How to test electric oil pressure gauge?

[The Veg]

During a recent drive I noticed that the oil pressure gauge was pegged at zero (and the needle was vibrating a little), but since the engine was making no evil noises and I hadn't looked at the gauge in a while, I figured that the actual oil pressure was fine and that the gauge was not functioning. I think it is related to some other electrical issues (the turn indicators stopped working too) but want to know if it's the cause or the symptom.

I tried isolating the green wire that sends power to the gauge (as well as to the indicator flasher) from the other green wires all connected to the switched fuse because that wire was causing a total voltage drop at the fuse, then jumped power directly to that wire from the hot side of the starter solenoid. The ignition switch was off, and the oil gauge needle rose to the first line. I then applied power to the wire from the sender and the needle sunk. Not sure what to make of these findings or if I've even done anything that makes sense, so is there a better way to electrically test the gauge, especially without having to remove it from the dashboard, since I can't reach the back of it?

[2cams70]

Whilst the temperature gauge is for advanced users only the oil pressure guage most definitely isn’t. Don’t run the engine any further without establishing what the actual oil pressure is.

[HCA]

Sounds like you might have a power supply issue.

Try again connecting the green wire you used to a hot point such as you did on the solenoid, but this time, PLEASE put an inline fuse in the test wire...

With the needle on the first line as you had it, start the engine. With any luck the needle will rise...

[Craven]

To test, remove connection from sensor at the sensor, measure sensor resistance to ground, with no pressure reading of around 200/250 ohms. Run engine reading should progressively reduce as pressure increases around 100 ohms with 40psi. Can’t say for sure actual resistance but you should get the idea. With ignition on check voltage of the removed connection wire, should read battery voltage.
You could also replace the sensor resistance with an appropriate fixed 100 ohm resistor.

[billwill]

You could also replace the sensor resistance with an appropriate fixed 100 ohm resistor.

I think Craven means, try that to test the gauge, not as a permanent 'fix'.

I posted this only because it is all too easy to misinterpret advice.

[Craven]

Yes very easy to read things wrong, the post starts TO TEST.

[2cams70]

The sensible thing to do at this point would be to buy yourself an oil pressure test gauge for about $40 from your local auto accessory store screw it into the oil pressure port on the block and measure your oil pressure. Do that before you continue operating the engine further. Although unlikely you could have an oil pressure relief valve stuck in the open position or something else could be wrong. Even if it is in fact the oil pressure gauge that is wrong having another gauge will enable you to compare one against the other.

[Quart Meg Miles]

Forget the oil gauge, sort the common problem with the turns indicator in the power supply. It's too much of a coincidence.

[Craven]

Is your fuel gauge working correctly, it uses the same regulated 10volt supply as the oil pressure gauge.

[john.p.clegg]

Is your fuel gauge working correctly, it uses the same regulated 10volt supply as the oil pressure gauge.

Which reminds me of a peculiar fault.... both the voltage stabiliser ( if original) and the oil pressure sender work with a vibrating bi-metallic strip , sometimes these can be affected by harmonics , causing ODD readings try feeding it with 12V to test if it plays up ( reading will not be accurate though ) .

John

[Andy8421]

For whatever reason, there seem to be lots of different +2 wiring diagrams out there. This is my understanding of how it is wired up:

One thing that was common from the diagrams was that the electric oil pressure gauge was fed from battery voltage, and not from the stablised 10v supply like the fuel gauge. This is strange, as a changing battery voltage will lead to different meter readings.

I guess that the argument was something like 'to have pressure, the engine must be running, and if the engine is running then the battery voltage will be 13.6V' If so, the wiring of the gauge is:

Switched battery supply }----------} Oil pressure gauge }---------} Oil pressure sender }--------} Earth (the engine block)

If I have got this right, going back to the OP's original post, then to test the meter, you need to ground the wire that runs between the meter and the sender, not connect it to a supply. If you connected the supply to this wire without first removing it from the sender, you might have fried the sender.

FWIW - Just an observation - using battery voltage (particularly on a car with a dynamo) to power the gauge is a pretty rubbish solution. The truth is none of the Smiths instruments are at all accurate, this would make the oil pressure gauge even less so.

[HCA]

... OP's original post, then to test the meter, you need to ground the wire that runs between the meter and the sender, not connect it to a supply. If you connected the supply to this wire without first removing it from the sender, you might have fried the sender.

.

I read it that the OP (note he also did not have indicators...) connected the green - +12V switched wire - to a hot point on the solenoid and this caused the needle to pop up to the first line...

This then indicates that what the OP had was an issue with the green feed - indicators and as Craven asked, fuel gauge plus the other items on switched 12V... He needs to sort these out, then see if it works and if not start down the sender wiring...

[billwill]

SEE: viewtopic.php?t=29442&p=195342

and;

[Craven]

Andy & Bill
At least 2 arrangements for Smiths electrical pressure gauge. I believe later arrangements use a pressure related variable resistor sensor and needs a voltage stabiliser. An earlier arrangements supply is at varying battery voltage however the sensor type used are bimetal devices that are essentially a pressure-sensitive voltage regulator integrally providing the required stability.

[billwill]

Yes, I'm aware.

Personally I prefer the system used in the 2-seat Elan, which is a REAL bourdon pressure gauge connected with a copper pipe to the actual oil.

Though I'm an Electrical/Electronic Engineer, I think that converting the pressure to use electrical signals there is just asking for reduced reliability, just more things to go wrong.