1991 Oldsmobile 88 heating/cooling

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 12 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's 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 this high-current circuit, a little brake light bulb will not pass sufficient current so it will be bright even when the short is gone and the fan motor is trying to run. Unplug the motor, then do this procedure. The test bulb will go out completely when you do something to remove the short.

Dandy. That starts to narrow it down by eliminating everything before the control unit. Can you unplug the fan speed switch separately? If you can, that will help you determine if the controller is shorted internally or if you need to look at the fan speed resistor.

Yup. Leave the bulb in there in place of the fuse, and keep unplugging things and moving wires until you localize what makes the bulb turn off or get dim. You've removed the short when the bulb is no longer bright.

The switch won't short. What is shorted is something in the circuitry that comes after the switch. It sounds like you need a different controller.

Yes. GM was famous for using a computer module to control the heating / AC system. Anything electronic is prone to failure. A module can short internally, although that isn't real common. If you unplug it and the test bulb goes out or dim, the short is in the module or something that comes right after it in the circuit.

A larger fuse is not the answer. Based on your description, I would unplug the fan motor, then run the controls like normal. You won't have any air flow, but if the fuse no longer blows, replace the motor. You'll find the old one spins very hard. When the bearings get tight, the motor slows down, and to get back up to speed, it has to draw higher current than normal, and that can blow fuses.

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.