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.

The light I put in place of the fuse went out when I disconnected the control panel at the on/off switch. Does this mean I should replace the control panel itself or keep troubleshooting the wiring? In checking the wiring visually, I can't find any breaks or exposures.

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.

The blower speed switch is separate from the controller. While testing with the fuse bypassed to find the short, one of the connectors I disconnected was for the blower motor resistor. When I pulled that connector, the light remained on leading me to believe that the resistor is okay. I also disconnected the blower relay and the light stayed on for that as well. I didn't pull the connector for the speed switch. Should I bypass the fuse again and test the fan speed switch?

Thank you

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.

I tested the rest of the switches and they were ok. The only switch that made the bulb go out is the on/off switch of the heat/ac. Do I need to do more troubleshooting or can I assume it is the on/off switch? If it is the switch, do I need to replace the entire control panel or just the switch?
Thank you

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.

I don't know if I understand what you mean by controller. Would that be the panel that has the defrost switch, fan speed switch and slider switch that turns the heat and ac on and off?

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.

I got a working a/c control switch and panel installed, put in a good fuse, turned the car on, slid the switch from off to max and the blower started working, but only for about 20 seconds. Then the a/c fuse blew again. The air would only come through the defrost vents when it was blowing with the correct amp fuse installed (25 amp). I put in a 40 amp fuse and the air blew through the front vents and defrost vents before the fuse blew again.

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.

I did what you suggested and unplugged the blower motor, replaced the fuse and tried the control and the fuse still blew. Are there any other circuits that could affect my ac circuit? Should I begin looking more closely at the wires themselves? If so, what is the best way to look for a problem with wires I cannot easily access?

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.

Thank you so much for helping me troubleshoot my issue. I have yet to resolve it but your advice and step by step instructions have brought me further than I would have gotten on my own. Thanks again!!