Oil Pressure Gauge

[Fred]

Fellow enthusiasts. I have a 1968 OTS that has been restored and has a new oil pump. For several years the oil pressure reading has been slowly dropping (before restoration) and now reading essentially zero, except in very cold weather at startup (10psi then). I have put a real gauge in the oil pressure sender hole and it reads 40psi at just above idle when the engine is 3/4 warmed up, increasing to near 60 at higher speeds. I bought a new sender unit and there was no change in reading with the original gauge. I bought a new gauge and sender, with still no change. So original gauge with 3 different senders still reads near zero. A new gauge reads near zero with two different "new" senders. I used a separate long wire from the sender to the gauge, bypassing the wiring loom and still reads zero. Any thoughts?

[Fred]

Forgot to add that all other gauges work properly. Thanks in advance

[bitsobrits]

Put in a mechanical gauge. I’ve seen them available to match the original style. The modern electric gauges/senders seem to be crap these days.

[mgcjag]

Fred..the electrical sender is earthed as its screwed into the houseing and sealed with a copper washer....if you use sealant or tape on the thread it may not be earthed...try earthing the sender body...but as above fitting a wet guage is the way forward...Steve

[lowact]

Fred, does the power supply to the gauge come from the voltage regulator, is it ok? What voltage do you measure at the power supply connection to the gauge?

[Fred]

Thanks for the input. I will check the voltage at the gauge and at the terminals of the voltage regulator. I'm just surprised that the oil pressure gauge (the 1968 original) reads low or zero while the other original gauges seem fine.

[Fred]

I didn't use any sealant or tape on the threads and there is a copper washer in place.

[johnetype]

The oil pressure gauge should not be connected to the output from the voltage regulator. Only the temperature gauge and fuel gauge are supplied by the voltage regulator.

The oil pressure gauge should be connected to an ignition switched +12 volts as per the wiring diagram.

The confusion often arises because the wiring loom uses the input terminal of the voltage regulator to supply the oil pressure gauge so it appears to be connected to the voltage regulator and indeed it is but to the input so it is supplied with 12 to 14 volts not 10 volts from the output of the regulator.

[Ruurd Schut]

I had similar problems, bought a mechanical gauge with long flexible tubing to be able to open the centre console. See www.classicquality.co.uk or mail tim@classicquality.co.uk

Very happy with the gauge, identical to the original one. Costs are ca. GBP 190 (shipped to the Netherlands)
A big relief, I was very afraid to ruin the completely rebuild engine as the oil pressure reading was extremely low and very unbstable.

Mounting is very straightforward, can only recommend this upgrade.

Good luck
Ruurd

[malcolm]

Yes, I had a few faulty senders which can be quite alarming! I fitted the mechanical one and was delighted - I always knew exactly what the real pressure was.

[Fred]

Thanks to all for the input. There is battery voltage to the gauge as described above. One of the sending units shows zero ohms between the terminal and the body while a second shows about 300 ohms. Wondering if anyone knows what the typical value is? Is there a typical resistance across the terminals at the back of the gauge? The second of my senders and the new gauge shows a fairly consistent 10psi above idle until fully warmed up - then less than that. Still a mystery to me. Hopefully not beating this to death.

[johnetype]

There's a good description of how the oil pressure gauge and sender work on pages P51 and 52 of the service manual.

An original sender will show infinite ohms between the contact and the body when there is no oil pressure (may not be true for some of the more modern "substitutes" but it is for the original as the drawing in the service manual shows).

Oil pressure gauge across the terminals: 25 ohms.

(For comparison:

Fuel gauge and water temperature gauge: 60 ohms

Voltmeter: 100 ohms)

[Fred]

Thanks John. i made a mistake in saying that there was zero ohms, when in fact it was infinite as you said. I will check the resistance across the terminals of the two gauges.

[Tjellvar]

Hi, I have the same problem with a new sender. Have change to a mecanical one.
The problem with a new sender is that the sender consist of a bi-metal that is bended by a wire that is open and close depending of oil pressure. A bi-metal is temperature sensitive and is heated up by the electrical wire. I think that an old sender has it's bi-metal compensated for the heating of the engin but not the new sender. My new sender show 60 psi when engin is cold and at 1500 rpm. With hot engin it was reading 10 psi.
The only solution I have was to put in a mecanical oil pressure system. I think Martin R has an instrument showing 60 PSI same as the original, but it cost.