Greetings, wondering if any 71 B body vacuum operated A/C system experts can guide me in installing a missing correct valve. I Just purchased the very original vehicle recently and constant heat is blowing out the a/c vents. Air does not divert anywhere via the vacuum controls and the temp is constant hot. Currently there is no valve connected to the inlet hose where once there obviously was, the bracket is there but heater shut off valve is missing. My research is showing I should use the center vacuum valve pictured though I’m skeptical. The vacuum hose is there at the bracket where the valve goes. Is this possibly something this easy to repair as installing the correct valve and I’ll be on my way actually able to use a defroster or likely diagnostic time ?[img]http://[/img]

Good luck, I have two 71 B bodies and I've just about given up on trying to get the vacuum controls to work! I truly hope you get it working so I can pick your brain.

Actually, my advice is to troubleshoot it one link at a time. I'd install a manual valve where the heater control valve would normally go, and then see if everything else works properly. If everything else works, then install a vacuum controlled valve and experiment.

In my opinion, the 8-hose vacuum control is really interesting! It's essentially an 8-bit control unit using vacuum rather than electronics.

Jim

JF, I appreciate your encouraging words and advice. There must be a more detailed reference source I'll need to locate regarding this system. The FSM is fairly vague in a breakdown of what each hoses' function is, etc. This should be interesting at the very least.

https://www.google.com/url?client=internal-uds-cse&cx=partner-pub-0048081641434287:nsck0a-6axg&q=https://board.moparts.org/ubbthreads/ubbthreads.php/topics/838986/re-71-b-body-w-a-c-underhood-bracket-question.html&sa=U&ved=2ahUKEwjnmcyTy__jAhVuUt8KHRuADVEQFjAAegQIABAC&usg=AOvVaw3sJeXZZpNUeIunm-tTgqql

Actually with a little further looking the FSM is good and there are several good threads here already regarding. Thanks

The center one is the one. The other ones are either a replacement or for a '72 and later years. My car has that center one that you posted. Yes, there is a bracket that bolts to the passenger inner fender from the valve. If the one you posted doesn't have one, you can get a direct bolt on bracket from here http://www.billrolikenterprises.com/proddetail.asp?prod=3441883%2DP . Also, a vacuum line goes to the nipple and runs across the firewall, around the master cylinder (or power brake booster if PB) and under your battery tray to a "vacuum can". Look at the attached photo I posted. It is in the upper left.

RM, thank you as well regarding.!

Problem identified. Little homework went a long way.

Hope you'll let us know how it works out.

I shall advise. Seems the only item missing is the valve itself. All the vacuum lines are there in the engine bay, and through the firewall, including the vacuum reservoir under the battery tray. Hopefully the diaphragm is OK. Repair pending.

The vacuum nipple on the heater/water valve is connected to the larger vacuum nipple on the back of the heater controls. Another hose that tees off of a hose coming from vacuum switch connects to the smaller nipple on the back of the control. The main vacuum feed is part of the vacuum harness and goes through the firewall on the passenger side near the floor, across the fire wall and up to the vacuum canister. Another vacuum line goes from the vacuum canister to the vacuum source on the intake. On my Charger, the "professional" shop I paid to work on it while I was overseas completely messed up the vacuum hose routing and left me with massive vacuum leaks and the heater control valve with a hose attached to it that didn't attach to anything else. It took a long time to sort it out.

A couple more photos

The valve on the lower right looks like what was originally on my '71 Charger, But when I replaced it, the replacement is like the lower center valve (the big plastic one.)
I was lucky the Heater/AC box was in decent condition, but the vacuum control switch in the dash was broken. I got a similar vacuum switch from (I think a dart?) and carefully opened both cases and swapped the rubber vacuum blocks because they are different, but the housings are the same. You should also have a vacuum reservoir under the battery tray, and the vacuum lines go through the firewall near the passenger side by the valve cover. My 2" Headers melted the vacuum lines.

Those 71 valves are famous for leaking. Had friends buy NOS ones and they started leaking. If you don't care about being correct you might consider using a 70 and up A body valve. Easier and cheaper to fine. Last a long time.
StratOvent cars use a different expensive valve so use the A body valve.

Luckily the vacuum system in this particular vehicle is unmolested and a fairly simple repair. The FSM is extremely beneficial. Good intel all around here as usual, well most of the time, in this instance, as well.

The correct valve is installed and no changes. The blower motor functions properly, however, there's no change of the air flow, it only blows out the a/c vents and will not divert into HEAT or DEFROST mode. Diagnostic for a vacuum leak is in order and/or a faulty diaphragm on the box, or vacuum storage can under the battery tray ?? The FSM is very explicit regarding although I need to obtain a vacuum gauge to perform said test. Anyone recommend a particular brand / model? Thanks regarding. Tommy

Changing the heater control valve, or even having the wrong one, won't affect the switching of airflow in the dash. Sounds like either the vacuum switch at the heater controls isn't working or has one or more broken off nipples, or there's a vacuum hose popped off one or more of the actuators on the heater box. Those are a PITA to try and check with the dash/heater box in the car. Hopefully, the photos I posted of my extra heater box will show you which hose goes to which valve.

Sounds like the control switch is bad. I think someone makes new reeds for it or find a new switch. Sound like the temp door actuator is not working along with the heat/defroster door actuator also. Default mode meaning if the system loose vacuum is hot defrost. You can check everything with a vacuum pump.

Make sure that vacuum hose to the water valve is plugged until you replace that valve.

Thanks for the insight. I'll check the hoses again, more closely as well as all the nips This is my.first time working on a vacuum system. Hoping the vacuum regulator IS bad, unless of course it's non available and I hope to score a new one.? Thanks again. Tommy

Mark, yes this is my scenario other than that the air only flows through the A/C vents, not hot defrost.
I can add that after I installed the new vacuum water valve, now the system is blowing only COLD air, even after normal operating driving distance & conditions.
Is it OK to ask on a discussion board if you have a vacuum regulator to sell perhaps? Thanks, Tommy

This hose was open to the atmosphere for who knows how long. I just recently acquired the fairly original vehicle and basically de-bugging the issues. I don't have any vacuum and all the hoses "appear" OK pending re-check. I need a vacuum tester ASAP. Thank you !, Tommy