Hi, all. I’m new here and hoping this will be helpful. I’ve had a problem for a while now that I’ve finally decided to tackle. There’s a lot, so bear with me.

The problem: AC goes hot when it experiences a loss of vacuum (taking off after a stop, climbing a steep hill, etc.). I cannot recreate this in park.

Details: I can hear something move inside the dash when I hit 1200 or so RPM after a stop, and it switches from any AC mode to the Panel and Floor heat mode. I’ve seen others have a similar issue with defrost. This is not that. All modes work normally until a loss of vacuum is experienced. Heat modes are entirely unaffected as far as I can tell. I still hear something move, but the mode does not change. I have recently replaced all engine bay vacuum lines. The vacuum reservoir holds vacuum even after the engine has been stopped for 20 minutes. Seeing as the HVAC vacuum line is on the check valve with the reservoir, a leaky hose seems unlikely to me (reservoir wouldn’t hold vacuum that long with even a slow leak).

The best I can reason it is this: I know that there is one vacuum supply line to the HVAC mode selector ****. I also know that, depending on the mode selected, the system redirects the vacuum to an actuator that moves the doors. Is it possible an actuator is leaky? What are the recommended courses of action?

 

This guy shows a variety of vacuum hoses. Might see something useful.

 

Check the vacuum reservoir / vacuum canister
then
Replace the one way valve
That canister holds vacuum while you floor it
Yours is so bad it is doing it at 1200 rpm

 

Defrost is the default in case of a vacuum keak

 

I have already replaced the check valve. The vacuum reservoir holds vacuum for hours after the engine has been shut off, so I highly, highly doubt it is part of the reservoir system.

 

If I disconnect vacuum completely, it only blows defrost. Other than that, mine is not switching to defrost. If I could tell you why I’d have it fixed.

 

As I understand things only the blend door could cause things to "go hot" if the air cooling system is functioning correctly (compressor, evaporator, etc.). So it seems like either the blend door is moving from cold to hot (it's electrically contolled), or the air is not getting cooled.

Vacuum doesn't seem to be in the mix. Unless the cool air is just being redirected. Which outlet are you checking for cold air? Maybe check another to see if the air is cold or not.

So, these three possibilities seem reasonable:

AC system stops cooling air or

blend door blocks air from passing over evaporator core or

cold air is redirected to vent you're not checking

 

Does your truck have the climate control unit or the manual one?

 

You sure you are looking at the right canister?
There are 2 or 3
It can only do what yours is doing for loss of vacuum
It's got to be losing it somewhere
Vacuum mode switch behind the dash is highly unlikely, but they do fail and come apart and the hoses fall off the back of the mode switch
Generally, that will not cause a problem just on accell
Maybe make sure the mode switch is plugged in all the way anyway

 

I think you’re right. I’ve done some digging inside the truck and everything vacuum related seems to functioning exactly as it should. Option three can be ruled out. I have checked every vent. Option one and two are plausible, but I’m leaning toward two.

Every single time it happens, I hear something shift in the dash. I cut the AC system off for 15 seconds, I hear it shift again, turn the AC system back on, and the air is ice cold again. I just don’t understand why the blend door would move every single time I accelerate but not any other time…nor do I understand why it would move back into its original position every single time.

It’s also worth noting that I knew there was a problem with the blend door anyway. I thought the actuator was bad, but after taking some panels off, I can hear it whirring when I turn the temp control ****.

 

It is the manual unit