Hvac vent setting issues...

[lenstar65]

Hello everyone..i hav a 99 ram 1500,3.9l...base model truck...when i accelerate my truck,the air blowing out the main forward vents switches to the defrost vents...if i decellerate it slowly switches back to the forward vents...i read another thread here that explained possible vacuum line issues...if im just sitting and i run thru the vent settings they are fine...only on acceleration do i hav a problem...we are having a heat wave and it has become pretty annoying..i can sometimes hear a door/flap moving around....any help appreciated...thanks....

 

[dudeman2009]

That is a textbook example of a vacuum line problem. The vents are spring loaded to return to the defrost position, so if you lose vacuum in the winter you will still be able to see.

Its one of two things, your vacuum check valve is stuck, or you have a vaccum leak after the valve.

The picture shows the location of the check valve. Also make sure you check all the lines going from this valve to the firewall, ensuring that nothing is cracked or leaking.

You can check for vaccum leaks by either getting a spray bottle of water or using what I prefer, brake clean. Either one will be sucked into a vacuum leak and change the idle rpm of the engine, brake clean more than water.

If that is all good, you should probably move to the back of your selector switch to see if its leaking there.

 

[lenstar65]

I took a good look at everything,so did my buddy..hee thinks the acutator,that little silver can,is bad..he said its not operating smoothly,kind of getting stuck..whats in there,a piston??..does that make sense,? All the lines and connections seem fine...he can move the linkage,and open the flap...trying to figure a way to prop in in the forward vent position,temporarily...any ideas?? Thank you for the assistance....

 

[dudeman2009]

I took a good look at everything,so did my buddy..hee thinks the acutator,that little silver can,is bad..he said its not operating smoothly,kind of getting stuck..whats in there,a piston??..does that make sense,? All the lines and connections seem fine...he can move the linkage,and open the flap...trying to figure a way to prop in in the forward vent position,temporarily...any ideas?? Thank you for the assistance....

I have never bothered manually moving any of the door actuators. However, some are spring loaded, some I believe are not. A little bit of resistance is nothing to worry about, the surface area of the piston coupled with even a few inHg of vacuum will allow travel along the length of the bore even with small obstructions.

You will have to specify which silver can, there are a few of them.
Pull the vacuum line of the can and stick your finger on it, switch through all the settings with the engine running. You will feel a vacuum if there is one present. If there is no vacuum, replacing that can is useless.

My next step would be to check for vacuum at that can, both at idle and under 3Krpm hold in park (shutting the engine off while holding the vacuum with your finger can also be useful in detecting the size of the leak). If there is none at high idle, pull the climate control assembly out of the dash and remove the vacuum connector from the selector switch. With the engine running put your finger over the black tube on the 7 tube vacuum connector. There should be vacuum. If there is, get another piece of tube (small dia. brake line works well for this, but even a bendy straw will work) and jump it to the yellow tube, this should switch to the panel vents.

As I said before, if the vents work while you are idling, but stop working when you hit the gas. You either have a large vacuum leak, or the check valve is broken.