Prior to 1989(ish) I think the e-fan for all V12 XJS variants were designed were designed to come on with the a/c compressor, or by coolant temperature, and allowed after-cooling.

Beginning in 1989(ish) the tie-in to the compressor circuit was eliminated...at least on some variants for some period of time. This was about the time the compressor circuits were changed, being different for coupes versus convertibles.

The wiring diagram for your particular year and model should reveal the answer...or at least a clue. I'm sure that, at minimum, it has a coolant temperature trigger....which for some reason I want to say is 94ºC but I can't swear to it.

Cheers
DD

 

Other thing to think of is there is a tendency for debris to collect in-between the condensor and radiator. It's only visible when the radiator is removed. I have pulled out rads and found them to be at least 1/3 blocked.

If the radiator has never been out on your car I'd start there and make sure the rad and condenser is clean before changing the fans.

The PO of my V12 obviously liked to go to the beach, as my radiator was plugged with sand. I took it out and held it up to the sun and I couldn't see anything through it. After cleaning (where I must have got 2 lbs of sand from the fins!) the car ran nice and cool and the temp gauge didn't budge no matter what. Previously it went into the read on a hot day.


I second this post. Clean the radiator out as a first, not last, resort. Then work backwards from there. Radiator cap replacement, new coolant.
I do suggest replacing the original white plastic fan with an aftermarket black plastic version. British plastics are not very good. An exploding fan will do immeasurable harm under the hood.
I still wish there was some kind of vents in the hood to let the excessive heat out. Maybe something like the hood vents on a 80's era Nisson 280/300ZX

 

Brad you wouldn’t be concerned about water getting into the engine bay? Maybe a recessed subtle upper fender vent with some ducting would work nicely

 

Actually the 70's era Pontiac Trans Am/Chevy Camaro Z/28 fender vents like you illustrated would be ideal IMO.


If money and space were not an issue I would get spare fenders and a hood to experiment with.


Keeping the V-12 cool is absolutely vital as we all agree. I just wish the designers had given more consideration to the weather realities of the US.


Cleaning the space between the radiator and A/C condenser is the first step though. No fan is going to be able to compensate for the lack of air flow over the radiator.

 

I will let you make of this what you will. Since, on Grant's advice I ditched the radiator foams surrounds and removed all obstructions to a clean airflow, not only has my cooling been better, and the engine bay run cooler, but no debris has collected between the radiator stacks.
Greg

 

The XJ220 was designed and built by a few blokes in a back shed; never heard of any over heating problems with that.
The XJS had a run of 21 years and when they left the factory floor they had been tweaked and modified to sort the problems out.
The only reason that an XJS can over heat is because of lack of care by previous owners.
Put it back to factory spec and it won't overheat.
I'm on my 5th XJS and if you add my old V12 LWB XJ12 I have never had an over heating problem, ever.
It's not rocket science.
I can say that safely because her indoors works for a rocket scientist.