A/C keeps blowing fuse

[grotflo]

Hopefully there’s a simple explanation for this. When I turn my A/C blower to high, it pops the in-line 15 amp fuse. Low fan works, but it isn’t a steady speed and it sounds like there’s something wrong. 

 

I currently have it hooked up to the fuse box at the following location (green wire with blue plastic visible):

 

 

Maybe that’s the wrong place? I also have a few other terminals open on the opposite  side (toward the front of the car):

 

 

FWIW, the car is a 1973. So maybe it’s caused by being hooked up to the wrong terminal, or maybe a bad ground? Anything else might cause this?

 

 

[Simeon]

Sounds like you could have short in the wiring. When running on the lower speeds the resistors used to reduce speed would keep the current lower but when the fan motor is directly connected it blows the fuse. Could be a short in the motor windings. Can you test the resistance of the motor with a multimeter?

[grotflo]

I can do that later today.  Is there a range of acceptable for resistance?  

 

And back to my original question, is it hooked up to the right terminal in the fuse box?  Does it matter?  My electric fan for the condenser is wired directly to the battery - should this also be?

[Stevenc22]

I agree there is bad wiring somewhere or a bad rheostat switch. I didnt even wire my AC with its own fuse, i just tapped power off the radio and have had no problems. 

[Mike Self]

The way to figure out what's causing a fuse to blow is to disconnect all the circuits served by the fuse, one by one until you find the problem child.  

 

You also need to look at what else is on that circuit; if there's another large current draw that is in use most or all the time, you may be pulling more than 15 amps.

 

Most A/C units I've seen have an in-line fuse for the blower and compressor; I think the Behr one is the only system that's tied into an existing fuse.

 

mike

[grotflo]

Thanks all.  Looks like I have some homework to find the culprit.  I'll also see if there's anything else pulling a lot of current on that fuse.  

[grotflo]

Update: I moved the connector to a fuse that’s isn’t drawing load (cigar lighter), then moved the ground to a more amenable location. Still blows the 15 amp in-line fuse on high fan. The blower motor has three wires coming off it - red, black, and orange. Orange controls the fan speed. Resistance between the red and black (which I assume is how I would measure resistance of the motor) is 0.6 ohm. Is that normal? How would I check to make sure the rheostat is good?

[Schnellvintage]

How current (amps) is the blower motor drawing, while it’s powered? If it has worn bushings/bearings (excess drag) and amps being drawn exceed the fuse rating it would blow fuses also. Does the blower motor fan move or spin freely by hand? Just my 2 pennies worth.


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[grotflo]

Yeah that makes sense. But the fan seems to spin freely. I don’t see anything glaringly obvious. It’s strange - I installed this A/C last year and it worked fine until recently. Not sure what would suddenly cause this. 

[Tsingtao_1903]

Do you have access to a bench top power supply capable of putting out 15A at 12V?  Set the power supply to 12V.  Hook up the fan speed controller and the motor to the power supply.  Turn up the speed and see what is the current draw at a given speed.

[grotflo]

Thanks for the suggestion. As I was reading over these comments, it seemed that for whatever reason, the motor was drawing more current than it should. That would indicate extra resistance somewhere. So I took a close look at the motor, and the brushes has a bunch of thick, carbony dust. I cleaned everything out and it seems to have fixed the problem, knock on wood. Thanks to everyone who helped me think about this. 

[jgerock]

1 hour ago, grotflo said:

Thanks for the suggestion. As I was reading over these comments, it seemed that for whatever reason, the motor was drawing more current than it should. That would indicate extra resistance somewhere. So I took a close look at the motor, and the brushes has a bunch of thick, carbony dust. I cleaned everything out and it seems to have fixed the problem, knock on wood. Thanks to everyone who helped me think about this. 

Does your car have the relay for the compressor and another for the blower?

[grotflo]

It has two connectors that go to the fuse box, if that’s what you mean. One for the blower, one for the compressor. Its an aftermarket evap/blower called the Mini-Kooler, not a Behr or Frigiking. Is that important that is has two relays?

[grotflo]

Looks like I spoke too soon. It’s on the fritz again. I’ll try what Tsingtao_1903 suggested. I sure hope it’s not the motor that’s bad - getting that unit installed in the glove box was not easy. 

[grotflo]

jgerock, this is how it's currently wired up.  Maybe I did something wrong?  

 

 

One thing that's not in this diagram is that I have a manual override switch for the fan.  It never kicks on automatically because I'm using R12a.  The high side pressure has to reach 255psi for the trinary switch to engage the fan, but R12a runs at a lower pressure so it never gets that high.

 

I think I understand the basics of A/C, but if I'm off base on something here I'd love to know.

 

 

Edited July 1, 2018 by grotflo