72 heater fan speed

[Chucks72Longbed]

Hello, anyone have any insight as to why my heater fan doesn't run in high speed? I just replaced a bad fan switch and now it runs in low and medium but not high. It's a 72 C20, 402 big block, TH400 with factory AC. Thanks

[BruthaMan]

There is a high speed blower motor relay that sits on top of the air box connected to the firewall, in the cab



Follow the orange wire in the photo. That's where the relay is. When this relay goes bad, high speed on the fan will not work. Solution: replace the relay.

[Wizo]

Check the Heater resistor. Its screwed into the inner air box up under the glove box. look like a group of 3 little springs. Couldn't hurt to check them with a volt/ohm meter. LMC carries them. Hope this helps.

[Chucks72Longbed]

This is awesome. Thanks a lot guys.

[Wizo]

Word.

[Ugly 61]

I had the same problem with the factory air on my 72 GMC. I put in a new blower motor, blower relay and fan switch still no high fan speed. I found that the orange wire to the blower relay had come off at the fuse box. I plugged this wire in to a terminal that is hot only when the ingition switch is in the run postion and then the blower fan works like it is suppose to. What the blower relay does is on high it bypasses the resistor and sends voltage directly to the blower fan motor.

[Chucks72Longbed]

More great info! So glad I found you guys! I just bought this sweet truck on 12-26-14 and love it. Always wanted one. One of my favorite chevy truck body styles. I'm am amazed how many parts are available. I'll post some pics soon.

[jimbosprint]

I just fixed the high fan speed that wasn't working on my truck thanks to a quick search that turned up this thread. A new relay solved the problem.

Once again, thanks for the help 67-72chevytrucks.com!

[Chucks72Longbed]

Yeah. It's so nice to have this group to bounce questions off of. Like sitting around in the shop shooting the breeze with all your buddies about trucks.

[Accelo]

The circuit is somewhat confusing. It makes a lot more sense if you know the fan is fed with two different circuits. On has the lower voltages provided but using the resistance in the ducting. The other is a full 12 volts.
Rick in Washington State

['72 C10 Super]

I have a '72 with A/C and the blower switch has only off, low and high
notches in the switch as you move it....? Does a non-A/C fan switch
have only low and medium, while the A/C has low, medium and high?

[BigBird05]

Not sure how to answer your question, but there are 2 different fan switches, one for A/C and one for non A/C. I just bought an A/C switch from Randy. There has been a for sale post on here for a non A/C switch also.

['72 C10 Super]

My fan switch only has 3 places, off, low, medium. At Medium it is bottomed out
in the slot where the switch is positioned. I don't seem to have a 4th spot??

I would be happy rigging it to low and high or medium or high.. but I definitely
need high for the A/C.

Any ideas???

['72 C10 Super]

Ok simple questions:

1. Does the fan switch have Off, Low, Medium and High?
2. Is there such a fan switch that only has Off, Low, High?