It is 55* out and when I run the heater the clutch on ac compressor stays engaged, hot air is coming out of vents as should but don't understand why ac is on, burns more gas.
Thank you.

 

The a/c compressor run in all climate control modes (except "off") to dehumidify the air. All incoming air is first refrigerated and dehumidified, and then warmed as needed.

In '89-ish, compressor circuits were modified on *convertibles* to make the compressor run at all times even with the switch turned "off". This was for fuel cooling purposes. The fuel cooler is tied into the a/c compressor plumbing. If the compressor doesn't run the fuel cooler will not work.

Jaguar predicted that in hot weather, when fuel cooling is likely to be needed, the driver would have the top down and the mode switch turned to "off"...disabling the compressor. Thus the modified circuit.

Later convertibles had, as I recall, a compressor circuit that engaged the compressor only *if* the fuel reached x-y-z temperature.

I also think later cars had a vent or economy mode which disengaged the compressor. Not quite sure.

Cheers
DD

 

If I'm following you right in hot weather with top down should I turn fan on so ac will cool the fuel?

 

Your 1990 model year convertible should operate the compressor if with the mode switch "off".

Or maybe yours has the electric fuel temp switch. I can't remember all the change points. I'll try to look them up later.

Cheers
DD

 

Doug, that's why i depend on you. I know nothing about a Jag, my Dad gave this great car to me and I love it with all the problems.
Thank you