A/C - H-6 System Information

SOURCE: Subaru Tech Tips

TITLE: H-6 Climate Control System Information

APPLIES TO: Legacy L.L.Bean
VDC Outback

SERVICE TIP:

Both the Subaru Legacy L.L.Bean and VDC model Outback use the Valeo/Zexel Automatic Climate Control System. This system contains a few new circuits not found on any other Subaru A/C systems. The following is some information you may find useful.

The first new circuit is a new two-wire circuit that runs from the Engine Control Module (ECM) Connector B134, pin 13, to the rear of the A/C compressor connector F82, pin 2, then from the compressor to a vehicle ground at connector B83, pin 12. This circuit was originally thought to be a compressor thermal limiting device but, upon further inspection, it was found to be an internal compressor speed sensor. This is a "pulse" signal that is emitted 4 times per compressor revolution.

Upon closer inspection of this new circuit, it was found that the black (B) wire that exits the rear of the compressor is the ground-side of the circuit, which ends at the vehicle ground connector (B83). The second wire, which is yellow (Y) in color, comes out of the compressor and then changes to white (W) at the first connector on top of the compressor. This circuit continues to the ECM as the input side of the circuit to the module. By monitoring this wire with a Digital Volt Ohm Meter (DVOM) set to the AC volt setting, you can monitor the AC voltage being sent to the ECM.

The New Select Monitor (NSM) was installed and noted the following air conditioning monitors:
A/C Switch ON/OFF (monitors driver input from a/c switch)
A/C LOCK SIGNAL ON/OFF (ECM lock out of compressor clutch engagement)
A/C MID PRESSURE ON/OFF (input of high pressure to ECM for fan control)
A/C COMP SIGNAL ON/OFF (command from ECM to energize a/c clutch)

Next the compressor speed signal circuit to the ECM was disabled and it was discovered that when a/c is requested through the switch by the driver, the compressor clutch engages for 5 seconds and then disengages for the remainder of the 'key ON' cycle. It was also noted that without the compressor speed signal input, the A/C LOCK SIGNAL changed from OFF to ON meaning the ECM had cancelled the compressor ON signal therefore disabling the compressor clutch.

Next 3 key ON/OFF cycles were performed, requesting a/c each time, with the speed circuit disabled followed by an ECM self test that revealed that no trouble codes were set due to the loss of this signal. This leads to the conclusion that failure of the internal speed sensor or an open anywhere in the circuit will render the a/c compressor inoperative. Again, by monitoring the A/C LOCK SIGNAL and the A/C COMP SIGNAL, using the select monitor, one can see if the ECM has locked out the compressor clutch circuit causing a no engagement concern.

The second circuit investigated was from the ECM connector B135/pin23 to the Dual Pressure Switch on the receiver drier connector B10/pin4/R wire. The Dual Pressure Switch continues to control the compressor clutch for high and low system pressure cutoff in the same manner as on previous systems. The fourth wire (red in color) is new to this vehicle application. It was noted that this new circuit uses a 5V reference signal from the ECM to the pressure switch, which can be monitored using a DVOM.

Monitoring this circuit with a DVOM and observing a/c system refrigerant pressures with service gauges revealed that the voltage changes from 5V to 0V at approximately 260PSI high side pressure reading. In conjunction with the circuit voltage change, it was noted that the ECM increased the engine cooling fan speed to combat the higher high side pressure. It was determined that this new circuit is for mid-pressure cooling fan control by the ECM, and again, a new feature to this vehicle. This circuit can be monitored using the Select Monitor A/C MID PRESSURE data reading.