Blower motor operates on its own

[Smross]

My newly acquired '79 C10's blower motor operates randomly on its own. With the heat levers all the way to the left and the fan on low the blower motor will still operate, seemingly on high. Yet there are times when I want to operate the blower it will not work. What the heck is going on here?
Thanks.
smross

 

[bucket]

Is it an AC equipped truck? From what I remember, there's a funky secondary blower switch that operates off the heat/defog/AC lever. It is screwy in my truck too, the blower only works on the floor setting and even then you have to get the lever in a sweet spot. I'd suspect the secondary switch is faulty in both our trucks, but really, the HVAC controller should be removed and looked over. A previous owner could have screwed with the wiring or there could be a damaged/melted connector.

It's also possible the blower switch (the one you move) is bad, but usually when they fail, they just plain quit working at all.

 

[chengny]

Blower resistor - maybe.

 

[Smross]

Thanks for the replies. No, it's not an A/C truck so I guess I'll try a new resistor. They are not expensive so it is worth a shot.
Thanks again.

 

[chengny]

I think (and don't quote me on this because I have never owned a pre 1981 truck), that on a 1979 the blower cannot be completely shut off.

The electrical control switch only has 3 positions (HI, MED, & LOW) correct? So that switch would not provide a means for totally securing power to the blower.

And, as far as I know, even though the distribution lever has an OFF position, it has no effect on blower speed. It only serves to move the various dampers - that direct the air flow into the cab - to their fully closed positions.

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On the other hand, if the truck is equipped with an A/C system, there is an interaction between the directional lever and power supply to the blower.

However, it is the opposite of a shut down; when the lever is shifted to MAX AC, the blower is supplied with full battery voltage. Even in the OFF position the blower still runs on LOW. With the lever in any other position, all blower speeds are available.

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[Smross]

Chengny, what you say makes perfect sense, however, when the fan does come on it is usually at a higher speed. I should mention that I cannot control fan speed with the fan switch; it either comes on by itself or not at all.

Also, if I move the two horizontal levers (temp and heat/defrost) all the way to the left to the off position wouldn't that shut the fan off, assuming everything was working the way it should?

 

[chengny]

Also, if I move the two horizontal levers (temp and heat/defrost) all the way to the left to the off position wouldn't that shut the fan off, assuming everything was working the way it should?

No, on a heat only system, moving the levers - to any position - has no effect on fan speed. There is no electrical connection (or any type of switch) associated with the temp or selector levers. Behind the face plate the levers only connect to cables.


You could pull the control panel from the dash and prove me wrong. But I can almost guarantee you it is going to be a resistor issue.

"High speed only" blower operation is a text book symptom of a failed/failing resistor stack.

 

[bucket]

I think (and don't quote me on this because I have never owned a pre 1981 truck), that on a 1979 the blower cannot be completely shut off.

That might only be the AC equipped trucks. I've only had one without AC, and that was years ago and I don't remember if the blower was always on a little. There was a bunch of air leaks and holes in the floor, so I might not have been able to tell anyway. I also drove without a back window for a while.

"High speed only" blower operation is a text book symptom of a failed/failing resistor stack.

This is true, but high speed will still work like normal. From my understanding, the blower only works on high, and only sometimes. I have a hunch there is more than one problem. Maybe the blower motor is bad too, only working when it hits certain bumps in the road.

 

[scottybaccus]

Pull the resistor and expect to see a broken coil that flops around and makes contact intermittently. It may be a similar scenario in the relay.

 

[chengny]

No relay - he has no AC.