oil pressure guage over optimistic - why?

[Shearwater]

with ignition 'on', o.p.g needle goes to maximum and stays there regardless of engine on, off, idle or revving so I guess a wiring problem, especially as I have renewed most of the loom and do not know if the guage was working properly before I started the work. There are three terminals at the back of the guage (plus lamp). Is there any point in changing the cables around? Can I do any harm? Many thanks in advance.

 

[steve28]

you should have pos neg and signal, most gauges read zero with just the pos and neg and the signal is what creates the variation.
try disconecting the signal cable to the sensor.

 

[FlorealIII]

The most common arangement is this: Power is switched to the + side of the gauge by the instrument switch. A wire connects from the other gauge terminal to the sensor at the engine. With oil pressure the sensor will vary its internal resistance and allow current to flow to ground. The meter will point to the corresponding PSI on its scale. If you very briefly connect a test light from the second gauge terminal to ground the meter should deflect : touch only very briefly! If you change the cables around you can only connect the battery + over the sensor to the ground. No harm done.

 

[Marsupial]

All the above are correct. BUT where was the guage connected before you started? Some car type guages run on a reduced voltage even though they a re rated at 12v. There would have been a small component that "trembles" thus supplying 12v for only a fraction of the time there fore delivering 7v (ish).

These guages under normal running would go full deflection at switch on then slowly fall back to a reading. It sounds like yours goes full (12) but never falls back (PERHAPS).

Have a good look at the old arrangement.

 

[sailinglegend420]

Had the same problem on my Yanmar 4JH3E. After changing the pressure sensor it turned out to be the damper that the pressure sensor plugs into. It had blocked up with gunge and just needed cleaning.