It’s pretty common with auto climate control systems to use a fan to pull air across the cabin temp sensor, looks like the XJS got one in 93ish in fact, but prior they seem to just use whatever suction the current blower fan speed is creating to move air across it. This generally results in less efficient sampling at low fan speeds. Anyone ever added a fan to pull air across that sensor or can think of any reason not to?
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Albeit I think it works fine the way it’s designed, a fan might improve the reaction time of the system.
I see no reason not to.
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The only fault that I see with the passively aspirated sensor is that it does take a while to overcome a hot soak; such as when the vehicle is fully warmed up and parked, it always blows cold air out of the upper level center vent on a hot restart unless the temp setting is increased. Really only an issue on 30-50F days.
Could just be a dirt covered sensor, hard to get to just to check it.
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if you like experimenting go for it… after a lot of things I’ve done on my xjs… I have removed various “experimental” stuff and went back to the original… too many gadgets to deal with…
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I have two 1990 V12 Jaguars with the Delanair MK III Climate Control System and in my opinion adding another fan is a solution looking for a problem, adding unnecessary complexity to an already complex system, and a waste of time.
But it is your car so have at it, but please document your modification for the next owner otherwise we will likely be hearing from him/her about why that fan isn’t working. And we will all be scratching our heads wondering “What fan?” 
Paul
Mine does the heat soak thing as well.
Otherwise, it seems to work fairly well when the fan speed is up and I assume it’s drawing air across the sensor fairly vigorously, but its response time seems slow when the fan is on low and the cabin temp is stabilized. I really noticed it again this year when on a longer drive and it’s cold out and trying to make small adjustments in temp, it takes forever for the system to respond. It does, but it’s in no hurry about it. The placement of the sensor isn’t ideal and it’s not moving a ton of air volume, based on Jaguar adding a fan to the later XJS’s and what I’ve seen other manufacturers do, it seems like an opportunity for improvement. Somewhere in my parts dept I have a spare Mercedes sample motor I might try to plumb in. Will report back when I get around to it.
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Did you try the “Manual” mode? The Delanair MK III has a manual mode if you pull out the mode selector know as detailed in the Owners Handbook.
Paul
Manual seems to work as it should.
FWIW I’ve never met an auto climate control that didn’t have quirks.
The plastic line that draws air thru the cabin sensor is barely long enough and can pull out of the “Y” in the blue blower output duct near the steering column in LHD cars. Making sure it was fully seated made the system much more responsive in my '87.
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