On my 1968 Cutlass Holiday Coupe with factory A/C I have a problem with the heater/AC fan switch (I think). The fan only works on the "High" setting. Nothing happens on the med or low setting. However, when I leave the car for a couple days the battery drains unless I take out the in line fuse under the hood. (which of course keeps the fan from working at all).

Do I replace the fan switch to solve this? Or is the problem someplace else? Any trouble-shooting tips?

 

Unfortunately, the 1968 factory Chassis Service Manual doesn't include a wiring diagram for the A/C system. The trick here is that there is a blower motor high speed relay on the firewall near the brake booster. When the fan switch is on HIGH, it activates this relay, bypassing the motor speed resistor and applying a full 12 volts directly to the blower motor. At lower speeds, the relay is not energized and the power feed runs through the switch and motor speed resistor before running to the blower motor to reduce voltage and thus motor speed.

If the blower works on HIGH, the motor switch is almost certainly fine. The two most likely causes of your problem are a bad relay or a bad motor resistor. Start with the resistor, which is bolted into the fiberglass HVAC case on the firewall. Check the connector terminals and pull the resistor out of the case and inspect it. I've had these crimped connections corrode, causing the problem you are seeing. If the resistor is good, chances are the relay needs to be replaced.

 

Here's the 1969 schematic, which should be essentially the same:



- Eric

 

Thanks for the diagram! Checked the resistor. (Tough to get to under the A/C hoses) it was fine, no corrosion. I will replace the relay.

Many thanks gentlemen!

 

Before replacing the relay, you should verify that with the key on and blower switch at a medium speed (anything except OFF or HIGH), you get voltage at the blue wire going into the relay. Note that with the resistor in line, you won't see 12 volts, but something less.