So after a coolant drain and refill I'm getting the red light - coolant level low.
Not the end of the world but 1) its annoying, 2) if fluid does drop below safe level due to a leak I would not know any different.
This had been documented elsewhere so I'm posting this as additional input.
After cleaning connectors no joy.
Unplugged and jumped harness connector using some insulated electrical wire (about 3 inches, take off 1/4 inch plastic sleeve each end - loop each end into harness plug sockets ) result - no coolant low warning.
Conclusion - faulty sensor or stuck float, but not wiring problem.
Decided to remove expansion tank and have a play:
- Remove sensor plug socket - squeeze lower retaining wire upwards and pull out
- Drain expansion tank using large syringe. You can partially drain the coolant system via the radiator lower hose, but this still leaves fluid up to the level of the bottom of the expansion tank so no real gain by doing this.
- Remove 3 small bleed hoses from expansion tank using pliers and flat blade screwdriver on the quick connects, make a mental note of where they all go
- Detach sensor from bottom of expansion tank, this should just pull and snap out (this makes the bottom hose removal easier as its got one of those user friendly Jaguar clamps)
- Remove 2 tank retaining nuts (10mm socket - they're not tight)

- Clamp and lower bottom hose clip about 2 inches over the hose with pliers, twist and pull tank to free it from bottom hose
- plug lower hose outlet with a rag to avoid spillage - the fluid will be up to the top of here
So with the tank out determine whether the inner float is free. It should slide up and down the spindle when tilting the tank up and down - you will here it move if it does. Its very difficult to see it with fluid in the tank...
If stuck use a suitable implement to drag the (white) float up and down the spindle.
I plugged the bottom outlet tube with tape and filled the tank with hot water and standard kettle descaler, let stand for an hour or two, after which it moved freely. Any c**p in the cooling system will tend to settle partially in the expansion tank, eventually clogging the float movement.
The external sensor spindle can also become scaled up so I soaked in descaler also after cleaning with fine wet & dry paper.
So now the float moves freely, to test the sensor output :
Reconnect sensor to bottom of (empty) tank
Using standard multimeter take mA readings : + probe on rh sensor blade with tank in upright position.
With tank in upright position (float in "empty" position) meter should register a positive reading (sorry I didnt note the figure but its not important).
With the tank in the upside down position (ie float in "full" position) reading should be zero. Any different then I would suggest the sensor is faulty.
The internal float has a small spindle magnet within which determines the signal sent to the sensor.
Hope this helps save a bit of time for anyone with similar problems.

AndyP

 

I would love someone, somewhere to post either a picture of these components or a diagram. Once you have this fault it is difficult to envisage what is the correct thing to look for, particularly with this 'swimmer' in the header tank. I think my swimmer has somehow come detached or something, but not sure what I am looking for without doing the complete strip down. If I was at home it would be simple, but working out of the country, facilities are low and my only option is to buy a new tank and sensor, which is the expensive option and I think the tank would need to be shipped from UK.

 

There's a huge thread on this - eventually it turns out the float loses buoyancy and sinks in cold coolant then floats again as the engine warms up.
If you get that then add buoyancy to the float.
BTW meter should be on 'diode' or 'continuity' for your test.

 

Heres an excellent analytical article which shows the components very well :

JagRepair.com - Jaguar Repair Information Resource

With the tank out of the car it's quite easy to see the float mechanism inside with a torch . The donut shaped float should have the small magnet within and should move freely up and down its spindle (you can move it manually with a plastic spoon or similar Jaguar tool). It should send a signal to the sensor depending on its position. Logic should determine what components are malfunctioning with the testing described. The sensor should be removed from the bottom of the tank and thoroughly cleaned (as stated I used 1200 wet & dry paper and descaler) prior to testing. The channel that the sensor "pole" sits should also be cleaned.
Either the float, sensor or wiring will be the cause of any problems.

 

Wow, thanks Kyle04,
first impression is that the float is bigger than I thought and I dont feel it with my fingers so maybe it is out of reach or has broken up?
I must pull out some fluid and have a look and today I bought a small pump to draw out some coolant. My other concern was that if the magnet had fallen out of the float where would it be, but it can only lodge in the tank.
Probably I will need a new tank but I also want to learn about the car as I love it.
My only other problem is an air bag sensor in the headlining I am told has stopped functioning (just above the mirror). Maybe it is easy to replace, we will see.
Thanks again for the link to the pictures.