Jonny Retrofit Posted April 16 Posted April 16 When using our small ECU in custom applications we often get asked about the orange wire in the loom and how/why it connects to the cabin blower. The orange wire is used to 'sense' if the cabin blower is on. Primarily this is used to stop the compressor from running if the blower is switched off. This prevents the evaporator from freezing due to lack of airflow. Simple enough.... There are, however, many different cabin blower fans ( e.g. fixed speed, multiple speed, variable speed etc). Additionally, some blower motors are wired as 'high side switching' and some as 'low side switching' High Side Switching In a high side switching system the motor negative is connected to chassis and 12V is applied to the motor positive through a switch. The orange wire should be connected to the positive side of the motor and the ECU needs to be set to detect a voltage threshold above a certain voltage to mean 'ON'. This is done using the BLV command in the ECU and the usual recommendation is anything above 4V should mean 'ON'. This allows for blowers with resistor packs where the slow speed usually runs about 6 volts. To enter this setting in the ECU, type BLV=4 <enter> e.g Blower Voltage > 4V means ON. Low Side Switching In a low side switching system the motor positive is connected to 12V. Motor negative is connected to ground through a switch. The orange wire should be connected to the negative side of the motor and the ECU needs to be set to detect a voltage threshold below a certain voltage to mean 'ON'. This is done using the BLV command in the ECU and the usual recommendation is anything below 9V should mean 'ON'. Note that on a low side blower the voltage will reduce closer to 0V as the blower speed increases To enter this setting in the ECU, type BLV=-9 <enter> (note minus sign) e.g Blower Voltage < 9V means ON. You can check the blower voltage in real time using the DIAG facility in the ECU to test your actual voltages and set accordingly. Automatic Compressor On with Blower Some simply installs want to minimise the amount of controls. After the above thresholds have been set, you can also enable another mode If you want the compressor to start automatically when the blower is switched on. Change the input button mode (BUT) to blower operation.
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