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Username Post: Oil Pressure Diagnostics - Electrical Help        (Topic#366517)
Posts: 5

Reg: 09-02-21
09-26-21 06:53 AM - Post#2827978    

1980 C20 350 w/TH350c - 100k miles

My C20 oil pressure gauge is reading low at idle and when driving. It fluctuates slightly but stays below the first has mark (15psi?). I removed the sending unit and did a continuity test. It had a static 16ohms resistance. My expectation was that it should read infinity. I purchased a replacement oil pressure sending unit. To establish a baseline and compare it to the old unit. I measured the resistance and was getting about 1.6ohms. Again, my assumption is that there should be no resistance when the unit is out of the vehicle.

Question: Is my assumption correct that an unplugged and deinstalled sending unit should read infinity?

I also tested the wire that runs from the sending unit back to the wiring harness on my (new) circuit board for the gauge cluster. My expectation was that I should have zero resistance. I did not. There was a small amount of resistance registering on my Klein voltage meter.

Question: If that wire casing is bare and touching metal, or crushed in some fashion would a small amount of resistance register?

The engine sounds fine. There's no knocking. 5w30 level is right on the mark. I am going to try changing the filter to see if there is any difference. I really would like to verify whether my gauge is reading correctly or do I actually have an oil pressure issue.

Thanks in advance for any help anyone can offer.

Member #271 Senior Chevytalk Moderator
Posts: 25563
Loc: Austin, TX 78748
Reg: 05-09-00
09-26-21 07:24 AM - Post#2827983    
    In response to Thumbpick330

Welcome to the site. The gauge is probably faulty. The measurement wire should have resistance. If it does not, then there is a short or broken wire somewhere. The same applies to the sensor. It should have some type of resistance. A replacement and old one should measure within a small range of each other. If there is no resistance, the sensor is damaged inside.

'83 Silverardo XST - ZZ4 powered
'95 Jimmy SLT (Bought @131,814)
'96 GMC Jimmy LS Ret. @236651 miles
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Posts: 5

Reg: 09-02-21
09-26-21 09:31 AM - Post#2827991    
    In response to gchemist

Thanks for the reply. Looks like I’ll need to purchase an oil pressure tester. I noticed multiple comments in many threads indicating that a Fram oil filter can give these results. I just replaced it with a K&N but no difference. Will see if I can get an external gauge to see what the pressure really is.

Posts: 889

Reg: 03-20-18
09-26-21 12:56 PM - Post#2828019    
    In response to Thumbpick330

make sure the filter is rated for your vehicle.

I been reading some of the newer filters are more dense to provide better filtration as a result messes with pressure.

Learned about it when trying to increase oil pressure in a 30+ year old van.

filters is something you don't want to mess with for every day driving.

when asking Questions its related to van I own: 1987 GMC 3/4 ton G2500 vandura, V6, No AC, standard length. 3speed trans(no overdrive).

Posts: 5

Reg: 09-02-21
09-26-21 01:59 PM - Post#2828031    
    In response to Thumbpick330

I purchased a Maddox oil pressure tester. At idle I’m sitting at a steady 30psi and it rises with load. Appears that the gauge is the culprit. I did notice that when I ran the truck with the wire disconnected the gauge maxed out above 60psi which is the expected behavior. I’m guessing there is a dead zone on the gauge itself. Thanks for your help and pointing me to the gauge itself.

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