Hi, bought the above car two months ago, very good overall condition, 53,000 miles, two prior owners in 24 years, all fluids clean and full. When driving around town the coolant gauge reads cool, really closer to cold than the normal, middle position. I just assumed it was a bad sender unit until I drove on a 150 mile round trip to a British car show last weekend, up some very steep mountain roads that make the temp gauge in my 2008 Mercedes E350 rise above normal. On that trip, it did rise much more closer to normal. This made me think the sender may be good and maybe I have a thermostat stuck open? I have moderate skills myself and would not be adverse to replacing the thermostat(s) and sender if someone can point me to them, or suggest other things to check. The electric fan does not run all the time, in case you are wondering, and in fact I have never heard it run unless the A/C is on, so maybe that is more evidence the engine is in fact staying too cool. Thanks!

Submitted by dhall@hallandh… on Sun, 09/30/2012 - 21:03

Thanks for the advice, Doug. I checked it out with my IR thermometer today and I was getting 175-185F everywhere I checked, so it is at normal operating temp. I pulled the gauge cluster and it was a lot cleaner on the back than expected, actually, for a 25 year old car, but the connectors on the plugs as well as the mylar circuit board were brown/grey, not shiny. A couple of the prongs on the plugs also appeared to be pushed into the plug too far during assembly and did not not have any "spring" to them, so I doubt they were making contact to the mylar board at all. I went over all the contact points with 3000 grit paper and got it all shiny, gently pried all the prongs out a little so they will make positive contact, and put it all together. When I turned the key to check everything, the temp gauge rose higher after the car had been sitting for a while, than it usually does when driving! So maybe I did some good. It's a start for sure!

Submitted by dougdwyer1@com… on Sat, 09/29/2012 - 00:48

"Too cool" is a seldom heard of problem on an XJS V12. :-)

First, the gauges on these cars are notoriously flaky. I would use an infra red thermometer on the coolant rails to get an approximate idea of actual coolant temperature.

But, assuming it really IS staying too cool then, yes, replacing thermostats would be the logical first step. Make sure to get correct Jag V12 thermostats....not semi-universal replacements. They need to be the correct length, have a rear by-pass disc, and a steam hole in the mounting flange.

Make sure to properly bleed the cooling system!

If it turns out that the guage is reading too low then the usual fix is to remove the instrument cluster and clean all the contact points and connectors on the circuit board and then add a redundant ground to the cluster.

The gauge sending unit, if you determine it should be replaced, is on the RH coolant rail just aft of the thermostat housing. Single wire...green as I recall.

Cheers
DD