Heater fan speed won't go on high

[Orion88]

My 71 Chevy has a very weak fan speed. My first assumption was a mouse nest somewhere in the ductwork or blower motor. Took all that apart and to my surprise it was mostly clean. I then took out the heater control panel and messed around with it. When I go to turn my fan on, nothing happens until I get to about 3/4 full speed. Then it finally starts blowing...barely. By the time I get to high it only blows like it's on a low-medium speed setting. My guess is the cable needs to be adjusted, but I couldn't find anywhere under the dash where you can adjust the fan speed cable. I don't think it's the relay because it would not work at all on certain settings. Instead it acts like the cable isn't engaging the fan until 3/4 of the way, so it simply cant blow on high speed because the cable is out of whack. Does anyone know how I can adjust the cable, or if it might be something else? As brutal as this winter has been in the midwest it sure would've been nice to have a fully functioning heater. Better late than never I guess.

[hugger6933]

Welcome to the board glad your here. Here we are some 45 years later and it is still a problem on the recent chevy trucks. The high speed fan relay. With you talking about the fan running in the high position, You may be looking at a ground problem. Now on todays trucks the problem ends up being the pigtail that goes into the fanswitch itself. Check there and also good grounds do wonders for these and any trucks. Good luck with it and glad your here. Jim

[LockDoc]

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The cable itself doesn't control the fan speed. The arm on the center control lever actuates the fan switch directly. There is a resistor in the top of the heat duct below the glove box insert that can go bad. Did you test the blower motor when you had it out?

Also, what Jim said.... did you put the ground wire back on from one of the blower motor mounting screws to the firewall?

LockDoc

[MARKDTN]

You need a good schematic. It is hard to diagnose electrical issues without one. A few things to check:

1-if you put 12v directly to the fan, does it run on high? (use at least a 12ga wire for this).
2-make sure the ground is good to the motor if it doesn't. If it won't go on high with direct power and good ground then the motor is bad.
3-make sure the fuse in the high speed wire is good and that you have 12v to it if you have one
4-If it will go on high with direct power and you have good power and ground, it's probably the relay. Those same relays were used into the '80s so they are pretty common. You can get another or retrofit to a later style.
5-look at the resistors, but I doubt it is that from what you describe.
6-look at the fan switch in the dash, they can get hot and build up resistance and/or fail.

[Orion88]

I didn't think to test the fan because it worked on low speeds. I did however clean the ground on the blower motor and reattach it. I will try running 12v directly to the blower motor and go from there. Thanks.

[LockDoc]

Originally Posted by Orion88

I didn't think to test the fan because it worked on low speeds. I did however clean the ground on the blower motor and reattach it. I will try running 12v directly to the blower motor and go from there. Thanks.

Sounds good. Post up what you find out so other members can benefit from your troubleshooting.

Oh, and to the board...

LockDoc

[Hutch]

Orion88
Check the ground wire from the relay inside the cab on the firewall.
It is very short
It is located up high between the glove box and passenger side wall.

There is a short ground wire that is supposed to go one of the shroud bolts that come through the firewall
Easy test is to attach a short piece of wire while it is on high from the relay to ground.

A lot of folks do not get the ground re-attached correctly while working on the heater coil.

The symptoms are
Low= Low
Medium= Medium
High = Very Slow

[Longhorn 70]

Mine had a similar issue. It was the ground for the high blower relay.

[cpetersen]

Need to move somewhere warmer.

[cool68]

my fan works only on high

[69GMCLonghorn]

As stated 15 times already, I'd bet on the ground being the issue. I am just now getting my rig back on the road after the heater started smoking thanks to 89 tons of leaves and crap in the heater system. Prior to this, it wasn't blowing very hard, same as on my last '69 GMC. That one, I just ran a big fat ground wire directly from the battery to the blower motor. Same thing you mentioned trying with the hot side. Problem solved!

