My A/C doesn’t work but if I turn my fan on it blows hot air all the time. And my legs feel heat even when the fan is turned off. I assume that when the temp is turned down it should be pulling in outside air. I see there is a heater valve on the center firewall. I pulled the vacuum hose off of it and there is no vacuum that I can feel. Since the hose comes out of the firewall I assume that is controlled from inside. The other hose going into the firewall has good vacuum. Is there supposed to be vacuum on the heater control when the heater is on or off? I also assume there is some sort of flap in the heater box that directs air? Any suggestions on how to diagnose the hot vent problem?
Don,
Your 84 Jaguar utilizes the Delanair MKII climate control system. What you describe is a common symptom of either a failed servo / feedback potentiometer…or more common a failed control amplifier.
The amplifier monitors / compares the values from the temp selector (65-85) and the ambient and in-car temp sensors. These do not commonly fail. The amplifier then compares the position of servo via the feedback pot to determine whether to rotate for more heat or cool. The servo contains numerous micro switches to control fan speed, recirc flaps, mechanical linkages for some flaps and 2 vacuum switches ( 1 controlling the hot water valve and the other controls the center dash flap).
While it appears from your post…you have fans and heat…but are lacking control. I suggest testing to start with the sensing system.
Remove left transmission vent/cheek panel by removing 2 screws in vent and then pushing panel slightly toward front of car to disengage rear retaining clips. First check the amplifier fuse located in the black ground wire terminated to the heater case. This should be fused 1 amp fast acting. If blown…replace and test system. Do not use a fuse higher than 1 amp. Damage to some amplifiers could occur. If no joy, you then need to locate the white octagonal plug…separate this plug…it may require a small screwdriver to pry apart. You will be testing the plug with the female sockets. With a 9 volt battery apply + to red and - to purple wires. You should hear a motor whir from the right vent panel. If not, servo could be at one of its limit switches. Now reverse the connections…- to red and + to purple. You should hear a whir of the servo motor. If no whir is detected, ( testing the servo from the right side wiring connector is prudent)…then a faulty servo is suspected. If you heard the whir…we need to move to testing the feedback pot.
With a multi meter set to Ohms, connect the leads to the orange and green sockets of the female plug. Now apply 9 volts to red/purple to run servo motor. Record Ohms at both limits of servo travel. While running motor, watch meter to visualise a smooth increasing or decreasing ohms reading from approximately 0-Ohms to approximately 2.2KOhms. Actual numbers can vary slightly. Numbers out of range or a radical signal drop during travel indicates a faulty feedback pot on the servo.
If you would like me to walk you through this on the phone…send me a message by clicking on my avitar and we can set up a time.
Cheers
Gary
That’s just nuts!!!
Thank you Gary for that extensive response. I may want to take you up on your offer to walk me through it when I get some time. I looked at your web site and like the idea of having manual switches to control heating. It sounds interesting.