as the sender grounds through a brass fitting and then into my manifold, and the whole engine is grounded via a braided 1" wide ground strap bolted directly from the head on the engine to my frame.

So, my sender range is 3-95ohm, and my gauge is internally resisted at 70ohms. What have I done wrong, or what do you think I need to do to get these to work? Individually they both work but it seems my issue lies with communication between the sender and my gauge. I am going to build some test leads for my old gauge, and hook it up to the sender while its on the tester so I can increase pressure to the sender via my test kit while watching how the gauge reacts.

Wanted to make a post here incase anybody had ideas, I am experiencing a similar issue with my fuel gauge as well but it is always maxed whereas with my oil pressure gauge, it does move with the pressure, just reads way too high when my PSI is above 30. From wha I understand, the gauge has to match the sender, so if I have a 95ohm sender I should have a 95ohm gauge. But, I cannot change the resistor on my new gauge as its internal. Hopefully I can get some advice from everybody here who knows far more about it than I do.

Thanks everybody!

Sounds like the sender is incorrect for the gauge.Did you get both parts from the same supplier and are they supposed to be compatible? I believe factory senders were 0-60 ohms,30 ohms was 30 psi 0 ohms was 0 psi and 60 ohms was gauge pegged.

Are you able to "Tee" in a mechanical gauge to get a 2nd opinion?

Maybe I'm simple minded, but if it's accurate at 30 psi and below, that'd be plenty good for me.

@bucket yeah I am happy its working accurately enough to tell me if I had no pressure, but it bugs me that it isn't accurate on the top end of the gauge.

@Rickf I have had a mechanical gauge in use for a couple months now, and is the reason I know that my new gauge is accurate under 30PSI and inaccurate above. Been able to compare them in real time while driving.

@Ricko1966 the sender part # is confirmed to be for my year/make/model/engine, as well as it is a stock style 73-87 gauge I am using from Summit. Both part #'s check out and they should be compatible, which is what makes my issue with the gauge a real head stumper.

Probably going to build a dedicated board to mount and test my gauge while having it connected to my sending unit while the sending unit is connected to my pressure tester. Might have to do some weird gimmicky thing with resistance to get them to communicate, not entirely sure.

76 to at least 75, maybe 76? were mechanical gages with a tube from the engine to the gage. No electric.

My guage pegs after a certain point of wot, and reads around 30 psi at idle, which I feel is acurate also. I've replace the guage & sender, even hooked up an 80 psi diesel guage, which acted the same way and was 30 psi at idle, pegged going down the highway. I can literally see an electric glich when a certain rpm is hit as the guage will peg out, then drop to normal when it idles. Sounds like we're experiencing the same thing.

I think I'd try adding a 20ohm resistor in series.

idahovette said:

76 to at least 75, maybe 76? were mechanical gages with a tube from the engine to the gage. No electric.

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'77 was the last year for it.

I’m having a similar issue. I swapped the 305 out on my 84 k5 blazer and at startup I get like 15 psi or so and goes to 0 at idle after warm. Tried an external oil pressure tester and got 52 cold and 26 or so warm at idle. I bought a whole another cluster to swap it and it was exactly the same reading. Can I added an another resistor to get this at least a little more accurate? Stresses me out when it basically shows 0 lol

jbro1988 said:

I’m having a similar issue. I swapped the 305 out on my 84 k5 blazer and at startup I get like 15 psi or so and goes to 0 at idle after warm. Tried an external oil pressure tester and got 52 cold and 26 or so warm at idle. I bought a whole another cluster to swap it and it was exactly the same reading. Can I added an another resistor to get this at least a little more accurate? Stresses me out when it basically shows 0 lol

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Are you still using your known working sender from the old 305?

No but I bought another sender and had the same reading. I sold the motor with the old one unfortunately otherwise I would’ve checked it.

jbro1988 said:

I’m having a similar issue. I swapped the 305 out on my 84 k5 blazer and at startup I get like 15 psi or so and goes to 0 at idle after warm. Tried an external oil pressure tester and got 52 cold and 26 or so warm at idle. I bought a whole another cluster to swap it and it was exactly the same reading. Can I added an another resistor to get this at least a little more accurate? Stresses me out when it basically shows 0 lol

Click to expand...

Mine consistently reads at max, way above 60. When I am hard on the throttle it will react and go down to just under 60, which I believe is reading accurate. When I decelerate and lower the RPM's of the engine using the brake in a higher gear, the gauge will react to the lower pressure. Example if I can slow the engine down to 500rpm the gauge will read around 30, which again I believe is accurate. Have not confirmed with my pressure tester as I took it off and have been busy with work since. Getting back into spending time on the square this week with some parts that came in so will be revisiting this issue here soon. Not sure why it reads at total max during regular driving and idle, I'm positive its something to do with the resistance between sending unit and the gauge. Both are new and spec'd to match, all the wiring is new as well. I cleaned off some of the sealer from the threads of my sending unit to try and improve its ability to ground through the engine, don't think it made much of a difference. Engine grounding should be fine, sending unit goes into manifold, grounds through the manifold bolts into the heads because one of the bolts into the head for my power steering pump is where my master ground cable is at.
Don't know enough myself to say if a resistor needs added and/or how much is needed or where in the system. I work with a bunch of electricians though so was planning on asking them sometime for some ideas of what/where to test parts of the system for measuring the resistance drops.

if your in doubt about the sender, put tubing on the sender (in your hand) with a syringe and pull. watch your meter and make sure it varies as you push/pull.