This question is about my '72 442 with factory AC. When I bought the car the fan didn't work on all speeds, but I replaced the switch, high speed fan relay, and also the inline fuse for the high speed setting and for the last year or so all fan speeds have worked.
Saturday I washed the car and went for a "dry off" drive with the AC on (it was near 100 degrees) and the fan on speed 2 or 3. The AC fuse blew after a few minutes, so bought a new fuse and installed it, but it happened again a few minutes later. I drove home, changed the fuse one more time, and put the car away to do some research.
I took the car out again yesterday with the AC on various fan speeds and had no problem.
I suspect that I may have gotten water on the blower motor and/or some other part of the wiring, although I didn't spray any water under the hood. I did spray off the wheel wells because there was some dirt built up from a prior trip.
Is it possible that water got into the blower motor or other wiring and then dried after some time?
I've done some reading of other threads on the forum and I also suspect that the low speed resistor may be the problem. Is there a way to check it or should I just replace it?
Thanks,
Jeff

 

I don't see how getting any of the parts wet could cause a fuse to blow, and I also don't see how any problem with the resistor block could cause a fuse to blow.

I'm afraid you may need to troubleshoot the whole system, from one end to the other, to find the short, unless it never does it again, which is also possible.

- Eric

 

Look for bad connections to the blower motor starting under the hood at the motor and the resistor (on the AC box under the hood). Pull the connectors and check for evidence of arcing, pitting, or burns. Check the motor ground wire, too.
Fast intermittent connections to the motor will cause the fuse to blow.
Also check the connections to the compressor clutch.
Heck, pull the resistor to make sure it is not trying to come apart.
If they are all okay, check at the master control switch inside (separate from the blower switch).

If all else fails, you may need to do a current draw test on the blower motor. 10-12A running on high is normal; 50A peak on startup to high speed.

 

An excellent point, and one that I had not thought of: A repetitive fast intermittent with a high enough frequency could cause a 25-30A draw easily, and overwhelm the "slow-blow" quality of the fuse.

- Eric

 

These fuses are regular fast blow. The slow blow ones have the little resistor thingie inside. Even fast blow fuses need quite a bit more than their rating for a certain time to blow.

 

I know that it isn't an official slow-blow fuse, but, as you say, nearly all fuses are designed with a certain amount of pulse tolerance above their rating, for starting motors and similar transient loads, but thank you for the clarification.

- Eric

 

Guys, would any of the ground wires be in need of checking and cleaning? Especially the copper ground from the engine to the firewall?

 

It's always a good idea to clean all the grounds, and the one from the engine to the firewall is very important.

There is also a short ground wire from the blower motor to the firewall, and it is always a good idea to clean that connection whenever you are in the area.

- Eric