I am confused about how the low coolant warning lite is activated. The expansion tank has a floating magnetic ring which activates a magnetic switch which applies a logic ground to the major instrument cluster low coolant input pin. When the expansion tank is full the float will be at the top of its travel. In this position is the magnetic sensor switch activated? If so it would apply a logic ground to the instrument cluster low coolant pin, which would be normal and no warning lite. Or does the logic ground at the major instrument cluster input pin cause the low coolant level to lite??

There seems to be different answers to the above. In other words does the floating magnetic ring activate the magnetic sensor only when there is low coolant in the expansion tank and it is at the bottom of its travel? If that is the case unplugging the connector from the magnetic sensor would float the input to the major inst. cluster, which would be normal operation and no warning lite? It has to be one or the other. I am getting a low coolant warning lite and I find that if I unplug the magnetic sensor plug, I still get a low coolant lite on. If I short the plugs pins the light goes off. I have replaced the magnetic sensor which had no effect on the problem. Right now the problem looks like it could be a defective magnetic float or an intermittent shorting wire to the instrument cluster!!

 

I had this debate on the X308 forum and lost the debate LOL. The switch when the float is at the top closes the circuit (normal closed) when the level drops the switch opens and the ECM no longer see proper fluid level.

 

Long story short............. Common problem. The only real cure is to replace the whole top up tank.

 

That is my understanding of the way it works. The problem I have seems to change every time I trouble shoot it. I currently have the sensor plug disconnected and the plug contacts shorted and no longer have the low coolant lite on. I am going to monitor it for several days to see if continues to work. I am also going to do a continuity check across the switch plug with my VOM meter. That way I can move the magnetic float up and down to see where and if it engages the sensor. This test should tell me if the magnetic float is doing its job or is the problem.

 

It sounds like the magnetic floats are the major item that causes this problem.

 

Yes. I tried everything I could think of to sort mine and in the end I just replaced the tank. Touch wood nothing since.

 

I just finished running the final tests on my expansion tank, for a false low coolant display lite. The test results are as follows: the magnetic float does not cause the magnetic sensor switch to activate. I moved the float up to the top and down to the bottom of its travel and my VOM shows a constant open across the magnetic sensor pins. I do not know how the magnet is installed in the float, but it either lost its magnetic power or it fell out?? The magnetic sensor switch was tested by checking across the pins with a VOM and putting a magnet near the shaft. When the magnet was near the top of the switch the meter showed continuity(short) as I moved the magnet toward the bottom of the shaft it read an open circuit. I have two magnetic sensors and both read the same when tested.

I found two expansion tanks on Barratt USA. One sells for $119 and the other for $64. I would have opted for the cheaper one but the more expensive ones weight is 1lb 6 oz. the cheaper ones weight is 13.6 oz.. It would appear that there is a lot more beef on the expensive one, so that is the one I will order. I currently have the magnetic switch plug contacts shorted together which eliminates the low coolant warning lite display.

 

Before I ordered one I found that these tanks are for an XK8 and will not work on an XKR. The part numbers are very similar MJB4400AB and MJB4400BA. The MJB4400BA is the correct part number for the XKR.

 

This I have found, is like several little Jag specific things...my coolant tank is fine and I have enough coolant...when i choose to go around a corner fast or park on a steep hill, the coolant light goes on...then after a while turns off. I have topped off the tank, but it's not low....so I just ignor the light if I have parked on a hill,e tc....