A/C goes to defrost when accelerating

airconditioning

#1

On my 1999 Dodge Ram, when accelerating with the A/C on, the air stops coming out the vent and comes out the defrost. When I lift off the gas, it comes out the vents again. This happens when both A/C or non-A/C vent is on. Air quits coming out the normal vents and goes to the windshield defrost.


#2

Sounds like a vacuum leak within the mode switch.


#3

This is a familiar problem with Rams. The setting is controlled by engine vacuum. Acceleration increases the vacuum and the the airflow setting changes by itself, temporarily.

Dodge has a TSB and a fix for this situation. It consists of installing a valve in the vacuum line somewhere. You’ll have to consult your dealer. Or you can consider this a minor nuisance and just live with it.


#4

There is a one-way check valve on the vacuum line from your engine to the climate controls. Either that valve has failed, or you have a vacuum leak somewhere in the climate control system. The air doors in your truck are vacuum controlled. Defrost is the default position if all vacuum fails.


#5

As SteveF says, this is a common problem on Dodge Rams of that vintage. I have the same problem with a '99 Ram Van. In researching this on the Internet I found there is an improved check valve/accumulator that costs less than $10. I still haven’t gotten one, but that’s because I don’t use the van very much and it’s not a big deal.


#6

I remember that one from back in the 1960’s You have a vacuum problem, likely a leak