73 Barracuda, factory AC car.

I have just rebuilt the HVAC box, tested all of the actuators, inspected and pressure tested the control tubes (using mouth vacuum/pressure) and charged the AC.

The AC functions and I get cold air if I have the temp switch in just the right place (I assume it is a spot where the valve is actually closed on both sides) so I know the system is working.

My box, however, is not functioning properly. I have checked the plumbing to the source line through the firewall and can move the heat/def door with the switch set to def and a vacuum pump. The door will also return when I go to heat. However, the fresh air door won't actuate in either direction, nor will the heat/AC door, which is currently stuck on AC. I was obsessively careful to photograph the box pre-disassembly and use numbered tape strips to identify the tube and its actuator. For the moment, I'm assuming the integrity of that part of the system.

I'd like to rule out my control switch, as it has visible electrical damage at the 12V connection from before I owned the car (of course the bulkhead connector also has damage at the ammeter wire. That said, the switch works electrically, in that it kicks the a/c compressor on appropriately (Max AC, AC and DEF).

Is there a method to test the vacuum switching of the HVAC Control Switch? I'd like to confirm its integrity before pulling the dash apart (again).

Also, any other suggestions?

Thanks
Chris

3895790 is the pn for a new one, about $120 new from several places, Summit has it in stock.


1973 Dodge FSM available online, should work to troubleshoot this. It's fairly detailed, chapter 24.

http://www.mymopar.com/index.php?pid=109

I'd suggest using a relay to provide power to the compressor to minimize future damage. Factory just ran it thru the switch, but in later years went to a relay. I'd think about one for the blower motor feed too.

Yeah, I believe my referenced electrical damage is related to an AC system malfunction. I have put a sanden compressor on, a parallel flow condenser and put the whole circuit on a relay triggered by the ac switch. Thanks for the link!

Chris

Good luck

Well, I checked the actuation of all of the doors from the cannon plug with a mighty vac pump and all the doors moved right around 8"HG, in spec with the manual, though I don't have an electric vacuum pump to test per the manual. I wasn't able to figure out how to test the switch its self without an electric pump, but everything is working from the switch. I gave the switch a little Alabama Rebuild and reinstalled. I'll test it on the road tomorrow and go for a new switch if it doesn't work properly.

Van's has the switch for 94+9.75shipping. Best price I've found so far.

Not sure about '73, but earlier control switch innards were made of soap stone, same stuff used for marking layout lines on steel. The soap stone pieces were cut in saw tooth fashion so that a pushed button moved the soap stone sideways to the bottom of it's tooth, actuating a vacuum line.

Soap stone pieces are slick (like soap) and low friction, but they eventually wear out after so many button pushings. They can be DIY rebuilt with new soap stone pieces of the correct silhouettes - if you can reverse engineer what the original shape was.

I made/have a diagram explaining how the b body 71/74 AC control switch works and what kind of signal drives every wire. If I can't find where is posted and have patiente will upload it from my laptop in a few hours ( not at home at this moment ). E body coul get diff switch ( dunno ) but wiring color codes is the same.

About using relay for the compressor as mentioned, despite that, blower is the big sucker, not the compressor.You can confirm that just checking how the ammeter flicks with blower on and then just feeding the compressor. Diff is NOTICEABLE. If later models got the relay for the compressor I think is due temp ciclying being controlled by turning on and off the compressor, not due the load. You can use a 4 relays setup for every blower speed ( low, mid, high, and heater which gets its own speed ). I had to made this to save the fan blower lever continuouslly melting the plug on back of my 74 lever selector switch. Have diagrams for that too.

Can't help on vacuum lines thought

And about the melting point at alt wire... well:

Parallel wiring upgrade AND better alt.

I talked there about the relays for the blower speeds.

A bit more maybe in better english than mine:

http://www.heritech.org/cuda/Charge.html

Thanks everyone, my little overhaul worked. Lightly dusted with a computer air can, clean with warm soapy water, lightly dried with computer air, NO lube per fsm. Emory cloth on the electrical contacts. Reinstalled, works great.

On the soapstone, that is an interesting setup, infinitely rebuild-able! E-Bodies are other style of switch, unfortunately.

Agreed on the blower motor. I actually had the blower motor relay circuit all laid out but my converter lockup setup ended up eating two relays to incorporate a brake kill (owing to power interrupt instead of ground interrupt on the brake light switch), and so ate up the last slot in my box. My thought was that between the headlights and compressor clutch being moved off the main circuit and with my big alternator and bypassed ammmeter I should be ok for now and I'll re-engineer the setup when I add in an electric fan and fuel pump.

I notice that for quite a princely sum, Painless has an add-on 8 relay box with a single wire power/ground with a bus setup and RF suppression.

just in case this is the AC wiring setup for 71/72 B body, valid up to 74 B body and several A bodies too. Pretty sure some E bodies used the same setup. However wiring colors and how they work are pretty much the same for all Mopars.

I decoded all the wiring functions and made this

( just noticed the writte up got some typo mistakes, LOL )