FYI, the OEM heater control valves can have vacuum on them, and still flow coolant. I have my old one out, and can suck through it with the valve held shut.
I've been hunting an AC issue that seems to be due to the controlling of my Heater Control Valve. I've had heat at any setting except when the engine is cold. Changing the HCV solved the issue in MAX, but only at a stop or low throttle. I think my EATC unit is cutting the vacuum signal most of the time, heating my cold air.
The EATC units are now getting old. It is very common to have the capacitors inside go bad, causing all kinds of issues. With a Mark VII, it is typical to have a failure of all of the main electrical features, the radio, EATC, and compass modules. There is an expert who charges $90 or so each to fix those.
I'm planning to hunt for an EATC repair place who knows these Explorer units well. I expect the answer will be to have them rebuilt. BTW, I see new old stock EATC units for as low as $75 sometimes. That may be a good choice.