I have a box of “bad” blend air door servo motors. Even tried to fix a couple – to no avail.
The "add-on" water shutoff valve works fine – but I wouldn’t keep it shut off for a long period of time. Been my experience that a lack of coolant flow though the heater core causes them to leak a lot faster (from rust/corrosion).
The SATC (Semi-Automatic Temperature Control) system was designed to have coolant going through the heater core full time – except it shuts off flow during “Max A/C” operation or when “Off”.
What I have done, is two things. First I purchased a 12 volt timer set to 4 minutes. The timer powers a vacuum solenoid (from a 4*4 front differential vacuum actuator solenoid). The vacuum solenoid shuts off manifold vacuum to the hot water valve for the first 4 minutes of vehicle operation, After 4 minutes, it allows for full engine vacuum to the hot water valve which should (if valve works correctly) stop any coolant flow through the heater core. Note: this was a temporary fix – but works fine for 10 months out of the year. Note: I did "plug" the (hard) grey vacuum hose/line going to the hot water valve, so I could use it in the future.
Second, my permanent fix. I removed the A/C servo, its short piece of wiring harness, removed the in-car temp sensor, Aspirator, its hoses and ambient temp sensor. I then replaced the A/C control head with a non-SATC unit. I also found cable that fits early ’80's FM-bodies (not sure about J-body), part number 3847430 (note – I’m not 100% sure this is the correct part number), lever 3846007 (also not 100% sure of this part number), Clip (p/n 6031132 – which came with my cable) and nut (a common ¼” hex nut.
I found another cable from an ’80 Aspen – but it was broken (so no spares).
If a person wanted to get rid of SATA, then I recommend finding an ’80 or early '80's FM (and maybe J) body and get the A/C control head (you only really need is the top lever of the control head), cable, lever and its nut. Then remove a few of the un-needed parts and presto, you now have working A/C and heater (but just not automatically controlled) - providing your A/C or heater was working correctly to begin with.
Now, if it was just this easy to get A/C working on my ’77 wagon . . .
BudW