fan motor speed issue

[hogrocket]

Wondering if anyone else has ran across this issue. I'm trying to get my AC/heater to work. Initially, I was getting power to the motor, but the motor wasn't coming on. The motor worked fine when off and tested on a battery. I put it back on and cleaned/tightened all the grounds and it now works. That was probably just a grounding issue.
Now my current problem. When I turn the switch on low and medium speed works fine however, when I put it in high it goes back to low or off. Anybody have any ideas? I thought it might be the relay or resistor, but I thought I would ask you guys first. Thanks in advance. M

[hogrocket]

New question--I was reading some older threads on this subject and found some mentioning a high speed relay. I know where the one is near the glove box, but where is the "high speed" one located? Could that be my problem. By the way this is a 70 K20 AC truck. Thanks, M

[Longhorn Man]

that one under the glove box is the relay you are looking for. Odds are, it is the problem.

[BLACK AND BLUE 67-72]

The blower motor resistor could also cause this type of problem

[FLEET72]

The resistor assy reduces the voltage at low and medium speed. High is the full 12V. If the motor does not run on high when in the truck you need to check the connectors @ the resistor and/or at the switch. Also it is possible that the switch is bad.

Check this thread for the wiring diagram: http://67-72chevytrucks.com/vboard/s...d.php?t=185665

[Hutch]

hogrocket
Mine had the same issue and it was the ground from the relay.
It is behind the glove box up high on the firewall between the heater box and the passenger side wall.
It is a short wire that allot of folks do not properly attach after a heater coil replacement.
It is a very short wire and easy to get left off.