Jaguar Repair: A/C system blowing fuses, poor conductor, fuse box

Question
Hi Howard, I'm trying to identify an electrical problem I have with my 1983 XJ6. Whenever I turn the climate control fan on the 50amp fuse for A/C - Blower gets incredibly hot and either blows or melts the black plastic around the fuses then blows. I now have quite a mess of melted plastic around fuse 3. The car is Australian and the fuse box I'm talking about is on the right side beneath the steering wheel. When troubleshooting I see that this is often caused by a faulty clutch on the compressor so I took the only wires I could see attached (2 wires going into side near clutch area) to the compressor off then started the car and ran the fan again. The fuse continues to get hot, much faster when fan is on high than on low so to avoid further melting damage I stop there. If I keep the car running and turn the fan speed switch to off the fuse cools down again.
Are you able to offer any guidance to help identify the cause? I've also read the Thermal Switch should be checked but I can't find that - the service manual says it's behind a valance?? where do I find it if this model has this switch?
As far as I know the compressor is the original part, it says ApcoAir 50-007 on it. I tried touching the wire coming from the clutch/compressor directly to the + on the battery in an attempt to hear the clutch engage but only got arcing on the battery terminal and no audible noise from the compressor. My goal is to get the A/C working properly again.

Thanks in advance, Brendan

Answer
Hi Brendan,
All the info that I have is for the US version. However, I believe most should be the same as the Euro and Australian versions. However, I don't believe your problem has anything to do with which version it is. Any and all times a circuit or a connector gets hot it is 100% due to high resistance. That narrows it down to either an excess amount of current going through or a normal amount of current going through a poor conductor.

Even though the two fans do draw a high amount of current they should not cause heat at any point in the circuit. The fan speeds are controlled by a resistor pack mounted in the heater/AC box on the left side up high. My diagram show no thermal switch in the fan circuit.

If it was my car, knowing what you have already experienced, I would check the function of each fan one at a time and measure either the amp draw or use a volt meter and note the voltage drop across the circuit. I don't have what the amp draw is but checking both fans should tell you if you have a failing fan because it would be very unusual for two fans to fail at the same time in the same manner. The 12 pin relay on the lower left side of the heater/AC box is the control of the fans so with a wiring diagram you should be able to pull each fan wire off and test each fan one at a time to see if a fan is failing and drawing too much current. Or do a voltage drop test across the two wires of each fan one at a time to see that they are about the same and sound like they each run fast connected direct to a power source.

The other possible cause is that the connections on and inside the fuse box have failed to make sufficient contact to conduct high amounts of current and thus the heat. Any and all times a high amount of current that is conducted through a circuit, the weakest point in the circuit will heat up unless all points of the circuit are good conductors and sufficient to handle the amount of current. (amps)

You were correct in testing the compressor clutch as a cause by removing it from the circuit. Especially since this system runs the compressor at all times that either AC or heat is on.
Howard