Hi All,

I've got an issue with my '95 X306 XJR whereby the oil pressure gauge always shows in the middle of the gauge.... and I mean ALWAYS, even with the engine off, the oil light is always off too after the bulb check.

It drops to zero when the car is powered off, but as soon as you turn the ignition on, even before engine start it's showing good oil pressure!

Now, I figured it would be a broken sender, but I unplugged the terminal from the sender just behind the oil filter and it's still reading good pressure, so I'm assuming the sender is fine (I'm assuming the sender is open circuit with no oil pressure then connects to ground once it senses pressure?). If that's the case then I can only assume that there's a short to ground somewhere in the oil pressure circuit wiring.

So far I've checked the wiring back to the connector in the kick panel and there's no earth leak I can find, there may be a fault in the short run from there to the dash but I doubt it. The other, more likely, possibility is a fault in the instrument gauge pack itself.

I've just got another gauge pack out of another car so I can plug in and test to see if it is the dash or the wiring. If it is the dash pack (the most likely scenario) does anyone know what would be the best way to swap the oil pressure gauge over into my dash? I want to keep my dash looking original and the original miles, and all of my displays/lights etc are still looking really good. Has anyone ever swapped bits between gauge clusters before? Is there a circuit diagram for the cluster out there?

Thanks in advance!

George.

 

Most likely the sensor is stuck. Disconnect it and see if the gauge goes to zero

 

What happens if you disconnect and ground the wire from the oil pressure sensor? The bulb should illuminate and the gauge should show full pressure with key to on.

Larry Louton

 

There is only one wire to the sensor, I'm guessing it geounds through the block.

I would have thought that the sensor with the engine off would be open circuit and if you disconnected the sensor the open circuit would remain and show no oil pressure even with the engine running. That would be the failsafe way.

If it is as you describe and the sensor is supposed to be grounded when there is no oil pressure then in that case it is possible the sensor has failed and the dash/wiring is okay.

 

So if you had a voltmeter, Connect the black lead to engine block and red lead to wire, You should have 12 volts with key on, 0 volts with key off. The oil pressure sensor provides the ground with the key on, Engine off. Open up your oil cap with key on and running. You should see some oil splash of sorts with engine running, If so you have oil pressure.

 

Thanks, I will check this. It may just be bad sender after all then, it never occurred to me that the switch could work the other way to what I was assuming.

 

Well it's looking like it is the pressure switch that's failed.

Does anyone happen to know the thread and the actuation pressure of the stock switch? I can order from a parts place overseas (I'm in New Zealand) but I'm sure I can find an equivalent part locally. So far google hasn't shown up any cross references to any brand I can find here.

 

It would appear this is incorrect.

I do indeed have 12V at the oil pressure sensor wire with the ignition on. However, grounding this does not appear to make any difference to what is shown on the dash, I still get normal oil pressure reading whether it is grounded or not.

It would appear that my sensor is working fine and is wired as I initially suspected (normally open). There is no earth through the oil pressure sensor to the block when the engine is off, however when the engine is running this switches to earth, telling me that I both have oil pressure and that the switch is functioning normally.

Given that I am getting 12V at the sensor but pulling this to ground does not trigger the gauge to fall or the low oil pressure warning, this would suggest that the fault is inside the instrument cluster somehow or i have a wiring fault that is backfeeding 12V into the sensor wire somehow. It is unlikely to be the wiring as I have 12V to the pin, which is supplied via the instrument cluster itself (just a single wire from pin FC9-21)

Anyone have an internal circuit diagram for the instrument cluster?

 

Hello Esprit. This is all I could find in my Jaguar schematic of Instrument cluster. Suggest first disconnect battery and touch ground cable to positive to see if hard reset will reset Instrument pack. Remove Instrument pack and disconnect cable and inspect multi wire connecter. Disconnect Orange and Slate wire at sensor and test for continuity to ground. Test Orange and Slate wire for short to 12 Volts and finally test for continuity end to end.
If you want open Instrument pack and follow Orange and Slate pin to oil pressure gauge and inspect for condition. Perhaps you could find a used Instrument pack and exchange just the circuit board for oil pressure gauge.

