Hello I've got a 1971 C-10 with a 396 or 400. I just hit a hundred thousand miles A/c works but I'm Having problem with my heater it's not blowing at all. I tried tapping it with a screw driver on the fan part but nothing does any one have a good place to start for me? Thank you for your time.
Use a test light to check for voltage from the body to the wire going to the fan motor. The ignition switch must be on, and try the speed switch in different positions. If you find around 12 volts, the motor has worn brushes and must be replaced. If there's no voltage there, follow the wire back to the resistor assembly bolted to the heater box. Check for voltage on all of those wires. If you find it there, the thermal fuse built into it is open and that assembly must be replaced. It usually burns open because the motor is tight and drawing high current. You can verify that by jumping the wire with the voltage to the wire going to the motor. The motor should run at the highest speed. If it's tight you'll hear it stop suddenly when you remove the jumper wire rather than coasting gradually to a stop.
If there's no voltage on any wire at the resistor assembly, work back to the fan speed switch and check there. If it's missing there, a fuse, wire, or connector terminal is open.
GM used a variety of circuits, and some of them used a high speed fan relay for the highest speed to reduce the stress on the switch contacts. With those circuits the fan will still run on the highest speed if the resistor assembly is defective.