Firebird Nation banner

AC Evaporator Purge Valve

784 Views 4 Replies 2 Participants Last post by  myk74
Hi all,

I have a 72 Formula 350. The gray vacuum line coming off the AC evaporator purge valve vacuum actuator

is hanging loose under the dash. Anyone have any idea where it plugs into? If it goes into the 9-port vacuum

switch, what is the port number. Why is it so difficult to find a vacuum diagram for the 72 AC system? Can anyone

direct me to a source or send me a copy? The 78 diagram is not correct for the 72. My shop manual is no help,

nor is the body manual. I have a assembly manual coming and a separate vacuum/electrical diagram manual on their way.

Hopefully one of these will help.


1 - 5 of 5 Posts
I can't find anything on a 72, what's different on the 78 chart?
Hi Bryan,

Thank you for your interest. its like trying to solve a jigsaw puzzle.

The 78 diagram shows a 5 port switch hookup and no hookup for the grey hose that goes

to the purge valve. I know there were several changes during the 70-73 years and more later.

Some of the cars had the 9-port valves, some had six port valves and some had 5 port valves.

Some had a second 2 or 3 port valve. I'd just like to know what my car is supposed to have

as opposed to what we found (which may in the end be correct - who knows).

I had a bad vacuum mode switch and thanks to "the parts guy", obtained a switch he refurbished.

After reading a large number of texts on the TransAm site, I am now beginning to understand the

beast a little better and that the switch isn't perefectly tight either. I have vac. tested with a hand

pump the individual vacuum lines (six that I can find) and all are reasonably tight in that they will

hold vacuum to a point and then slowley leak down. When I try hand pumping the lines via the switch

according to the 78 diagram, the only valve that seems to work is the AC on the upper cowl. I can

see the door close with vacuum and open as it leaks down. Can't get the kick panel door to work

either. So I'm now wondering if I'm on the correct ports. I'm also wondering if I'm reading the port

numbering correctly i.e., does the number correlate to the port to its right or to its left???? Egads

man - its driving me nutso LOL.

Here are port assignments supposedly for a 71 AC car that I obtained off the Trans Am site:

Port-1 White (AC)

Port-2 Brown (inside/outside air)

Port-3 Vacuum Source (engine intake manifold w/check valve)

Port-4 Blue Defrost

Port-5 Gray (Purge Valve)

Port-6 Open

Port-7 Yellow (Heat)

Port-8 Open

Port-9 Open

Tomorrow I plan to back the car out of the garage and try the switch per the 78 diagram and see what

works and doesn't work with the engine running and constant 22" of vacuum. If that doesn't do it, I'll go with

the above and see what happens. I did eliminate the rubber plug and now have the lines going direct to the

switch itself. I will also retest all of the lines individually.

Love the car. Had a 70 Ram Air IV brand new and never forgot it. this one is a Formula 350 that at my

76 years is plenty of "giddyap" but the thought of Ram Air IV heads, cam and intake manifold is intriguing

at some point.

Would appreciate any light you can shed on this enigma.

See less See more
I just saw a heater control assembly on firebird central and it says it's "Exact fit for all 70, 71, 72, 73, 74, 75, and 76 Trans Am models with air conditioning". I have my 74 heater control pretty accessible. I'll try to pull it and see what's connected to it sometime today.
Here's the one from my 74, doesn't look like it will help you much. But you should have a port marked 5, which I would assume is where the purge line would go.

White is on 1

Brown is on 2

Black is on 3

Blue is on 4 (behind yellow in the pic)

Yellow is on 7


See less See more
1 - 5 of 5 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.