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Old 07-24-2017, 01:06 PM   #1
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78 C10 oil pressure readings and grounds

I have a 78 C10 big ten 2wd with a GM 350HO, all other parts are factory in readily good condition. Have owned it for 3 years and drive it occasionally. From the day I bought it the oil pressure gauge always read high and would flutter consistently. I considered that to be an issue with the sending unit and the stock factory other problems or symptoms. Recently I changed the oil and filter and watched the gauge for a few read a little higher than normal but same behavior....however a few days later I noticed it dropped by roughly half and at idle shows almost zero at 160 degrees when in drive with AC on. No other problems or symptoms have surfaced in the motor which is tight and solid. Now I have read opinions on oil pressure which span left to seems to follow the 10 psi per 1000 rpm. I also understand the sending units can go bad and that proper grounding will wreak havoc on issues like these.

Last night I started troubleshooting the gauge and sending unit first. The truck has a main battery ground running from the battery to the alternator bracket. It then has a ground strap from the pass side cyl head to the firewall. I also see a ground from the battery to the core support. Those seem to be good. I checked resistance of the sending unit (bottom of distributor) from the sending unit to the bolt on the neg ground at the alternator and aside from it fluttering quite abit it reported around 40-45 ohms at idle (750 rpm). When I moved the test probe to the battery neg post it dropped to 20-28 ohms. Now this seems suspect to me but I can't for the life of me figure out why. The cable is good, connections are good and tight... could it be a battery issue and is this possibly the answer to my seemingly weird pressure reading?

Later this week, I will be testing oil pressure at the engine with a real mechanical gauge to rule out an engine problem. Any other thoughts?

Last edited by miket007; 07-24-2017 at 01:58 PM.
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Old 07-25-2017, 10:54 PM   #2
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Re: 78 C10 oil pressure readings and grounds

I agree, the first thing to do is check the pressure with a master gauge & see what it really is doing. If OK I would replace the sending unit. 9 times of 10 that is the problem when the gauge acts up. There is a lot of difference in oil pressure when the engine is cold & when it is hot.
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Old 07-26-2017, 11:39 AM   #3
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Re: 78 C10 oil pressure readings and grounds

Just about all electrical senders work based on resistance to ground. They either have a high resistance to ground ( temperature) and get lower as the engine gets warmer, or a low resistance to ground and increase as the engine temperature gets warmer.

The gauges work based on 12 volts input, which should be a constant, and the variable resistance to ground from the sender. If the input voltage varies it would cause the gauge to flutter and if the gauge is not grounded well it can cause the same problem.

I would disconnect the sending wire from the sender and watch the gauge to see if it is steady and if the reading changes I.E. full high , full low, or no movement. If it still flutters then it is most probably the gauge, either a poor connection on the input voltage or the gauge ground, or the gauge itself.

If there is no movement on the gauge needle then I would ground the sending unit wire
to a good ground and watch the gauge needle for movement or for a steady reading. If the gauge doesn't flutter then I would suspect the sending unit.

I agree with Wrench that using an oil line pressure gauge to compare with the electrical gauge is a good idea and easy to hook up, if the testing I suggested gives no results.

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Old 07-26-2017, 02:19 PM   #4
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Re: 78 C10 oil pressure readings and grounds

Thanks guys...before I posted when I put a meter on the sending unit I checked the gauge for behavior while disconnected and it is behaving as it should....

I put a mechanical oil pressure gauge directly in place of the sending unit and got plenty of pressure at all RPM levels including idle, in gear with AC on. Motor and oil pressure is fine...the sending unit was leaking it appears. So today a new sending unit will be installed and I can cross this one off my list.
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A/C, PS/B, Tilt, GM 350HO/T350, 12 bolt posi, Two-Tone
full length Headers, Lowered with factory ralleys, 2.5" duals with Dynomax 17749's

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