Tell me about the fan clutch
 

The last several times I have driven the car, the fan clutch seems to be making a lot of noise. It makes this horrible squealing/screeching sound. At least I think it's the fan clutch, but I recently checked all the belts and made sure they were all nice and tight. It sounds like a squealing belt, but with more screeching, and seems to happen most when starting from a stop, especially when accelerating hard. Also happens loudly (and more easily) when gunning it in reverse.

I can't really understand how the fan clutch works from reading the manual. From other internet websites, there is some silicone in there that becomes more or less viscous with temperature??

The car starts to overheat slightly when stopped at a light sometimes, too. By overheat, I mean going past 70ºC where it's normally at.

Why is it making this screeching sound??? What should I check? Can something be adjusted on this clutch, or can the silicone be easily renewed?

Basically you have its operation sorted. The silicone stuff gets less viscous as it gets hotter, and the channels inside close off so the fan is driven faster.

I have never heard one scream??, groan or rattle, YES, but scream, NO, but I suppose anything is possible.

The front pulley has a rubber insert between its 2 pieces and they do fail, and SCREAM, like a loose fan belt. I am not sure on the XK engine, been tooooo many years now, if the water pump belt is on the outer ring of that damper???.

I do know that the V12 alternator belt is and flat batteries are common when they fail.

The incorrect section belt can have it bottom out in the "V" and that will cause a scream.

If the belts are "older" than new would be my first line of attack, and work up from there.

Going waaaaaay out on a limb, is the trans fluid full, as our S2 (same trans) screamed once, and it was 2 litres down, no idea why, but a top up stopped the noise.

This clutch starts squeaking shortly before it fails. They cannot be repaired, you just have to buy a new one. Failure is lock-up, the clutch is designed to slip some once a certain revs is reached, so it doesn't absorb so much power of the engine.

I had to replace one on my XJ6 around 1989

Check your alternator belt too... It's a little tougher to get to, being that you have to get under the car, but when it's loose it squeeks really bad when you take off...

To test the fan clutch... Just open the bonnet and give it a spin... The fan shouldn't go around more than 1 to 1.5 turns.... If it spins more freely than that, it's time for a new fan clutch. There isn't any way to adjust it, or repair the silicone.
If you think you need a new one, let me know. I think I have a couple of new ones on the shelf...

Thanks
David
Everyday XJ

The alternator belt is brand new and I tensioned it nice and tight. I will check on it.

I will also do the fan clutch test.