heater control vacuum hose

[Steven]

Good afternoon you guys, I am hooking up the Factory A/C and heater on my 71 SS 454,  my issue is there is only one vacuum hose coming thru the firewall which I believe goes to the vacuum ball, all the doors operate correctly when the vacuum hose is connected to engine vacuum, what I don't see is a vacuum hose for the heater control valve to stop the flow of coolant to the heater core. the vacuum diagram I see online shows 3 vacuum hoses coming from the bottom side of the HVAC controller, mine only has 2 vacuum hoses.

Am I missing something? I would think there would be a vacuum hose going to the heater control valve from the HVAC control.

[jft69z]

Assembly manual for 71 shows that heater control valve was possibly on early designs, then removed from later drawings (assembly manual page C60; sheet 1A-16 and 1A-20). 1972 Montes don't have that control valve at all, and only one vacuum line. The superseded drawings listed above show the elimination of that water valve as time went on.

If you don't have the provision for the second vacuum line coming out of the firewall, and it appears original, my guess is that it wasn't used on your car.

[Steven]

Thank you for the reply, is the assembly manual your talking about in this forum?

[Canuck]

I think Joe may be right here. I know that 1972 didn’t have the valve and maybe it was eliminated part way through the 1971 production year. My 1971 SS454 has it which was built in September 1970.

[jft69z]

52 minutes ago, Steven said:

Thank you for the reply, is the assembly manual your talking about in this forum?

No, something I had to purchase.

I had to get all 3 years (70, 71, 72) because there are subtle, (and not so subtle) differences in all the years. Sometimes it's part numbers, different equipment, options, updates, etc. 

Gm Service manuals and Fisher Body manuals are really a must if you're doing any amount of work to these cars as well. 

[Steven]

Thank you. I guess I will see if the vacuum doors blend the amount of heat coming from the heater core. I will post what I find out

[jft69z]

1 hour ago, Steven said:

Thank you. I guess I will see if the vacuum doors blend the amount of heat coming from the heater core. I will post what I find out

It should work fine, as long as everything is in good working order.

I've seen where people have removed that coolant control valve & replaced it with a ball valve, stating that in the summer the A/C wouldn't get cool enough due to hot coolant going thru the heater core (they close the ball valve in the summer). All things being equal, theory suggests that if the valve worked, no coolant flow thru heater core, A/C would be the only thing going into the interior.

All fine and good......except.....GM engineers removed that control valve in mid-71 we'll say, and didn't use it at all in 1972. My 72 makes about 36 degree cold air thru the center duct on a hot/humid summer day, with hot water still circulating thru the heater core. When I rebuilt the car, I went thru the HVAC unit, tested all the actuators and blend doors, and also replaced all the internal foam seals, etc. 

Every time I see someone say they need that ball valve to stop flow thru the heater core, I just keep going back to my car. That just tells me they have another problem somewhere else, and the ball valve is a band-aid. Keep in mind, that heater core is also another heat exchanger. I've seen where that saved the day in the heat of summer, stuck in traffic, and the only way to help save the engine was to turn the heater on full blast. Yeah, it sucked being in the car, but if you can't pull over or stop due to the traffic situation, it's a nice option to have if needed. 

 I haven't done any research yet to see if the interior HVAC unit is different on a '70 vs. a '72, but a quick check at SS396.com shows the interior assembly available as one part number for 70-72, which tells me the system should work fine without that coolant valve. Just a thought...

https://www.ss396.com/monte-carlo/AC-1400F.html