A/C Puzzle

[ironman86]

I seem to have a problem with my A/C. Here's what happened... I was driving down the highway at about 70mph when I needed to pass another vehicle, so I accelerate, and when I do so, the air stopped blowing from the vents and starting blowing out of the defrost. As soon as I let off the gas, the air started blowing out of the vents again. The A/C control is set to MAX AC. Now, I'm assuming that there is a vacuum leak somewhere which is what is causing the air to blow out of the defrost when I accelerate instead of it blowing from the vents.

Now, my question is, where do I look for this vacuum leak? Where could the vacuum leak be that would case this sort of problem? Is there anything else that could be wrong, causing this problem? Thanks for everyone's help.

 

[martin]

Sounds normal to me.

 

[AllanD]

Ok, ducts switching under load?

Your A/C ducts are all controlled by Vacuum pots
and the engine provides the vacuum.

when you go to higher load there's less engine vacuum.

There is supposed to be a check valve that prevents that from happening,
but on an eight year old truck things wear out, and apparrently the check
valve in the vacuum supply line (probably before the vacuum reservoir)
to your A/C controls.

It's a pretty simple and rather common failure

AD

 

[ironman86]

Is this check valve visible on the vacuum supply line or is it hidden within?

 

[dusto2]

i have seen on a 01 b3000 which is the same, i found that underload the ac would switch like that, do to it loosing vacuum at the heater valve

 

[ironman86]

i have seen on a 01 b3000 which is the same, i found that underload the ac would switch like that, do to it loosing vacuum at the heater valve

What did ya end up doing to fix it?

 

[dusto2]

replacing the heater valve it was a customers car i would have capped it off till winter if it was my truck

 

[ironman86]

I want to get this fixed because it's summer-time and I use the A/C all the time. It's quite annoying when the A/C air starts blowing out of the defrost instead of the vents.

 

[snake]

Passenger side down by the frame and toward the front is the plastic "egg" vacuum reservior. Not sure where the one-way valve might be. You can buy a hand operated vacuum pump and trouble shoot it pretty easily.


You have trouble-shot it properly, it is losing vacuum which causes it to default over to defrost...

 

[ironman86]

Is this the check-valve that everyone is speaking of? The vacuum line I'm holding up [in the picture] is coming from a vacuum reservior.

 

[AllanD]

That's your problem

there is supposed to be a check valve there but that's just a double barb.

AD

 

[ironman86]

I wonder why it would be missing. Do I just buy a generic check valve or is there a specific one I should be looking for?