Oil Pressure Interference

1959 - 1978, M151, M151A1, M151A2, Technical questions and discussions, regarding anything related to the M151.
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bazookajoe
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Oil Pressure Interference

Post by bazookajoe » Sun Aug 05, 2018 7:12 pm

I was driving my A-1 today and noticed that the oil pressure had dropped off to just about zero, but when I turned off the lights it went back to normal, but the needle was really bouncy.....any thoughts, I also had recently replaced the three lever light switch, your thoughts???

Rickf
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Re: Oil Pressure Interference

Post by Rickf » Mon Aug 06, 2018 6:32 am

Check your gauge connections behind the dash, especially the ground. I doubt it has anything to do with the light switch, totally different circuit other than the ground.
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone

mdainsd
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Re: Oil Pressure Interference

Post by mdainsd » Thu Sep 13, 2018 3:20 pm

It is a grounding problem with the gauge to the instrument panel and/or the grounding of the panel to the rest of the jeep. I alway make another "star" circuit (Like the one that powers the gauges) but has a ring terminal to go on each of the gauges mounting studs (one per gauge), and a long lead that gets attached to the body up behind the gauge panel. Rock solid gauge performance after doing that. All my M series have that and they all work as they should.

What is happening to yours (more than likely) is that the gauge panel is not grounding completely when mounted. If you turn on the lights, the panel come on, and if the ground isn't solid, a small voltage is present on the panel. The gauges rely on the steady voltage across one half of them compared to a variable voltage caused by the variable resistor in the sending unit. If you have a digital voltmeter you can check this very easily, turn on the lights and put one probe on the gauge panel and one probe on a known good ground, the battery "-" terminal being the best. Bet you find a volt or two there and its enough to screw up the gauge reading.

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