I have a 98 Ram 1500 4X4. When I accelerate, the A/C air flow shifts from
the vents to the defroster ducts. Is there a vacuum control module, and if so,
where is it located?

 

IIRC that problem is usually caused by a bad check valve. If youtrace the vacuumaccessory lines overnear the brake booster, behind the motor, andnear the firewall, you will see it. It'sabout as big around as a quarter.

 

Tried the check valve, no joy. It only does it when truck is in drive on the hwy.
It does not do it when it's in park.

 

you need a vacuum can its about 2 inches long and about 1/2 in diameter i will post the part number tomorow morining after i get to work it gos on the passenger side right at the fire wall just to the left of the drier. I sell like a10 to twenty of those a summer.

 

Canister will help, I also had a leak which made the problem worse which i tracked down to the rubber block that plugs into the vacuum switch on the transfer case. The harness (from engine to transfer case and steel lines) was pretty cheap, $40 from DC, won't get it from anyone else, switch on mine was sticking also, wouldn't hurt to replace it, about 15$ from DC. I believe there's also a TSB for some years which stated that battery acid can drip onto the lines to the cruise unit (located underneath batt) which can cause problems.

 

the part number is 4677204 there only like 7 or 8 bucks

 

Is that the part # for the vac reservoir that is under the cowl? Is it prone to cracking or what goes wrong with it? Mine is doing the same thing & I only noticed it because it is warming up & I finally used my a/c.

 

There is more than one check valve. Do you have hydroboost brakes?

 

thats the one under the cowl on the passenger side its the one thats

 

I cant seem to find this vacuum canister. I have the same problem and was gonna pick up one of them and replace it, but I cant find it. There isnt anything close to being like that to the left of the drier on the firewall. Are they only located there on certain models. I have a 97 ram 1500 360.