This is where the light bulb gets the best results. When a fuse blows, all you know is there was too much current, but you don't know why, and you often don't know when it blew. With the light bulb in the circuit in place of the fuse, the bulb limits current to a save value, and it gives you a visually indication of what's happening in the circuit right now. You can wiggle wiring harnesses, select various settings to turn things on and off, or unplug things to see what makes the bulb change between dim and bright.
The fan motor was the most likely suspect. You unplugged that, and you replaced the controller which was another good suspect. That leaves the wiring and the mode door actuators if they're electric. Most actuators in the early '90s were vacuum-operated and couldn't cause electrical problems. Unfortunately there's no easy way to locate an intermittently-shorted electric actuator. You'd have to unplug one at a time, then wait to see if the short still occurs. The problem is they're hard to get to. On most cars the heater box has to be removed, and that's a real big job.
Monday, October 26th, 2015 AT 10:09 AM