Hi,

'96 XJS 4.0 six.

I was wondering if there was an out of car test I can do to see if my temperature sensor(s) are working right.

Whole story: A year ago I replaced the thermostat with one from a chain parts store to try and fix a chronic but minor hot running problem. Didn't really get to test it properly afterwards. Last fall I started driving the car regularly, now that I live in CA again. Shortly afterwards I noticed that the temperature reads low. Like VERY low. But I figured better low than high. So I drove like that for a few thousand miles until May. Then I started to get a code P0116. Says that's either bad temperature sensor or thermostat stuck open. Figured that it might be the thermostat, because replacement parts are excrement, and I've also been having a problem with little or no heat from the AC. So I got a more expensive Waxstat thermostat mail-order from SNG Barret, and put that in. But I haven't refilled the coolant, and if there's a test I can do that involves taking the sensor(s) out I should do that now, because later I'd have to drain the system again, which is a special pain in these cars as you all know. (I keep saying sensor(s) because it looks like there are two in the engine side of the thermostat housing).

Alternatively, how can I test the old thermostat? I tried boiling it on the stove (don't tell my wife) and I couldn't see anything moving. Does that mean it's bad? What was I supposed to look for?

Thanks!

-mB

 

There are two sensors. One is for the dash gauge, the other is for the ECU. For the thermostat if you put it in a pan of boiling water you should see the plunger move and the disk open slightly - it's not a huge amount. Similarly, when cold it should seal fairly tightly.

To test the ECU sensor, simply measure the resistance across the two terminals. The attached chart shows the curve. It's a bit hard to read, but at 100C, the resistance should be around 200 ohms, at 20C about 4000 ohms.

 

It should expand to 43mm? When boiled to 90c. Dont quote me but ya if a Tstat doesn't move at all that isn't good, Jaguar cars must have the correct length and air bleed pin at the top, sensors can be hit and miss my SNG replacement sensor read 15% higher then the original. Better to buy a digital gauge like autometer and simply train yourself to remember what the barrel gauge equals in the digital readout.

Alternatively my mechanic has an aircraft mechanic friend who can tune the Jaguar barrel gauges to perfection...at great cost of course.

 

Hi there-

both the OEM thermostat and the ECU temperature sensor are available new from jaguarclassicparts in the U.K. at a very reasonable cost. If your units are original, I would personally replace them and rule that out