1989 Chevrolet S10 Blazer AC issue

Question

I have a 1989 S10 Blazer with a 4.3 L V6 engine. It has an AC that is giving me some trouble.

Observed Behavior: When i slide the mode selector to any AC mode , it blows the ac fuse.

Ive replaced the following

1. Replaced the AC Relay and the blower motor resistor. It worked for around 10 minutes of driving and then the fuse blew up. ( The weather was around 35 degree C, the engine temperature was around 100 degree C)

Also noticed the following behaviors

The fuse does not blow up if its on heater or on vent mode and the temperature is slid to heat.

This has been a yearly issue during summer and the only solution I've got is replacing the AC blower motor. But its coming to a point where I'm replacing it on a yearly basis.

I thought id check here before I go change my ac blower motor. Could I get some suggestions to help troubleshoot the root causes?

Thank you

Answer 1

Most vehicle ac systems use a diode (usually next to the ac fuse) to prevent the ac relay from internal arcing when ac is turned off. When power is removed from the ac relay, the compressor magnetic clutch creates a reverse current from the collapsing magnetic field. This current is fed back to the relay contacts and will create arcing, eventually pitting the contacts. The diode absorbs this current. At times, these diodes break down and short circuits within itself and simply shorts out the ac fuse as soon as ac is turned on. The simplest way to troubleshoot a faulty ac diode is removing it, replace the blown fuse and try ac again. If ac runs without blowing the fuse (again), the diode is faulty.