Faulty oil gauge?

Hi all
Have a little issue with my CAV and its oil pressure gauge,

When the ignition is turned on, the needle travels the extent of the gauge from 0 to 100

Is the fault with the gauge or the sender?

Also, where is the sensor?

Thanks
Will
 
Assuming SBF engine...Sender unit is found just in front of oil filter, or may be at end of short hose if original owner mounted it remotely from that oil gallery fitting. Sender will be about 2"+ in dia if its for electric gauge. Possibly has never worked and might be sensor for oil light ( About 1.25" dia ) which could explain gauge readings your observing. Some folk like to run both oil light and gauge in which case you will need a Tee fitting to enable use of both type sender units.
 
Hi mike
I have mailed joy at cav, my car is a pre 100 car and they are not 100% sure of the method used to install the gauge/ sender

Will
 
The car has both a light and a gauge
The light goes off once the car is started

The gauge, did work fine when I purchased the car, it then started to work intermittently, the needle now just goes to the full extent of the gauge as soon as the ignition is turned on

Thanks for your help

How can I establish if it is the gauge or the sender,

Thanks
Will
 

Mike Pass

Supporter
Look round the engine and locate the sender.
Assuming it is electrically operated, ignition on and remove the wire so it is not touching anything and check the gauge reading. Then ground the wire and check the guage again. This should cause the gauge to move across the scale.
If the gauge has some serial number or makers name you might be able to look up the spec to see what input it needs to drive it. It can then be fed suitable inputs to check it’s operation.

If you have a multimeter connect it between the contact on on the sender and ground and measure the resistance with the engine stopped and then running. You can get cheap multimeters from Ebay.

Cheers
Mike
 
Hi mike

I have removed the wire on the sensor previously and the needle still traveled across the gauge to max,
I did not ground the wire though
Will try grounding it out tomorrow

On that basis, when I turn the ignition on, the needle goes to max, so

if I ground the wire our at the sensor end, and the needle drops to zero, would that indicate a sender fault
If I ground in he wire out at the sensor end and the needle remains at max, would that indicate a gauge fault?

Thanks

Will
 

Mike Pass

Supporter
Hi Will,

If the needle drops when you ground the wire at the sender end that would indicate that the sender is most likely the problem. The best way to be sure which end is at at fault would be to test them individually with a meter to see what is going on. It is a bit difficult to diagnose the issue without knowing exactly what parts you have. The best way forward is to test the resistance of the sender with a multimeter and see if it is changing with changes in oil pressure. As you will need new parts at some point have a look ( take pics) at the sender and gauge to try to find out the make, model to check on specs and find replacements. Post the pics and someone on the forum might recognise them.

You could buy a small multimeter from EBay etc which is a useful item to have around.e.g. Draper Pocket Analogue Multimeter for General and Automotive Use DC AC Voltage | eBay

Alternatively run the car round to the nearest auto electrics place as they will have all the test gear and may well stock suitable spare parts.



Cheers
Mike
 
Will, if you have a pre-100 CAV you most likely have the Smith's electric oil pressure gauge.

The Smith gauges tend to last a lot longer than the sensors. So, I would guess it's your sensor which has gone bad. As you probably know, the sensor is located just forward of the oil filter on a SBF. Sensors vary, but typically they work by varying the resistance with oil pressure....with resistance (measured in ohms) in the range of 10 ohms to up to maybe 100 ohms. I don't know what sensor you have, but that's the typical range for many.

Personally, I would just install a new sensor and see if that fixes it. The sensors are usually inexpensive and easily changed. If that doesn't fix your strange gauge reading then I would think the next possibility is a grounding or short somewhere in the wiring between the sensor and gauge. The early CAVs had a few short cuts in the wiring....as I recall, the full voltage of the lights went through the switch rather than through relays, for example. This kind of thing can cause localized hot spots and melt downs....which can affect other circuits in the loom if wrapped together.

Hope this helps. It's not a surprise the fine fellows at the factory couldn't help - it's my understanding that almost all of the early cars were shipped without engines so the engine installer would be in charge of what sensor got utilized.
 
Last edited:

Doug S.

The protoplasm may be 72, but the spirit is 32!
Lifetime Supporter
The car has both a light and a gauge
The light goes off once the car is started

The gauge, did work fine when I purchased the car, it then started to work intermittently, the needle now just goes to the full extent of the gauge as soon as the ignition is turned on

Will, I had that issue on a '65 GTO after I loaned it to my younger brother. It came back with what sounded like a rod knock but the oil pressure gauge was pegged, which would not be likely with the excessive clearances that would cause a rod knock. I had the crank ground and the rods resized, but still the gauge would go all the way to the highest reading soon as I turned on the ignition switch. I took off the sending unit and could see that there was a piece of metal of some sort wedged inside the opening on the sending unit...I guess that shorted it out, because as long as the ignition switch was "on" the gauge showed 60PSI..whether the car was running or not.

Lesson learned...make sure you have the right kind of pressure sending unit (or that the oil pressure gauge is a mechanical one) and even then don't trust it if your gauges tell you something that doesn't make sense.

Doug
 
Hi guys

Tried a few things today,
1) took the wire off the sender unit, turned the ignition on and the needle travelled the full extent of the gauge
2) did the above and grounded the wire to the sender unit to earth, the needle travelled the full extent of the gauge again

In desperation I turned the ignition on and put my hand up behind the gauge, the needle dropped from maximum to minimum

Started the engine,the oil pressure light went off and the needle went up to its usual position, about 3/4

So the issue must have been a poor connection on the back of the gauge

Thanks for all the advice guys, well chuffed it's sorted

Thanks

Will
 
Lesson learned...make sure you have the right kind of pressure sending unit (or that the oil pressure gauge is a mechanical one) and even then don't trust it if your gauges tell you something that doesn't make sense.

Doug

And maybe don't loan your car to your younger brother as well???
 

Doug S.

The protoplasm may be 72, but the spirit is 32!
Lifetime Supporter
Yep, Cliff...that never happened again!

Cheers!

Doug
 
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