My '89 XJS has an HVAC quirk.

Recently I had the A/C fixed and she blows good and cold.
Yesterday I had her out about town. It was warm so the A/C was on 65. She blew cold from the dash vents and the foot-wells. The fan knob was on normal, so from what I can tell the system functioned normally.

This morning was a tad chilly. I turned the temp knob to full hot and the blower shut down as I expected. Shortly (about 2 minutes) thereafter it kicked back up, before the coolant temp was up. The dash and foot-well air were both ice cold. Eventually the foot-well air was good and hot. The dash air, however, stayed cold.

It seems like a stuck flap to me, but I'm not sure how easy they are to access by hand. And, from what I've read it is vacuum actuated and not electronic.

Any suggestions where I should start? I'm going to buy the Delanair III book, but that will take a little time to get to me.

 

start with the vacuum hoses. more often than not theyre the issue. i believe the one your looking for is on the passenger side firewall.

 

Very good, thanks.
I had a look briefly @ lunch. I assume it's the smaller of the two. So far everything looks in-tact. On it goes.

 

The 89 has the later of the HVAC units, the way it works is:

1) Cools air comming into the HVAC unit
2) Walms air to required temperature, this is accomplished with blend flaps driven by servo motors.
3) There is a slide control under the radio that allows hot air to the foot well and cold air to the centre vents - check the position and operation of this

 

With the slide all the way to red and the temp on full cold, air was cold from the dash and foot-wells.
With the slide all the way to red and the temp on full hot, air was cold from the dash and hot at the foot-wells.

 

Ok I suspect you have a problem with the blend flap servo motors.

To check this remove the blower fuses from each blower, this will allow you to hear the servo motors running. Turn the ignition on (do not start the car) put the AC into manual by pulling the Temperature knob out and move the temperature control from cold to hot, you should hear the motros running.

These systems can be a PITA to fault find I know mine has never worked properly - I will be converting it to a full manual system as part of my restoration.