What I wanted:
- Normal operation when desired
- Force off mode when making short trips below ERDTT temp.
- Force on mode when making long trips (further than full charge) but still want SOME heat (ie: ~34°)
Well, this is exactly what I've got now. I've got 3 resistors I can choose from:
- a fixed ~10500Ω which make it think it's 30° (lets call this resistor A)
- the stock thermistor (lets call this resistor B)
- a fixed ~9500Ω which make it think it's 34° (lets call this resistor C)
I ran a pair of wires from the connector that plugs into the stock thermistor (I'm calling this the ECM wires) to the passenger compartment. I also ran a pair of wires from the stock thermistor into the passenger compartment. The ECM wires go to the input side of a DP3T switch. The middle output goes to the stock thermistor wires, the left output is bridged by the A resistor, and the right output is bridged by the C resistor.
Now if I want the car to be in "auto" mode, I choose the middle position. If I want to force ERDTT off, I choose the left position, which forces ERDTT off, REGARDLESS of outside temp (car thinks it's 34°). If I want ERDTT to run, even if it's ABOVE 32°, I choose the right position (car thinks it's 30°).
What's even better is that when running in "auto" mode, if I'm near my destination, I move the switch to the left position so the ICE won't cycle back on just as I'm pulling into my parking spot (this has happened more than once). Unfortunately, this is the one tidbit that doesn't work (yet). Apparently, once ERDTT is engaged, it won't give up if the temp only rises to 34°. I need to pick a different resistor value to make it think it's warmer (I know it will give up if temp is 42°). 42° is my next target value, and I'll update this thread when I have a chance to figure out that value. I know a resistor value that will make it think it's 60°, but I don't want to swing too far from reality because the car is too smart. I had a value that gave 119°, but that threw a DTC that indicated the refrigerant was low (it wasn't, but at 119°, the static pressure of the refrigerant should have been higher, but it wasn't because it was really only ~50°. I'm speculating on this last part, but there's also no reason to stray too far from the 32° trigger temp.
Oh, and I also used 'stock' connectors to connect to the thermistor and the ECM wires. I'll post those part numbers next.