Mitsubishi A/C Compressor Clutch

I took a hotline call the other day that reminded me of an odd problem on some Mitsubishi vehicles that I have seen many times now.

Some of the vehicles involved are the Galant (1999 to 2004), Eclipse (2000 to 2011), Lancer (2002 to 2005) and the Outlander equipped with manual air conditioning system (2003 to 2006).

The A/C compressor clutch will not come on if the blower motor is on a speed other than low. For instance, if the blower is on low speed, the clutch engages but the blower is inoperative. If the blower motor is turned to a higher speed, the blower works but the A/C compressor clutch does not engage. Occasionally the technician may not realize (or be informed by the driver) that the compressor clutch problem is related to the speed of the blower motor.

The A/C compressor clutch is energized by the automatic compressor controller (sometimes called an A/C ECU). The automatic compressor controller must see that the blower is on before it engages the A/C compressor clutch.

If you check a wiring diagram, you can see that the automatic compressor controller receives an input from the blower switch on the low speed circuit. When the blower is set to low, this voltage goes directly from the blower speed switch to the input of the automatic compressor controller. But if the blower is set to a speed higher than low, the voltage goes from the blower switch to the blower resistor, through the low speed resistor and then to the automatic compressor controller. If the low speed resistor is open, the automatic compressor controller never sees the voltage and the A/C compressor clutch is not energized. This obviously also explains the lack of low speed blower operation. The fix is simply to replace the blower resistor.