Car has about 140k, unknown previous history, relatively new to me. Overall decent but not collector condition. A daily drivable survivor. Decal on the fender says A/C was professionally converted to r134a some time back.

Symptoms: A/C is functional but weak. At 90F I can manage a 15 degree temperature drop in the cabin when the ACC is set to MAX. Additionally, I sometimes get hot air from the footwells only when ACC is set to anything below MAX. It behaves like a binary system in that state: either all hot or all cold. No mixing based on where the dial is set. This binary state comes and goes.

Troubleshooting: A/C pressure reading are 100psi static and 27L/200H @78F and 65% humidity. Just about normal. Compressor appears to cycle normally. I have a 3-port heater control valve aka mono valve (single solenoid, 1 pipe in, 2 pipes out). I’ve bench tested, disassembled and cleaned, and cross-tested the valve against a functional spare, it seems to work. However, when the binary hot/cold condition occurs, the mono valve is not being energized to close. When I override with 12V to the valve, the ACC appears to function as expected. I’ve also tested the internal air sampling vacuum pump that lives next to the glove compartment and pulls in air in the sunroof/domelight console. Functional.

I think have at least 2 issues: 1) weak AC performance, 2) faulty ACC

Help: it seems that the mono valve is not getting the correct signal to close at the right time, resulting in improper air mix and temp. Setting ACC on max seems to override this, so I know the circuit is good, but then I don’t have air temp control. So it’s an electrical issue? What the next troubleshooting step here?