how does voltage effect oil pressure gauge

[natwales]

I have 1988 merc 5.7 L. Can anyone tell me how the oil pressure gauge on the dash works? Would decreased voltage cause the gauge to display higher or lower oil pressure? Last season I'd get 40 - 60 psi depending upon rpms. This season its 60 -80, but with the engine off it reads 20psi so I'm thinking its most likely a corroded connection somewhere, but that would mean lower voltage = higher reading and that surprises me.

 

[Haut Medoc]

Re: how does voltage effect oil pressure gauge

It's not about voltage, it is about Ohms...(Resistance)...​

 

[natwales]

Re: how does voltage effect oil pressure gauge

are there any circumstances where increased resistance (less voltage, no?) would cause the gauge to reader higher than normal?

Or what else might cause the oil pressure gauge to read 20 psi with engine off?

 

[Alumarine]

Re: how does voltage effect oil pressure gauge

Or what else might cause the oil pressure gauge to read 20 psi with engine off?

Your oil guage itself might be off by 20 lbs.

You could hookup a manual guage to the block and find out.

 

[jclemmojoe]

Re: how does voltage effect oil pressure gauge

Keep in mind guys that most of our boats have electric gauges and non-regulated voltage to the gauges, therefore, it is possible that when you supply 16 volts as opposed to 12 volts to the gauges you could get slightly different readings. I notice a difference mostly on my water temp gage, but there'e a slight difference on oil pressure also.

Just my opinion from past experience.

 

[Bondo]

Re: how does voltage effect oil pressure gauge

Disconnect the wire from the Engine end,+ Ground it........ It should Peg.....

It should Also be at Zer0, with the wire Off the Sender.........

 

[natwales]

Re: how does voltage effect oil pressure gauge

Solved my problem, here's what I think I've observed:

Less voltage supplied to the oil pressure gauge (In this case due to the resistance of a partially corroded ground wire) causes the oil pressure gauge to read higher than normal.

Am I getting this right?

 

[Bondo]

Re: how does voltage effect oil pressure gauge

(In this case due to the resistance of a partially corroded ground wire)

Sounds like a likely Scenario to Me...........

Neglected Grounds are Quite Common...........

 

[Coors]

Re: how does voltage effect oil pressure gauge

Yep, bad ground = more ohms.

 

[whywhyzed]

Re: how does voltage effect oil pressure gauge

Solved my problem, here's what I think I've observed:

Less voltage supplied to the oil pressure gauge (In this case due to the resistance of a partially corroded ground wire) causes the oil pressure gauge to read higher than normal.

Am I getting this right?

I don't think so, but close.

Modern gauges have an internal voltage regulator that drops the 12V supply to 8V or more commonly 10V out the signal wire to the sender so that regulator cycling or lights being turned on etc. doesn't make the needle move around. The oil pressure gauge runs on such low current, that the corrosion on the supply cannot affect accuracy. It will just peg back and forth (bouncing to destruction) if there is a problem with power or ground.
Corrosion on the sender wire absolutely could cause the symptom described though.
I'll bet you bumped something on that sender wire when you "fixed" the power.

If you go back to the 1950's or 1960's before the internally regulated gauges that could be a problem.