AC/heater circuit keeps blowing fuses

BSPERAN

MEMBER
1989 CHEVROLET S-10
2.8L
V6
4WD
AUTOMATIC
190,000 MILES

The AC/heater circuit will blow a fuse after about one to thirty minutes (random) just with the fan blowing. I have tried to narrow it down, unsuccessfully. The fuse blows whether I am using the AC, the heater, the defrost or just the fan. I have replaced the blower motor three times. I have replaced the HVAC blower switch. I have removed the blower motor resistor, but it tests fine. I have even removed most of the dash to examine the wiring and unwrapped the bundle leading to the resistor. Nothing burned and nothing out of the ordinary. On a mechanic's recommendation, I tried detaching the wire harness going to the AC compressor, but it will still blow a fuse. I have gone through three or so packages of fuses by now. I am at a complete loss and about to give up.

Interestingly, these problems started shortly after I had a mechanic fix my AC. (It would not cool. If I remember correctly, the compressor was replaced.)

1 Reply

CARADIODOC

MECHANIC
33,916 POSTS

A simple trick to finding a short is to replace the blown fuse with a pair of spade terminals, then use small jumper wires to connect them to a twelve volt light bulb. A brake light bulb works well. When the circuit is live and the short is present, the bulb will be full brightness and hot so be sure it is not laying on the carpet or against a plastic door panel. Now you can unplug electrical connectors and move things around to see what makes the short go away. When it does, the bulb will get dim or go out.

For intermittent problems like yours the bulb may be dim already. Watch what takes place when it gets bright. That is when the short is occurring. It could be due to the rocking of the engine when you shift between reverse and drive. It could be due to the body flexing when you drive over bumps in the road. The bulb limits current to a safe value when the short occurs, in this case about one amp. If the circuit will not work with a brake light in the circuit, try a headlight bulb. The low beam filament will limit current to five amps, and the high beam filament will allow about six amps to flow.