Smiths Oil Pressure Gauge

All, another plea for help - this time regarding the oil pressure gauge on my trialer (3.5l V8 with fuel injection from a range rover I think?). With the ignition off I get a reading of zero, with ignition on but engine not running I get full scale on the gauge. I'm thinking that the gauge is probably OK but the sender has failed? Questions is - how to I test it to prove things either way please?

On the engine picture, I'm guessing the gauge sender is the higher one, further away from the oil filter and the nearer one with the white wire is the connection for the warning light? The warning light does go off when the engine starts

Thanks in advance.

Could be the wire earthing out.

If it's an electric gauge working off the existing standard sender and electrics I guess it would mimic the oil light,
ignition on
Light On
Gauge full on.

But a standard sender is either on or off so would not give a variable current for the pressure reading so most likely you don't have a standard sender or the real sender is elsewhere.
I would check sender first four variable current output and if non. Swap it out

Flossie said:

Could be the wire earthing out.

Click to expand...

Thanks Flossie. So if I'm understanding this correctly, the gauge gets +12v from the battery and the sender unit gives a variable resistance to earth in order to give the reading? I should see differing resistance between earth and the connector of the sender unit when engine stopped and running?

pete1987RRC said:

If it's an electric gauge working off the existing standard sender and electrics I guess it would mimic the oil light,
ignition on
Light On
Gauge full on.

But a standard sender is either on or off so would not give a variable current for the pressure reading so most likely you don't have a standard sender or the real sender is elsewhere.
I would check sender first four variable current output and if non. Swap it out

Click to expand...

Thanks Pete, I think there are two senders above the oil filter with a separate wire to each, will do some tracing back to the lamp and the gauge.

bin it and get a proper mechanical one - you don't want to be second guessing oil pressure.............................

RichardIStone said:

Thanks Flossie. So if I'm understanding this correctly, the gauge gets +12v from the battery and the sender unit gives a variable resistance to earth in order to give the reading? I should see differing resistance between earth and the connector of the sender unit when engine stopped and running?

Click to expand...

Yes.

Another daft question I'm afraid - what is a "good" reading for a 3.5l V8?

Above 25 cold and around 15 hot both at tickover is alright.................................... but get an accurate gauge to be sure..........................

waldamar said:

Above 25 cold and around 15 hot both at tickover is alright.................................... but get an accurate gauge to be sure..........................

Click to expand...

Thank you