1973 Chevrolet Custom Deluxe Air conditioning

There isn't much to the system. Start by measuring the voltage across the two fan motor wires. If you see full battery voltage on "high" but the fan runs slowly, it is tight and will need to be replaced.

The only other parts are the fan speed switch, resistor assembly, and usually a high-speed relay under the hood. If the relay has burned contacts you'll just be missing the highest speed. If the thermal fuse is burned open on the resistor assembly, you'll only have the highest speed.

Given the symptoms I'd suspect the motor first. When you measure the voltage across the wires, they have to be plugged into the motor to be accurate. If you find less than 12 volts, measure between the ground wire and a clean rust-free point on the body. You must find 0 volts there. If you do find some voltage, the ground wire or connection needs to be cleaned or tightened.