classicretrofit Posted October 30, 2023 Posted October 30, 2023 Our most frequent support issue is a non starting compressor. The majority of cases are either wiring problems or assumptions about how the system works. So let's start with some basics. No need to connect a computer to check any of the following: 1) Battery voltage. The compressor will not operate at all below 11V or above 16V. Please note that battery voltage checks with a DVM (digital volt meter) may not prove that you have sufficient voltage at the point of compressor start. For example, if you have a fully charged battery (above 12.6V) it is tempting to start the system without running the engine. Although this may work, the battery may also immediately dip and the compressor not start. It is alway best to start the engine. 2) Start up sequence. When the car ignition is switched on, the ECU must first see a battery voltage above 12.6V before the system is deemed to be 'Ready'. We use this to detect an engine start as we don't want the system to operate without the engine running (it would drain the battery). Below this voltage, the blue light will pulse indicating 'Standby' and the button will not respond to presses. When the system is 'Ready', the blue light will go solid (although dim). Now, the button can be pressed and the blue light will go bright - the compressor should now start although it may take up to 30 seconds. The contact relay will also click (you will hear this first). 3) Blower voltage detection. The voltage on the orange wire (connected to the blower) is monitored to detect if the fan is running. The compressor will not start if the blower fan is off (to prevent the evaporator from freezing) and the system enters 'Standby' (pulsing blue light). Some cars switch the 12V to the blower and some switch the GND. The 911 switches the GND so a voltage below 9V on the orange wire represents ON. If in doubt simply disconnect the orange wire to ensure 'ON'. This is the default setting (although it can be changed in the ECU software for fans that switch 12V - see BLV setting). 4) Blue Button/Switch. The button's blue light is controlled by the ECU, not by the button itself. It needs supply of 12V (red) and GND (black). If the ECU is not running or in the wrong condition (e.g. engine not started), the blue ring light will NOT respond to button presses. This is normal. 5) The condenser fan is controlled by the compressor current draw detected by the ECU. Do not expect the fan to come on when you press the blue button. The system pressure needs to build, which takes more amps from the charging system which in turn switches on the condenser fan. The condenser fan will only run if the compressor runs and consumes more than 35A ( default setting ). 6) Battery and current monitoring during running. Once the compressor starts it will ramp up to speed. The speed of the compressor will be faster in hot weather. The compressor may slow down either due to current monitoring ( usually to maintain 75A or less) or because of sag in the battery voltage. 7) Idle setup. If the idle RPM is low, the battery voltage will also be low and then the compressor may stop. The ECU default threshold is < 12V at the battery for more than 1 minute. The compressor will then wait for 1 minute before starting. The worst test conditions for the AC are to leave the car to idle after it has been sat - get out and drive the car for 20 minutes to recharge the battery. Then, you can let the car idle and observe the battery voltage with the AC on. If it is below 12V, then increase the idle so it is around 12.5 or better. If you can't achieve that without a very high idle then check all the ground points and 12V high current cabling including alternator and starter motor connections. As a benchmark, all our alternators can put out 70A at an idle of 950 RPM on an SC. Note, however, that some earlier 911s have lower geared pulleys so might require a higher idle (or a pulley swap). These cars are meant for driving! The timers and control scheme in the ECU have been designed to keep the AC running in typical driving scenarios. If the compressor is stopping during driving you most likely have charging system issues. The above checks will get most systems running without a laptop using the default settings as delivered. If you still have issues, there is a more in-depth diagnostic procedure documented here.
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