Hi everyone, looking for a little guidance with what all I can figure is an electrical gremlin. Recently did an overhaul to the A/C on my '90 F150, converting to R134a from R12. It's got the 300 six and Mazda tranny if it makes a difference, and had factory air. I thought I was doing this "the right way" by what I was told to do in that I replaced the compressor (the clutch was toast anyway), accumulator, blew it out with nitrogen during the process, replaced o-rings and under-hood refrigerant lines, changed low pressure switch over to the 134a type, installed a high pressure switch on that side (didn't want this thing to self-destruct, it always seemed to want to run high on that side, say 300+), and installed a pusher type fan with relay (and fuse, pulled power from the starter relay so that part is separately powered from the rest of it) in front of the condenser. Was told something about the R12 condenser having tubes too large to properly "condense" with the R134a? Not really sure, but did it anyway for good measure. Pulled a good vacuum on it and charged it to 80% of the R12 charge.

This thing cooled like a dream for about 3 months, and I mean you could hang meat in the cab of the truck, thankfully over most of the summer (it's hot down here in SC!) Then, about a week ago right out of the blue, I noticed the A/C wasn't working. Checked everything which appears to be as it should be and the fan motor will still run, just nothing at the compressor, no power. First thing I checked was a fuse, and low and behold a 30A fuse was melted in the fuse box under the dash. It didn't appear to be blown (although I understand that doesn't necessarily mean that it's not), but it was unbelievably hot and deformed from the heat. It's the one in the #9 position labeled "Heater; A-C Heater" below the dash under the steering wheel. I replaced it with a new 30A fuse, and still no power at the compressor, hence still not working. The fuse got too hot to touch within about 2 minutes again, and I just pulled it out with a pair of needle-nose pliers.

I've taken a cursory look over the wires that are "easily" seen and didn't see where anything looked out of the way, but that's not to say that the issue isn't buried in the dash somewhere I suppose.

I'm pretty decent with a wrench, but beyond checking for power don't really know what I'm doing with a meter.

My questions are:

1) Do you all think I'm on the right track that there is some sort of short somewhere with in the wiring with this thing?

2) Any threads, websites or advice anyone may have that might give me a little more insight into what I need to be doing with this meter to trouble-shoot the presumed electrical issue more effectively?

3) Anyone had anything similar happen, and if so what was the outcome and/or solution?

The old girl is a secondary vehicle so no real urgency, but I'd sure like to get this thing straightened out. If for no other reason so that the old Ford doesn't whip me!

Thank you much for any help and insight you have.

All the best,
Chuck Turner

 

I know this is an old thread, but I have a burnt out F9 fuse socket too and wondered if anybody had advice. I read somewhere about bypassing it with an in line fuse, but there doesn't seem to be wires on the back side of the fuse box going into the back of the F9 position????