2001 QC, 2WD, 4.7L, AT

Been this way for probably 5 years and I've just never tried to debug it... now that I'm here and asking questions about another issue I'm having I figured why not ask!?

So, basically all fan speeds work (blow proper amount of air) but the AC will not run on the lowest fan speed... only on the higher speeds... if the AC button is activated the AC light comes on... but if the fan speed is put on 1 the light goes off and the air is no longer cooled... but if you change to higher fan speed (any of them) then light comes on and air is cooled again...

Thoughts?

 

wow...you've got me. I don't think the two functions are even linked in the CTM or PCM....

That's a head scratcher.

 

Have you ever replaced the blower motor resistor? When it goes bad, that's the symptoms.

 

Thanks KDodge... I have not... not familiar with it... easy, difficult, cheap, expensive?

 

Easy and cheap. Google it.

 

Will do!

 

As stated this is a cheap and easy fix and I may just do it.. however, with a small amount of research (on a very common problem) I'm not finding where the AC working or not at certain fan speeds is related to the resistor... the resistor fix is typically for when fan doesn't blow at all or only on some speeds (typically the higher speeds)...

KDodge75 - have you read or experienced AC not working on certain speeds being tied to faulty blower resistor or are you maybe confusing my issue with the blower not working at all on certain speeds? For clarity, my fan blows as it should on all speeds... it's just that the AC only works on speeds above the lowest speed.

I haven't read much and am certainly not experienced with AC units, but there seems to be low speed and high speed specific circuitry in the AC system and maybe I have an issue with the low speed circuitry...

 

So far as I know, the A/C shouldn't care what speed the fan is blowing at, so long as it's blowing. My resource for wiring diagrams seems to be down at the moment though..... so, can't verify that.

 

If/when you are able to confirm, thanks! I tend to agree with your comment after some google searching...

For additional clarity - if the AC button is activated (as you all know it is simply an on/off switch) this happens:
1. Turn Fan from off to speed 1 - fan runs properly but no AC (AC light does not light and air is not cooled)
2. Turn Fan from speed 1 to speed 2 - fan runs faster than on speed 1, AC light comes on and air is cooled
3. If you turn fan to speed above 2 then fan runs faster, AC light remains lite and air is cooled
4. If you turn fan speed from 2 to 1 the fan runs slower, AC light turns off and air is no longer cooled

toggling AC button on any fan speed above 1 has the expected affect... AC toggles between on and off (light follows as expected)
toggling AC button on fan speed 1 appears to have no affect (light never changes and air never cooled)

 

I'm looking through the FSM, and I think i have noticed something: This little device called the "Evaporator Temperature Sensor". No description is given other than it's used for freeze-up control.

In theory, the evaporator could be getting too cold when there's a lack of air going past it (low fan speed), causing the HVAC controller to kick it off.