Jaguar Repair: Header tnak warning light, rabbit tank, coolant level

Question
Hi Howard,
Just when I thought it was all sorted...
I had ordered a new header tank for the engine bay as the PO had fitted a VW Rabbit tank on a Heath-Robinson bracket. This led to a loss of coolant to the expansion tank but no proper vacuum to draw it back again. Today I fitted the new tank (brown plastic with pressure cap) and noted that the sensor wire had been fed to earth against the inner fender by PO as the Rabbit tank did not have a warning system. This caused the warning light to go out after a few seconds which seems to indicate the circuit/module is fine.
When connected to the terminal on the new tank though, the light stays on. I suspect the system operates by sending a signal to earth through the coolant somehow but where is the other point of the circuit? Is it made by adding a connection to earth from the bolt in the center of the tank? I can see no other reason for having a bolt and nut through the middle of the tank and would appreciate your advice on this as I feel the low level warning is vital to keeping an XK motor from getting (expensively) hot under the collar...

Answer
Hi mark,

I don't know exactly how the coolant sensor unit on the Aux fuse panel operates other then it must sense a ground through the coolant itself to the head and block. I don't believe anything in the reservoir tank has much to do with the ground. I think the center bolt is to hold the large plastic tank together under pressure. If you wanted a ground in the reservoir tank it would have been much easier to do it at or close to the sensor.

I have found several Series III Jags where the sensor wire plugs on to the spade connector. The spade connector was loose and not making good contact.

On one of my ser. III cars it gets low on coolant and the light works and comes on but just after driving a short distance the expansion of the coolant makes the coolant level come up and shut the light off. I have searched for a leak and never found one and did an extensive tests of the head gasket etc. and never found a leak. I finally found the problem in the cap. It tested at the correct pop off pressure but would not seal in a vacuum which is needed to draw the excess coolant from the overflow tank behind the left front head light and I finally spotted a over flow of coolant out on the ground from the overflow tank telling me there was no vacuum being formed in the reservoir tank to draw it back so I kept filling the over flow tank until it over flowed.

I know this don't help you with your light problem but is just one more item in the system.

I have not played with tests of the light system but I do believe that you could try by taking a open container of water and put a wire from the head to a piece of metal in the water and then put the sensor with it's wire down into the water to see if the light goes out. Then connect the sensor wire to a plain piece of metal and do it again. If the light goes out then that tells me the sensor is nothing but an electrode and not a sensor.

If nothing works I would look at the wiring of the unit plug on the Aux panel. It has four wires, White w/blue tracer is the dash light. Green is the power from Ign "On". Black is ground and white w/red tracer is the sensor.

Let me know if you want diagram of the system and I will put it up on my site for you.

Let me know,

Howard