Hi I have a customers XJ8 in my shop which runs perfect and the engine temperature stays exactly in the middle of the gauge until you put the air conditioning on and after approx 5 minutes the engine temperature goes into the red but the actual engine temp is normal only the gauge reads high. when you switch off the A/C the temp gauge drops back to normal immediately. the radiator fans work fine,and as I said the engine temperature does not alter and stays normal. I have never seen this before, anyone have any ideas or seen this. Thanks for any help. Cheers Harry

 

Viking, what year XJ8 are we talking about here. That may help us narrow things down for you. Sometimes it is the silliest things that can drive you batty.

 

Hi Guys, the car in question is a 2006 XJ8 and when I could not find the problem, the customer took the car to the local jaguar dealer whom are as baffled as me at this time. Thanks for the help , any answer for future symtoms would be appreciated. Cheers harry

 

Viking, based on what you are describing and looking over the wiring diagrams, I would have to venture to guess that your issue is related to the positive wire of the coolant sensor rubbing against the hot pipe of the A/C system leading to an increase in resistance of the wiring, leading to the computer seeing it as elevated coolant temps. But, when you turn off the A/C, the piping cools down fairly quickly, restoring the wiring to more normal temps, and therefore normal temp indication.

The only other thing that it could possibly be is having both the ECM indications and the A/C controls going through the same boot in the firewall and you have managed to damage the boot, leading to wiring damage and so far, all you are seeing is the indications that you are with that car.

 

One hidden potential fault on these cars related to electrical faults are the earthing points (US = grounds). There are two (or it might be three), at the front of the car, visible when you take off the panel covering the radiator at the front of the engine compartment. These are subject to salt corrosion due to their exposed position. Just undoing the securing nut can break the stud off.

Might be worth a check, but that is all my knowledge exhausted now!