As Bob suggested, put in a bigger fuse. If the normal current draw for
your fan is 20 to 25 amps, and you've got 12 gauge wiring running to it,
then you can put in a 40 amp fuse without too much problem - if you
ever have a short, the current will go way above that and blow the fuse
quickly, before the wiring has a chance to get hot.
What you can do is turn on the AC, then use a voltmeter to measure the
voltage drop across the fuse. If you're getting a 0.5 volt drop across
the fuse, then assuming you are pulling 20 amps, you are generating 10
watts of heat in the fuse.
One last note: When the engine is running, the car's electrical power is
supplied by the alternator. If you really wanted to isolate the AC
demands from the Lucas wiring, then you should have run your 12 gauge
wire directly to the alternator output. As it is, you're asking the
alternator output wire, the connections at the solenoid, and the battery
cable to supply an extra 25 amps whenever you turn on the AC. Also, for
a 25 amp sustained load, I think 12 gauge is a little skimpy in a street
vehicle. 10 gauge would be a better choice.
Best regards,
Theo
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