I have a 1994 XJ81 car that has problem with the heating & defrost or demist system. When I turn the control knob to defrost I can hear a blower running but there is just no airflow at all out of the system to any of the air outlets. It’s not really safe to drive on a damp day since the inside of the windshield has to be constantly wiped with a rage or towel. One other possible symptom is that hitting the manual button on the heater/ac control panel does absolutely nothing. I am used to a small red led lighting up when this button is pushed on my other MY94 cars.
Anyone got any suggestions about what I could look at to start troubleshooting this problem?
I can only speak to the earlier XJ40s but first check to see that both fans are running.
First pull the left hand fan fuse, do you still hear a fan running. Now put the fuse back in and do the same with the right fan fuse. if one of the fans have failed you wont’t hear any fan noise when the good fan fuse was pulled. Because of the design of the system if only one fan is operating you get almost no airflow at the vents. If both fans are operating …
… then it’s most likely a problem with the demister vent assemblies. Very prone to broken linkage which keeps the vents closed. In your case both assemblies would have to have failed or you would be getting high volume air out of the good vent. Also it’s possible (but pretty rare) that both vent vacuum actuators are bad. it’s also possible that the actuators are not getting any vacuum through the line. Both actuators receive vacuum through the same line that T’s off to each actuator…
Pull the centre vent off and jam open the vacuum operated flap inside. Take a flashlight and have a look into the cavity. If a demist flap has broken (extremely common and symptoms sound like it) you will see the flap - a piece of plastic over a foot long - loose and sitting in there at an angle jamming the rotation of the blend door which directs the heat and A/C.
To remove it, use a long needle nose pliers and withdraw it out through the centre vent. Often you’ll have to cut away one of the vertical plastic struts in order to make enough space to wiggle it out, but they really don’t serve any useful purpose in my experience.
Once you get it out your system will behave pretty much normally. I say pretty much because the flap does prevent misting to a degree during A/C operation.
This is the quick “fix”.
The other thing you can do is remove the dash, remove the flap from the blend door and repair the flap hinge. I’ve done both methods and when my dash out repair failed last time I just reverted to pulling it out of the centre vent.
It’s been like that for a couple of years now and to be honest even in summer with full blast A/C I can’t say misting has been an issue. This is my daily driver BTW.