[kensousa]

Originally Posted by Hutch

Orion88
Check the ground wire from the relay inside the cab on the firewall.
It is very short
It is located up high between the glove box and passenger side wall.

There is a short ground wire that is supposed to go one of the shroud bolts that come through the firewall
Easy test is to attach a short piece of wire while it is on high from the relay to ground.

A lot of folks do not get the ground re-attached correctly while working on the heater coil.

The symptoms are
Low= Low
Medium= Medium
High = Very Slow

I am having the same issue where Low and Medium work, but High does not.
I followed the thought process above but had a different set of symptoms.
The ground was already on the relay up near the glove box.
When I removed the ground, as I was removing the relay to replace it, it was with the ground off that High = Very Slow.
Ground on = nothing on High.

I also tested 12V directly to the fan and the fan definitely works in high that way.
I also ran a ground wire directly from the battery to the relay as a test and the symptoms remained the same.

I went ahead and replaced the relay thinking that the issue might still be in there. When the new relay went it, the symptoms are exactly the same.
Low = low
Medium = medium
High = very low with no ground and absolutely nothing with a ground.

Would anyone have another set of troubleshooting steps to help me pinpoint the real problem?

[airdale94]

Originally Posted by cool68

my fan works only on high

If you only have high it's your blower motor resistor. If you have everything but high it's your blower motor high speed relay, or it's in your switch or wiring.

[cwilkie]

I have a similar issue as well. The fan speeds are fine. Even with the fan power wire unplugged hot air still gushes into the cab. The faster I go the more hot air i get. Low medium and high work fine. I changed the resistor tonight with a new one but it made no difference. It is like the heater core don't close.

[LockDoc]

Originally Posted by cwilkie

I have a similar issue as well. The fan speeds are fine. Even with the fan power wire unplugged hot air still gushes into the cab. The faster I go the more hot air i get. Low medium and high work fine. I changed the resistor tonight with a new one but it made no difference. It is like the heater core don't close.

That problem would be with the control levers/cables/diverter. You may have a mouse nest inside the heater box (inside or outside of firewall) that is keeping the flapper from closing.

LockDoc

[cwilkie]

The problem was with the heat control cable. It was not long enough so it was not cleaning the heater door all the way. I was also missing the plug/grommet for the cable that goes to the heater core door two. So the problem was in two places!

It's fixed so now the question is to still get an aftermarket ac or buy a gear vendors overdrive next spring...

[gpgofast]

Originally Posted by kensousa

I am having the same issue where Low and Medium work, but High does not.
I followed the thought process above but had a different set of symptoms.
The ground was already on the relay up near the glove box.
When I removed the ground, as I was removing the relay to replace it, it was with the ground off that High = Very Slow.
Ground on = nothing on High.

I also tested 12V directly to the fan and the fan definitely works in high that way.
I also ran a ground wire directly from the battery to the relay as a test and the symptoms remained the same.

I went ahead and replaced the relay thinking that the issue might still be in there. When the new relay went it, the symptoms are exactly the same.
Low = low
Medium = medium
High = very low with no ground and absolutely nothing with a ground.

Would anyone have another set of troubleshooting steps to help me pinpoint the real problem?

I know this is an old thread but I thought I would reply. For these symptoms it is one of two things. Either is likely but only one will cost you money.

The high speed circuit for the fan blower on AC trucks comes directly off of the fuse box wired "Non Fused IGN". There should be a wire plugged into the spade terminal there that goes to your cigarette lighter AND to your high speed fan position. If you do NOT have power to your Cigarette lighter, you have a problem with this wiring. If you trace this hot wire all the way to the relay on the far right of the truck behind the glove box and you have 12V, then the problem is with this relay. I actually had BOTH problems which made troubleshooting doubly difficult in truck that has been sitting for 40 years and someone in the 1970 hardwired the heater blower. I had no power to the relay AND a bad relay.

If your cigarette lighter doesn't have 12V odds are your high speed fan won't work.