a/c bypass?

RobS · Feb 6, 2012

So my latest ??

When I hit the gas to pass, or when I'm towing any kind of load, uphill..yada...the a/c or heat will transfer to the defrost vents...anyone elses trucks do this?

bigbadram1500 · Feb 6, 2012

Bump for ya. Sounds like maybe a vacuum issue.

rambob · Feb 8, 2012

mine does it also.
I read some stuff about replacing the check valves (vacuum)

they are only about $5 from the dealer.

I replaced the two I have and it didnt fix the problem.

ut yes, from what I understand. its all vacuum related.

and if you have cruise control, it affects it as well.

rambob · Feb 8, 2012

I think I may have a vacuum leak somewhere.

I read the plenum gasket leak may contribute to it as well.

RobS · Feb 8, 2012

i dont think the plenum is leaking, i looked all around it, and down under the throttle body...no oil or anything, like ive read you'd see...i'd hate to pull it off and find out it wasn't the problem...as far as the vaccuum line, which ones did you replace? under the dash or elsewhere...i'd rather start with that first...

rambob · Feb 8, 2012

two check valves

part # for the check valves is 4549209

pass side right near thottle body - on the line going into the vavle cover or around there.

the 2nd is on the pass side up near the fire wall.

rambob · Feb 8, 2012

be super careful with the vacuum lines.

soo many people have broken them - they are old and brittle.

the easiest way for me was to pull the entire part hose out of the engine near the TB, and then cut a slit with a knife. pull the CV out, and trim the hose back to a good part past the slit, then push the new CV on and re install.

the small diameter hose are even easier to break. luckily, the washer hose is a good fit over them.

rambob · Feb 8, 2012

google "wild vents" and you will see plenty of info.

but what I have told you is the most common fix

good luck

RobS · Feb 9, 2012

ill look into it today, Thanks Ram!!

kslongrider · Feb 15, 2012

Did anyone come up with a solution. My '96 V-10 has been doing something similar for years. Putting any kind of strain on it, like going up a hill, the a/c jumps over to the defrost and blows warm air. Not a big deal but it is annoying. So far I haven't found any vaccuum leaks.

rambob · Feb 15, 2012

kslongrider said:
Did anyone come up with a solution. My '96 V-10 has been doing something similar for years. Putting any kind of strain on it, like going up a hill, the a/c jumps over to the defrost and blows warm air. Not a big deal but it is annoying. So far I haven't found any vaccuum leaks.

you can check the check valves by blowing through them in both directions. it should stop air flow in one direction.

sometimes I have heard they may be installed the wrong direction.

a/c bypass?

RobS

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