XK8 Oil Gauge reading

Hi Everyone,
My XK8 oil pressure gauge reads 1\2 scale ( ie normal pressure) as soon as the ignition is turned on and the red oil light comes on. As soon as the engine is running the oil light goes out. If I disconnect the lead to the sender unit at the oil filter, neither the light, nor gauge register, so presumably this one sender operates both the gauge and the warning light. Given that there is only one wire, how does the sender unit operate both functions individually? It used to work properly, only registering pressure on the gauge after the engine was running.
Is the problem in the sender unit?

Gauge is nothing more than a glorified warning light anyway, it’s only there to bring you comfort. You can fit a mod for true oil pressure gauge if you wish.

Does the gauge not actually read oil pressure? It used to take some time to register after starting the engine, but if it uses the same contacts or whatever to perform both functions, how come one part works and the other doesn’t? I would have thought that if the internals had failed and caused the gauge to read all the time, the same would happen to the light. Thanks for the reply, BTW.

Others may be able to better explain it but basically the gauge runs mid scale regardless of variations in oil pressure unless there is catostrophic loss of oil pressure, more than likely signaled by the same sender the light uses.
The real gauge mod for oil pressure reading as far as l know requires the fitting of an additional sender. I don’t think oil pressure is available through the OBD2 data because of the lack of a sender for pressure reading.

Absolutely correct. There is no pressure transducer as standard and the gauge is designed to sit in the middle of the scale as confidence measure, nothing else. Virtually the same is true of the coolant temperature gauge which has a huge range of temperatures where it will show mid scale.

I haven’t tried them personally but I see JagWrangler make kits to convert both gauges so that they tell something like the truth.

Jaguar clearly didn’t think the oil pressure gauge (and voltmeter) were that important since you didn’t get either if you opted for factory fitted navigation.

Unfortunately, Baxtor is correct. The gauges do not read like you would expect. Same applies to the Temperature gauge as well as the Voltmeter. You can upgrade the gauges as mentioned, or can get some info via an X60 gauge plugged into the OBDII port, or use Torque on an old smart phone also hooked to an OBDII port ELM327 sender. IMHO, the temp gauge is much more important, as high temps are a killer for these engines.
Gauge Modification
X60 Gauge

Thanks for all the replies.Thinking back, the oil pressure gauge always read mid scale, unlike older models where the gauge would read high until the oil warmed and would then settle around 40psi, so clearly you are all correct and the gauge is really there for cosmetic reasons ie make the dash look good.If the sender switch has failed, I’m not sure why the light is still working correctly however. I have a new switch on order ( pretty cheap so I don’t think they have any smarts in them)so I will see if that restores normal service and update you.
Thanks everyone.
BTW I think the voltmeter probably does actually read battery voltage as I did have electrical issues at one stage that were heralded by the voltmeter. A new battery was the cure.

Ford (and Jaguar) started this ‘oil switch’ thing about 1996 because people were complaining about the gauge being all over the place reading high or low.

There was a a campaign in the mid 1990s to replace the senders and reconfigure the INST PKs to read 1/2 scale.

TSB 15-13 was for the AJ16 and from the mid 1990 onward ALL Jaguars were fitted with SWITCHES instead of transducers.

15-13 Oil Pressure Gauge – Fluctuation.pdf (33.8 KB)

Wow, that is very interesting. Still doesn’t explain why the gauge reads as soon as the ignition is on but the light functions correctly, but we will see if a new switch fixes that. Shame the manufacturers cater to the lowest common denominator ( ie people who can’t read a gauge properly) but I guess as long as you know what the story is , all should be fine. Just don’t buy a car thinking the engine condition is good based on the oil pressure reading.
Thanks heaps for clearing that up.

Just for clarity, I may have been incorrect on the Voltmeter gauge . I was comparing readings yesterday between my X60 gauge and the voltmeter, especially when stopped at a light and all the fans, AC, etc running, and I could see a voltage drop at the gauge, and also saw a corresponding drop on the stock Voltmeter. Still learning … :thinking:

Thanks for clearing that up.

New developments! I’m still waiting for a new oil pressure switch, but meantime the issue has resolved itself. So, with ignition on but engine not running, the oil warning light is lit and the oil pressure gauge reads zero. With the engine started, as oil pressure builds ( which can take a few seconds if the engine hasn’t run for a while) the gauge starts to move and the oil light extinguishes when the gauge reads 1\4 scale deflection; it then settles at 1\2 scale during normal running.
My question is this: If both the light and gauge are controlled by the same switch with a single lead, why does the light not extinguish as soon as the gauge begins to react, not at 1\4 scale? Old type switches broke the circuit when operated, removing the earth signal, thus extinguishing the warning light. How can this switch , on one hand remove the signal to turn off the light whilst at the same time provide a signal to operate the gauge?
The only way I can see this work is if the switch MAKES a circuit to activate the gauge with a set of break contacts within the gauge to extinguish the light at the 1\4 scale point. BUT, if this is so , why, with the original fault, did the gauge read directly to 1\2 scale ( ie normal running) but the warning light behaved as normal? It should have extinguished at 1\4 scale.
Very confusing. Maybe there are voltage sensing electronics involved?

The oil pressure switch provides an input to the Major Instrument Pack. This is where both the ‘gauge’ and the light is controlled from. The behaviour of both will depend on the programming of the MIP. Whoever wrote that part of the code may have just tried to make the whole experience seem a bit more believable than it really is. I suppose if you don’t know the truth, it’s more comforting to watch the gauge come up from zero to mid-scale in an analogue sort of way and at some point between the two the light needs to go out as would have happened on vehicles with a real gauge and separate pre-determined pressure switch operating the light.

Hi Eric,
Thanks for the reply. It sounds plausible enough, but if the switch was failing, presumably it would fail to send a signal to the MIP and both the gauge and light would misbehave. This would indicate the issue may be more an electronics problem, bummer! Never mind it’s working now. Much prefer the old ways, I could understand them. Have bought a Series 1 Daimler Sovereign as a project because you can actually work on them yourself.