Fan/blower speed not working at med/high setting
 

Tackling the next problem on the car. The heater/defroster blower swith only works on the low speed. Is it the motor or the switch I will need to replace?

Most likely culprit is your heater resistor. The way your heater fan works is thusly:

power is applied to the switch, switch selects high, med, or low speed. High is a direct connection to the motor, med is a low resistance between the switch and the motor and low is a high resistance between the switch and the motor. On early cars the resistor is on the front of the heater box, don't know where it is located on a '71 but follow the switch wires and it will lead right to the resistor block. Replace the resistor and you should have all three speeds back. Note after removing the resistor you will see the elements burned out. This could have occurred from a motor near failure. I would take the fan motor out and have it checked by a motor shop to make sure its not running excess current.

Who carry the new resistor? I noticed John's Mustangs listed the part not in stock and checked a few other suppliers without luck.

Took out the resistor and tested it with my ohm meter. Got a different reading on each prong (4.5, 2.3 and 1.5). I'm going to assume the resistor is fine and check the switch. I'm not crazy about taking out the heater unit to get to the motor. Since the motor works on 1 speed, you think it's the switch. :confused:

Likely. Try this: remove the umbilical from the resistor so just the prongs are showing. Jumper 12 volts to each one of the prongs in turn. Fan should run at high, medium, and low speed. If this works, then switch is likely bad...problem can only be in the umbilical or switch.

Note: do not try this on other model year Mustangs. Some require power to be connected to the resistor and some require ground to be connected depending on year model. According to a schematic I have, your's requires 12 volts to be applied. Hold 12 volts on tab only long enough to hear motor speed up. If it doesn't speed up immediately, then remove jumper immediately.

Ok, ran the test as suggested. Motor and resistor working as expected. I tried the ohm test on the switch and only 1 speed is available. Is there a switch replacement available or do I need to visit the wrecking yard? :confused:

Sounds like you've got a handle on this, but i just wanted to mention that it's been my experience that when the fan only works on high, that the resistor is bad. Any other combination of speeds not working is almost always the switch.

:)

Take care,
~Chris

Older catalogs show the switch was available in the recent past but it sources seem to have dried up. I would first call all of the usual Mustang parts vendors in hope of finding one...then those who specializes in NOS Ford parts like fomoco.com Lastly, I'd look in the salvage yard. There's a good chance your switch is sitting on someone's parts shelf, you just gotta find someone who has one.

I noticed you can take the switch apart by removing the back cover by bending the little prongs. Not sure once inside anything can be done. I won't go there until someone had attempted this and suceeded. Since I have one speed, I can live with it. In the meantime I will visit the wrecking yards. oh fun.....

I tried this on my '65 two-speed fan motor switch and it failed although I have been successful at repairing switches like this in the past. Success depends on how badly the contacts have been pitted by past switch actions. Even if you are successful at repair you should look for a new one since expected life of a repaired switch is short.

Note that the switch for '66 has the same three positions and you might be able to adapt the '66 switch to your application. NAPA also has dozens of switches in their catalog with descriptions. Its possible you could find a suitable alternate switch.

Took the switch apart and boy, there was so much gunk (years of dust mixed with ??) on the contacts. Cleaned them, put it back to tegether and working like new.:D

Thanks for the troubleshooting tips.

The gunk was electrical contact lubricant. Back when it was electrical motor grease. Today they use silicon paste. You can buy the stuff at Radio Shack in small tubes...believe they call it "transistor grease" since its used a lot in mounting high power transistors. If you assembled it dry, don't take it apart to redo it since you might break the tabs off the switch body. Next time, perhaps using a little of this stuff will extend the contact life.

Good to hear your repair was successful. Happy Mustanging!