My 1984 Trans Am (manufactured late 83) went into the shop last week to have the AC fixed. It has not been used since at least 1992 when the car was stored in a garage for 20 years. Unfortunately the shop could only get the AC to work in defrost mode as they said there was a crack in the system and it needs a new HVAC vacuum control module. I would be very grateful if anyone could shed some light on the part number for this, and / or where I could obtain one. Thanks in advance for your help.

 

The ducting the dash is all controlled by vacuum. There are a few different actuators, a bunch of plastic vacuum lines, and the valves in the control unit mounted in the dash. If you have a leak anywhere, the HVAC defaults to defrost.

You need to find the leak before you'll know what to fix.

 

Their is no vacuum control module, it's just a rotary switch on control panel where you turn it on. What did they replace? That might tell us if they knocked something off.

 

 

Thanks for your replies, guys. Just to confirm this is for a 84 Trans Am, we think the leak is from the HVAC vacuum line harness. Here is a photo of the part that needs replaced. Any help is appreciated.

 

My mechanic has said it is the vacuum harness that connects to #5 on the diagram. Thanks.

 

You could go to a GM Dealer and see if they can locate that harness for you. Some times you get a NICE counter guy(if he's not busy) who will get you a printout of all the dealers in the country who MIGHT HAVE ONE. If not you'll have to settle for used. Look in the "Parts For Sale" thread and see if anyone is breaking up a car for parts.

 

Stick your ear to the unit and listen for a hiss. If you do, the following link will probably put you on the right road:

https://www.thirdgen.org/forums/inte...anent-fix.html

JamesC

 

That's some good info

 

Make sure the black hose has vacuum while running before chasing things or spending money.

 

5 and 18 in the diagram are both vacuum control valves. They're a stack of parts with a spring and a plastic push-in clip that holds it all together. It's pretty common for that clip to be missing one or both of the tangs that hold the valve together. The bolt fix linked above just replaces the plastic clip with a bolt.

The vacuum lines themselves usually aren't a problem, at least not the ones inside the car. The harness in post #5 is usually just pushed on with a couple tin push-nuts to keep it from coming off. Unless it's been damaged, it's unlikely to be the source of the leak.