Hi all,

I just realised my oil pressure switch is on the fritz. Even when the engine is off I get no oil light after the bulb test and the gauge sits on the normal mark with the engine off.

I can order a new one in from the UK or US, but would rather get one locally if I can.

Does anyone know the thread and the actuation pressure for the switch so I can try and match it with a local part?

 

The oil pressure gauge on these cars is a so called "idiot" gauge, designed to show "normal" oil pressure on the gauge when the engine is running provided the oil pressure falls within a predetermined and wide range of values. The reason the gauge stays at normal pressure whether at idle or 100mph is down to this rather than some fantastically effective oil pressure relief valve. To retain this "feature / functionality", it is important that the sender is a precise match for the original transducer used by Jaguar. These cost around £35 in the UK, although I appreciate you will face postage costs etc too, and I suspect it is the best way forward.

 

I think you misunderstood, I know it is just a pressure switch, but the switch doesn't seem to be opening even when the engine is off, so it has failed.

 

I've just looked into this further and the pressure switch seems to be working fine. If I unplug the wire from the sender, the gauge is still reading normal oil pressure and no oil light, even with the sender switch unplugged and the engine off!

Next question, I'm assuming there should be 12V on the feed wire to the sender, and the sender switch pulls this to ground to indicate oil pressure? I will try testing tomorrow, but I am assuming that somewhere along the line between the gauge and the sender switch there is a short to ground.

My car is a later one that had the switch in place of the transducer from factory, so there is no adaptor harness that I can see, just a single wire out of the loom into the back of the sender.

This could be a tricky one to chase down, I'm hoping it's solvable. First port of call will probably be to try to pull the instrument pack out and de pin the wire to the sender to ascertain whether the fault is internal to the instrument pack or whether it is in the line itself.

In the end, it's not a major, who knows how long the car has been driving like that (I've only recently got the car back on the road in my ownership), and these cars don't tend to suffer low oil pressure commonly at all from what I've read.

 

I've pulled the wiring diagram up and I've managed to trace the harness connectors and I've gone all the way up to the FC6 connector under the dash, which is the last one before the instrument pack.

I can't get continuity between ground and the relevant pin on that connector which means there seems to be no ground short between that connector and the sender. Looks like it's either in the short run between that connector and the connector into the back of the instrument pack or it's inside the instrument pack itself. I'm leaning towards the instrument pack at this rate on balance of probabilities

I guess my next two questions are:

1, how easy is it to remove the instrument pack for testing
2, Is there a circuit diagram for the instrument pack internals?

 

Hi Esprit,

Sorry to join your thread late.

It's easy to remove the instrument cluster. I think the instructions are in the Workshop Manual, which you can download here:

Jaguar X300 Workshop Manual

The procedure is very similar to that in the XJ40, and you can find those instructions in the Haynes manual available here:

Haynes Jaguar XJ40 Repair Manual

I am not aware of a schematic that shows the instrument cluster internal circuitry.

A common problem is that oil contaminates the threads of the oil pressure switch and interrupts the ground path for the switch through the engine block. It might be worth removing your switch, cleaning the threads in the block with a battery cable terminal brush, and cleaning the threads of the switch with a small wire brush. Flush both with electronic contact cleaner and allow to dry, then reinstall the switch with no thread locker or sealant, and test.

Cheers,

Don