Larry Louton

 

Thanks Larry, that's excellent info and mirrors what I planned to do. Thankfully I have a spare cluster out of a 96 Sovereign here which I will swap over entirely to see if the fault goes away or if it remains (which will indicate whether it is a car wiring short or a fault internal to the cluster).

If the failure is cluster based I will strip down my spare cluster and try to see whether itnis possible to isolate the bits I need to repair the oil pressure circuit in my original gauge. I would rather not have to swap over the whole cluster permanently as I would like to retain my odometer.

 

The mileage is logged in the car, so swapping the cluster will not change what is displayed.

 

That's interesting, thanks, if that's accurate I might just be able to swap the circuit board between my instrument clusters as I think my original cluster is in better cosmetic condition and I think more of the LED displays work.

 

Sadly this doesn't appear to be the case. I plugged in my donor cluster and it showed the mileage of the donor car, not mine. Bugger.

It also demonstrated that the oil pressure gauge issue is definitely internal to the cluster as swapping the cluster over results in normal oil pressure gauge/light behaviour.

I disconnected the circuit board of the donor cluster and hooked it up to the car without anything else. Using a multimeter, I could tell the oil pressure warning light signal was high when the engine was off and went low when the car was started, so the circuit board seems to work independently of the gauge itself. Resistance between the oil signal pin on the cluster and ground is consistent between the two so it isn't a ground leak to the earth pins.

It would seem I can fix it by swapping over the circuit board as a whole but then it would seem that my mileage would read incorrectly.

Really need a cluster circuit diagram to go much further with it. I've tried tracing the pins on the circuit board but the tracks on the board don't all seem to be at surface level, so it's more than likely a multi layer pcb.

 

Oh, I have swapped mine without issue but I may just have been lucky …

 

There is a module in the instrument pack which deals with the recorded mileage. It can be swapped.

 

Any idea which module that is?

I've spent a couple of hours studying the circuit board and identifying the path for the oil pressure circuits. My instinct is that it is likely to be on the input side of the chip, and likely to be a bad joint/short or a bad component. It does go through a capacitor or two and a Zener diode so will focus my attention there. Going to go to a scrappy and salvage some harness connectors so I can power the cluster up on the workbench and probe voltages of my cluster and my donor cluster at various points in the circuit to try and chase the leak to ground. If I can find a ground leak I should just be able to swap the affected component between the two clusters with a soldering iron and that'll hopefully fix it.

 

Look on the Jaguar classic parts website. The part number is JLM1932. You should be able to swap it between instrument packs

 

I have recently purchased my first 1995 XJR-6 and will be with the car for the first time around 27th May so I can not provide any info as such but having recently sold my 2000 XK8 , I can confirm that the Oil Pressure , Temp Gauge were electrically jigged to sit at bang on the centre line of the gauges on this model due to early customer complaints about suspected problems with varying readings. There are after market kits available to rewire these Instruments to show the " real time true " temperature and oil pressure readings. Just throwing this into the equation but could be a million miles away.

 

Cheers, yeah im aware of that. Would be nice if it worked as an oil pressure gauge again but for now I would just be happy to have a functioning oil pressure warning again.

 

Found the fault!

That little bugger there, Resistor 148 on the circuit board is bad. Supposed to be 220 Ohms but measuring at 15M Ohms, which will just be the board leakage.



Wasn't expecting it to be a resistor, but it definitely is. This is preventing the feed voltage being fed into the sensor circuit, which is why it is always seeing zero volts and thinking that the oil pressure switch is grounded (which would mean good oil pressure).
Good news is that I won't need to cannibalise my spare dash for parts, I can buy another resistor at JayCar if I don't have one at work already and then solder it in.

Should be a pretty easy fix then from there. Assuming that the dead resistor is a cause and not a symptom.