A/C electrical testing advice

A/C control pot test:

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No idea re the replacement, but it would have to cover 0-10K Ohm. It's a 3 pole unit.

The heater valve can be tested with 12V and a ground; it's either open (0V) or closed (12V) depending on whether the power is on or off. Just pull the plug and test with 2 wires. You should hear it move.
With a multimeter, you can also test the the input signal in the plug: Permanent 12V in the red/white wire as the ignition is on. The negative is switched on/off by the ECU, not be the pot.

The ECU measures cabin temperature, HVAC bleed air temperature and evaporator core temperature (to kill the a/c compressor in case of freezing). It compares the cabin air and the bleed air temperature with your desired temperature as input by the knob and then decides to employ either heater, a/c or nothing, depending on the sign and value of the difference between measurements and your desire. It then cycles the relevant unit with decreasing interval and operation length until the desired temperature is attained. Here the heater logic. The A/C is similar. It won't employ both at the same time. So there is point, when moving from full hot to full cold, where you hear the A/C clutch go off and then the heater valve go on, and vice versa.

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In theory. In real life, you probably need A/C full on hahaha.

 

Welcome. Good your A/C hardware is working. This limits the trouble shooting to electrics or electronics.

Under the front hood on the electric board check:
- Fuse 6 (yellow 20A) and relay d (brown new BOSCH) that powers the condenser fan
- Fuse 18 (yellow 20A) and relay s (brown new BOSCH) that delivers 12V to the hcav ecu, a/c clutch, heater valve, interior and cabin air suction fan. If your interior fan works, you can assume that fuse 18 and relay s are fine.

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The HVAC power leaves the board via a brown/black wire in connector W in the right side, the 3rd from the bottom. Check the plug.

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Under the dashboard in the middle of the car close to the front firewall, there are 3 relays belonging to the hvac.
- Left relay (brown new BOSCH): Interior fan full power relay activated by the knob's microswitch when all the way up. For lower fan speed settings, the regulation is provided by a power transistor.
- Middle relay (brown new BOSCH): A/C clutch power
- Right relay (purple new BOSCH): Heater valve power. The original SIPEA 0440 relay has different attribution on the pins - if you use a new BOSCH unit, you need to switch 2 pins else the heater valve will not work.

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The HVAC ECU also includes a mini relay that switches the A/C clutch's 12V in the PCB. That relay may get very hot and may unsolder itself from the BCP, or burn out. Take the ECU out and check it visually.

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Good luck!

 

Welcome Mark. Let us know.