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

[Richard]

Originally Posted by 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

A/C fan relay just provides a direct 12v to fan in the high speed mode. Would not cause this, if bad no high speed. I would bet a dollar it is the resistor if nothing obvious at fan switch. 50+ years on a part can cause problems. Just changed the resistor in my friends 67 Chevelle with A/C, same part. All is good now.

[MARKDTN]

I would also tend to at least take a hard look at the relay. I have replaced a whole bunch more relays than resistors over the years in a wide variety of GM cars and trucks.

[dmjlambert]

I'm a little slow, still replying to your "Blower motor fan relay maybe?" Yes perhaps. You can (kind of) see in the diagram HO455 posted that the resistors send power from the 14-gauge dark blue wire at the high-speed relay. Voltage normally passes from the resistors through the normally closed contacts of the relay to the fan. I could not follow what Richard was saying but I think he was trying to say there is not a normally closed contact in the relay. If that is what he meant I'm afraid that is not the case. It is a single-pole-double-throw SPDT relay.

At the relay purple is common, dark blue is normally closed, fat orange is normally open, skinny orange is relay energize signal from switch, and black is ground for the coil that energizes the relay.

Fat orange is always hot 12V from the CIG terminal on the fuse panel. The relay is only energized for fan on high.

When the relay is not energized, power flows from the switch to the resistors to the relay NC terminal to the relay common terminal and out to the fan on the purple wire.

When the relay is energized a small amount of current flows from the switch to the skinny orange wire to energize the relay, and a large amount of current flows from the fat orange wire connected to the CIG terminal to the NO terminal on the relay, then out the common terminal of the relay to the fan on the purple wire.

You can test before buying the relay by disconnecting the relay connector and use an alligator clip jumper wire to jump from the dark blue to purple wire on the connector. Then notice your fan works on low and medium. The high position will give you low motor speed.

[Richard]

I apologize for not looking at my relay and wiring and causing confusion about the NC contact. Thank you dmjlambert for the correction. Even with the correction provided the intermittent medium still did not make sense as both low and medium go through the NC contact. If both low and medium do not work and you have changed the other parts. Use the information in dmjlambert's post to check operation. It can only be failed contacts in the relay or the fuse mentioned.

[kna4977]

I’ve got medium and high now. On low the fan doesn’t blow at all but the resistor turns red. Also turns red on medium but the fan also blows.

[kna4977]

Cleaned the relay checked fuse and wiring. All is good. Hooked it all back up and briefly had medium and high. Never low. Relocated a ground to see if that made a difference. Now I just have medium. No low or high. I’ve ordered a new relay that should be here today. If that doesn’t fix it I have no idea what’s going on.

[dmjlambert]

You're making progress. The bearings could be going out on the blower motor, or your squirrel cage fan maybe rubbing on something. If taking the blower motor out I recommend treat the rubber hose that goes between the motor and the evaporator casing with care, because it is not reproduced. Mine was cracked and I restored it to reasonably good condition with super glue and black RTV.

[kna4977]

Originally Posted by dmjlambert

You're making progress. The bearings could be going out on the blower motor, or your squirrel cage fan maybe rubbing on something. If taking the blower motor out I recommend treat the rubber hose that goes between the motor and the evaporator casing with care, because it is not reproduced. Mine was cracked and I restored it to reasonably good condition with super glue and black RTV.

I’ve tried new switch which it needed anyway. The old one worked but was dangerous and would spark. New resistor no change, fuse and grounds are good. New relay no change. Anything else it could be besides the blower motor itself? Parts are cheap and I can return the ones I don’t need.

[Rust_never_sleeps]

you could hit the positive term on the blower motor with a direct battery wire and see how it spins before you pull it apart

[kna4977]

In the process of replacing the blower motor as that’s what ended up being the sole issue. Have the old one out and the new one painted and prepped to go in. Looks like the motor was sealed at some point likely many years ago with light gray RTV that is brittle and cracking now.

What did the factory use? Would it have had a gasket, strip caulk, rtv or? I’ll need to smooth the blower motor box surface before I reinstall. Also the old motor has the squirrel cage, flat washer, star or windmill like washer and a square piece that I believe inserts into the back of the squirrel cage but not sure. How is it supposed to go on? When I pulled apart it all kinda jumped off then fell in the floor so I’m not sure what’s right.

[Rust_never_sleeps]

THe book refers to "non-hardening sealer", so probably seam sealer back then.
You could use whatever you want now

[kna4977]

Originally Posted by Rust_never_sleeps

THe book refers to "non-hardening sealer", so probably seam sealer back then.
You could use whatever you want now

Non-hardening sealer to me sounds like strip caulk. The same stuff they used around the upper line on the a/c. Nasty stuff, very black and sticky. Like tar but a little more solid. I have some I bought for a restoration 10 years ago that's been left in a hot garage since and untouched that is the same as it was then.

[HO455]

Here's a couple photos on how the cage assembles to the motor.

For more information on non-AC heater boxes see the page in the link and the previous page too.

http://67-72chevytrucks.com/vboard/s...698377&page=42

You can get the correct non-hardening sealer at NAPA for almost $50 or you can get screen door sealer for $8 as shown.

[kna4977]

Originally Posted by HO455

Here's a couple photos on how the cage assembles to the motor.

For more information on non-AC heater boxes see the page in the link and the previous page too.

http://67-72chevytrucks.com/vboard/s...698377&page=42

You can get the correct non-hardening sealer at NAPA for almost $50 or you can get screen door sealer for $8 as shown.

Thanks that’s how I did it but mine has a flat washer and a star washer too. Put flat on them star then the alignment clip. I used 3M strip caulk that I’ve had for years.