Cabin Fan slow on AC setting, fast on eveything else

[Chief Brody]

What could be the issue if the blower motor only kicks into low when the dash knob is slid to AC ?

It blows at highest speed on Vent and Heat...but move it to AC and it's the lowest setting...the low, med, hi switch does nothing at the AC setting.

 

[diesel4me]

Some trucks had a hi blower relay on the heater box...
I thought it controlled the high speed regardless of the A/C control settings though..

 

[dyeager535]

That does seem a bit odd, I'd check out the relay and the blower motor connector underhood, if there is one.

Check out all the heater wire connections and that relay. I don't recall how the AC heater setups were wired, but it has to be outside of the switch itself since the settings work otherwise.

 

[Chief Brody]

Isn't there a relay switch of some kind attached to the air conditioning tube that comes out of the drier...that's be low side wouldn't it?

 

[dyeager535]

Hopefully someone can help you with that, just not familiar enough to know.

 

[diesel4me]

Isn't there a relay switch of some kind attached to the air conditioning tube that comes out of the drier...that's be low side wouldn't it?

I'm not very well educated with A/C,but I think that switch is the "low cut off" switch,that shuts off the compressor when it gets too low on freon--and it has two prongs--this is the switch you have to "jump" when you go to add freon ,if its too low to let the compressor kick on by itself..it has nothing to do with the heater though..

 

[Chief Brody]

it seemed to work until I put that new ac/heater control unit in the dash from LMC...but it looked right...

 

[frankin5]

it seemed to work until I put that new ac/heater control unit in the dash from LMC...but it looked right...

That explains it. I would put the old one back in to see if it works properly. If so send it back and get a new one, or try a new source . It sounds like it is defective.