Checking oil pressure gage

[Eric S]

My wife has a 1969 US specs MGC since 2014
The oil pressure gage never worked. Actual oil pressure is OK as we never had a problem.
A friend, now gone, once told me that I should check the instrument first. To do so he advised to ground (providing I remember correctly) the wire where it connect the engine but I am not sure how to do it. Shall I leave the wire connected and bring a grounded wire to it or disconnect and put the end on any ground. I assume both should work.
After this test I'll know if the instrument need repair/replacement or if I need to replace the connector in the engine...

[simplyconnected]

Do you have a Shop Manual for your MGC?
Any information I would have pertains to Ford cars and trucks. Many times, Europeans do things differently.

[Eric S]

Yes I do. I assume you're looking after the electric diagram?
I asked as I thought any instrument would be tested the same!?

[simplyconnected]

Yes I do. I assume you're looking after the electric diagram?
I asked as I thought any instrument would be tested the same!?

Let's see your wiring diagram. Is your MGC 12-volt, negative ground? They can be very different, even within Ford cars. For example, 1955 Ford cars and trucks had 6-volt, Positive Ground, 1956 Fords had 12-volt Negative Ground, but 1957 Fords used a CVR on the Fuel and Temp gauges while the oil indication was simply a 12-volt light.

The oil pressure gage never worked. Actual oil pressure is OK as we never had a problem.

Put a mechanical gauge on the engine to check pressure. Just because you haven't noticed problems doesn't mean your pressure is ok. Don't want to wait until you notice there are oil pressure problems, - Dave

[Eric S]

It's a 12V positive ground.
This afternoon I pulled the wire from the pressure sending unit, grounded it and the instrument went up.
The unit has 2 post to plug in a wire, maybe the wire is on the wrong one. Can't check for now as head is out to the shop for checking and surfacing after a head gasket failure.
So either it's the wrong post or the unit failed.
I will ask on MGC Facebook page.
Just in case I am posting the wiring diagram.
Is a mechanical gage more reliable than an electric one?
WIRING DIAGRAM (later cars).jpg

[simplyconnected]

Your diagram shows TWO batteries connected in series and it shows your car is NEGATIVE ground. It also labels the components with numbers but there is no key.

I found another 'American' MGC' diagram that makes more sense because it shows your fuel, temp, and oil gauges:

C:\cmgc\public_html\techtips\mgb\pdf\wiring_schematics_MGB_M GC_V8.pdf (chicagolandmgclub.com)

This diagram shows this:

image.png

This setup is similar to the way Ford wires theirs.
If I were you, I would MEASURE your sending unit with a volt meter when the engine is running. If the sending unit passes zero power, you probably have a bad sending unit.
If your Temp and Fuel gauges are running, then the Gauge Voltage Stabilizer is good. Since all the gauges are wired in parallel, you can swap sending unit wires between gauges just to prove your gauges to be good.
Since you grounded your oil gauge and the gauge moved, I would replace the Oil Pressure sending unit.
If you find no power at the sending unit, check the gauge. - Dave

[Eric S]

my mistake it's NEGATIVE ground.
So the sending unit is on the ground wire from the instruments (arrow on the diagram above)?

But why I should find "power" at the sending unit then if it's on the ground wire?
As I said, there is 2 posts on the sending unit and just one is used. I see from other pictures on the web that only 1 post is used. Can it be the wrong post used?

IMG_20230429_165649_305.jpg

[simplyconnected]

With a meter, check if one post is already at ground potential.
This is a guessing game again... If one post is grounded, it will make your gauge stay at full scale. The other post will make your gauge go up/down according to the engine's oil pressure.

My knowledge of European cars is limited but 'electricity' is the same all over the world. I see your oil pressure switch for sale (here) for under ten bucks. Get one. - Dave