I have had this problem a few times in the past but now it is on as soon as I start the car. I ordered a new sensor and installed it, but I still have the low coolant lit on. I tried shorting the sensor plug connector but still have the same problem. It appears that I have a broken wire or a bad connection at the computer module.

I think I have a wiring diagram but of course cant find it. It looks like an easy fix to splice two new wires to the computer plug from the sensor plug? The wires from the sensor plug looks like they get funneled to a wiring harness then go to the computer module. If the one of the wires are broken or have a bad ground, running new wire should fix it. If there is a corroded pin on one of these pins at the computer module, I should be able to find it if I knew where the connector is.

I think the problem is with the positive connection as when I checked the sensor plug I did get a continuity to ground on the black wire and an high resistance on the red wire. Does anyone have any ideas on fixing this problem and where the connector for them is on the computer module. Where is the computer module located?

The low coolant sensor was very easy to remove. You can get at it by not taking anything apart. You just got enough room to pry the old sensor out and then take the connector off. It doesn't take a lot of effort to pry the connector out just wiggle it around and it will pop off. I was hoping the new sensor would fix the problem, but our Jaguars always make it a challenge.

 

With the vehicle running you should see 12 volts on the power wire. If you have 12 volts then it is probably not the wire. Have you checked to be sure that the float is actually floating. They build up debris and will stick in one position.

 

The float is working fine. I assumed that when the magnetic sensor closes it applies a ground to the computer module, as one pin of the sensor plug was ground the other pin measured open. When the sensor was installed and the magnetic switch closes due to the floating magnet, it applies the ground to the module?

 

Although the float is working fine, it's likely that the magnetic rod has dropped.
you can't see or feel it, but it would trigger the sensor even though the coolant level was correct.
on the 4.2 you can force the float to the bottom with your long finger, thus sliding the rod back into place.



this would only be a temporary fix.

 

I did some troubleshooting using the wiring diagrams and have come up with the following: The diagram shows a logic ground EM1BR coming in to a splice EMS38 then to one side of the sensor EM55-1 a black/pink wire and going to through the low coolant sensor magnetic switch EM55-2 orange/green to a connector EM1-19 then to the major instrument control cluster FC26-23.

If the logic ground is applied to the instrument cluster the low coolant lite will not lite, but if the low coolant sensor is open the lite will come on. I assume the logic ground signal comes on when the ignition key is turned on. I am not sure if it is controlled by a relay or something else. When I shorted the contacts in the sensor plug EM55-1 to EM55-2 this should have applied the logic ground to the instrument module FC26-23 and turned out the lite, when I turned on the ignition switch. This did not effect the coolant lit as it still was on, along with an open bonnet warning which is normal if the bonnet is open.

I am an old retired electronics engineer and have done a lot of work with electronics, but I think I will have to take this problem to my local Jaguar dealer. I could probably wire a logic ground from another source to the instrument cluster, that would by pass the coolant sensor, but that is not a good idea. Does any one have any suggestions where the logic ground comes from? If it goes through a relay I could replace the relay which might fix the problem.

 

Thanks for the advise I did drain out the fluid from the reservoir and could see the float and the rod it runs up and down on. Everything looked OK but I was not looking for the magnetic ring that is part of the float. They must be glued in and over time could separate! I assumed that when I took the connector off of the sensor and shorted the pins and still had the problem that it was a wiring or connector problem? I will definitely double check the float just in case I over looked something. Thanks for you input.

 

Thanks for the picture. I double checked everything and apparently that is what happened to cause my problem. I have a new tank on order. I plan to open my old tank like you did so I can see where the magnetic cylinder went. I would think there would be a couple of these magnetic cylinders mounted vertically in the float? Did you notice that there were more than one that you show in your picture? When I was troubleshooting this problem I had thought I had shorted the pins on the sensor plug, but the first time I tried it I didn't haver a good short and still had the lite on. I assumed I had a wiring problem but I finally tried shorting the pins together as nothing else made sense and this time I got it right and the low coolant warning lite went off. That plug is hard to get to as the wires are short so it is easy to screw it up when trying to put a paper clip across the pins.

 

IIRC there is only one rod, it's held in by friction and with the hot/cold cycles over the years the float becomes bloated and drops the rod.
the float can only move up and down the shaft about one inch. in the photo it's at the highest point.
the sensor fits inside the shaft, so when the rod drops it presumes the float is at the bottom and triggers the low coolant light.

only solution is a new tank, but the're easy to replace.

 

I changed my mind about buying the expansion tank from Barratts. I bought it from Jaguar Parts Unlimited. The price was a lot cheaper for an OEM part, $118.00 plus $8.00 shipping. I had to horse trade on the shipping cost and got them to send it via USPS @ $8.00 instead of UPS @ $26.50.