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.

[HO455]

Originally Posted by Richard

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. .

The A/C relay supplies 12dc power to the blower in high speed from a different power source in order to prevent arc damage to the switch from switching the higher amperage (Compared to a non-A/C blower motor) A/C blower. But in addition when the A/C relay is deenergized in low speed and medium speed, voltage from the switch through the resistor is routed through the A/C relay's normally closed contacts to the blower motor.

When you have intermittent problems the likely suspects are switches and relays. In the OP's case we shouldn't assume there is only one problem. It is quite likely the failure of the low speed is caused by the resistor especially if all the wiring harness is undamaged. But the intermittent medium speed problem could be caused by bad contacts inside the A/C relay.

Just my opinion which with a sawbuck will get you a cup of Joe in most places.

[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.

[Richard]

Originally Posted by HO455

The A/C relay supplies 12dc power to the blower in high speed from a different power source in order to prevent arc damage to the switch from switching the higher amperage (Compared to a non-A/C blower motor) A/C blower. But in addition when the A/C relay is deenergized in low speed and medium speed, voltage from the switch through the resistor is routed through the A/C relay's normally closed contacts to the blower motor.

When you have intermittent problems the likely suspects are switches and relays. In the OP's case we shouldn't assume there is only one problem. It is quite likely the failure of the low speed is caused by the resistor especially if all the wiring harness is undamaged. But the intermittent medium speed problem could be caused by bad contacts inside the A/C relay.

Just my opinion which with a sawbuck will get you a cup of Joe in most places.

A N/C contact does not exist in the relay. It is a SPST relay (single pole, single throw) When static no current flow is allowed, when energized the contacts close. In this case the relay only closes when switch is in high speed. There are 3 terminals present on the A/C fan relay. One on each side of the N/O contact, one side typically connects to the cigarette lighter circuit and the other ties into the feed to fan. Yes, the wires from the resistor also connect to the fan feed. But power does not back feed through the resistor. It cannot do that if the fan switch is in the high position. Also the power cannot feed through the relay in the lower speeds as the connection there is N/O.
The third terminal on the relay is the positive side of the energizing coil which connects to the high speed terminal at fan switch. Power only flows through this wire if switch is in high position. The negative side of the coil grounds through the relay case. Just trying to provide help for understanding of how the circuit works.
There really is only one problem. Diagnosing electrical faults can be tricky. It is best to eliminate all possible causes when diagnosing, not add them.

Edit- Not sure how I missed the intermittent medium speed comment. Low and medium speed are both controlled by the resistor. Would bet 100% it is the culprit. 0% it is the high speed fan relay. Maybe replace fan switch as well.

[HO455]

I stand corrected. I was going from memory and not the schematic.

I apologize for any confusion I may have caused anyone.

[kna4977]

I can’t quite read what is punched into the cardboard (I think it’s B H L) on the fan switch, but the bottom prong with a dark surround is loose and suspect. It’s the one with B.

[HO455]

According to this schematic B appears to be the 12 volt positive to the switch.

[kna4977]

Replaced switch and resistor still just blows on high only. I thought it ran on medium but I was mistaken. Any other ideas? Blower motor fan relay maybe?

[HO455]

There are two power sources for the blower. The one for the low and medium speeds goes though the fuse labeled heat and AC on the fuse block. See photo #1.

If your fuse block looks like mine did (Photo #2) you may not be getting voltage or enough voltage to run the blower due to rust and corrosion. Both at the fuse and on the back side where the wire is riveted to the fuse holder.

Do you have a voltmeter?

[kna4977]

Originally Posted by HO455

There are two power sources for the blower. The one for the low and medium speeds goes though the fuse labeled heat and AC on the fuse block. See photo #1.

If your fuse block looks like mine did (Photo #2) you may not be getting voltage or enough voltage to run the blower due to rust and corrosion. Both at the fuse and on the back side where the wire is riveted to the fuse holder.

Do you have a voltmeter?

I'll check that fuse, but I'm pretty sure its good there. I do have a voltmeter. I tested the blower motor relay and it appears to be bad.

[HO455]

Before doing anything with the relay I would check for battery voltage at the "B" terminal on the switch.

With the black lead of the meter connected to a known good ground turn the key on. With the fan switch in the off position check the voltage at the #14 BRN wire on the "B" terminal. You should have the same voltage as the battery. Then do the same test with the fan switch in low speed. You should still have battery voltage. (Within a couple tenths of a volt.) Do the same test for medium speed.

This will verify that you have the necessary 12 positive at the switch.

[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]

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.