Heater Fan doesn't blow on low

[kna4977]

A/C cab fan motor doesn't blow on low at all. Will blow on medium sometimes and will blow on high always. Where should I start?

[Richard]

Originally Posted by kna4977

A/C cab fan motor doesn't blow on low at all. Will blow on medium sometimes and will blow on high always. Where should I start?

The resistor in the ducting, fan switch and connector on fan switch.

[kna4977]

Originally Posted by Richard

The resistor in the ducting, fan switch and connector on fan switch.

Where is the resistor in the ducting? Fan works just not on all speeds. Connector looks good.

[dmjlambert]

In the heater core box attached to the firewall inside the cab. Here the box is shown removed and upside down while I was working on it. There is a little door through which you can peek at the resistor board, but to get real access you need to take more apart.

[kna4977]

Originally Posted by dmjlambert

In the heater core box attached to the firewall inside the cab. Here the box is shown removed and upside down while I was working on it. There is a little door through which you can peek at the resistor board, but to get real access you need to take more apart.

Is that part serviceable? Is there a modification/work around to eliminate the resistor altogether?

[Richard]

What dmjlambert said. In his provided pic you can see two screws, They hold the "S" duct adapter in place. Remove those screws and the ones securing the "S" duct to have access. Not real difficult. The resistor is available at many vendors, here is one of my go to places https://classicparts.com/1964-72-a-c...-w-factory-air

[kna4977]

Originally Posted by Richard

What dmjlambert said. In his provided pic you can see two screws, They hold the "S" duct adapter in place. Remove those screws and the ones securing the "S" duct to have access. Not real difficult. The resistor is available at many vendors, here is one of my go to places https://classicparts.com/1964-72-a-c...-w-factory-air

I see the two screws is there a schematic I can't quite picture where its at and how it's oriented with that single pic.

[Richard]

Originally Posted by kna4977

I see the two screws is there a schematic I can't quite picture where its at and how it's oriented with that single pic.

Look at the heat outlet on the floor above the transmission hump. Left to right- Heat outlet, "S" duct then the fitting screwed the the box shown. As mentioned, pic shows box upside down.

[RustyPile]

Originally Posted by kna4977

I see the two screws is there a schematic I can't quite picture where its at and how it's oriented with that single pic.

Before you start digging around in the heater case, read Richard's post #2 again.. Check the heater switch and/or the plug connecting the wires to it.. Not only a bad resistor, but also a bad switch or corroded/loose connection will cause the symptoms you describe..

[howzzzit]

Originally Posted by RustyPile

Before you start digging around in the heater case, read Richard's post #2 again.. Check the heater switch and/or the plug connecting the wires to it.. Not only a bad resistor, but also a bad switch or corroded/loose connection will cause the symptoms you describe..

Good point. Mine would work on random speeds whatever it felt like doing that day. Removed the original switch and the contacts had what appeared to be old lube or something built up around the contacts. Kind of looked like ear wax. Changed switch and cleaned connector with electrical contact cleaner and it works like it should. I'd start in the switch/connector area

[HO455]

It's also possible the AC fan relay is causing problems.

Here are some threads to help.
First has some good tests for components

https://67-72chevytrucks.com/vboard/...d.php?t=848097

This one is a great discussion on how the AC fan circuit works along with a wiring schematic.

http://67-72chevytrucks.com/vboard/s...d.php?t=813743

And one more.

https://67-72chevytrucks.com/vboard/...d.php?t=731399