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I've got a 2002 Subaru Outback 3.0 VDC Sedan with 158,000 miles.

A couple of weeks ago I started having a problem with the A/C system. The system will not consistently work when the engine is not warmed up. The system can be set for the A/C to be on and both radiator fans run yet the compressor will not run. After the engine has had a chance to warm up a bit the system will run normally after turning off and restarting the engine.

I've done the following things: reseated A/C fuse, reseated A/C fuse, reseated connector to the compressor, and reseated connector to the receiver/dryer. (Unplugging the connector on the receiver/dryer turns off both fans.)

Here's a typical scenario (temps in F, pressures in psi):
- Ambient air temp ~ 65
- Low side pressure = 75 psi without compressor running
- A/C compressor does not come on even though the temp setting is set to lowest value (65 F) (both fans are running)
- Run engine for ~ 5 minutes
- Low side pressure now ~ 50 psi
- Turn engine back on - A/C system works normally
- Low side pressure = 35 psi w/ compressor running and 45 psi w/ compressor off.

Since I thought the low side pressure might have been a bit too high I vented some freon to drop it about 5 psi.

A couple of other instances:
- Ambient = 72; A/C worked right away; low side 20 psi w/ compressor on; 40 psi w/ compressor off.

- Ambient = 75; A/C worked right away; low side = 80 psi with system completely turned off (I didn't take running pressures)

- Ambient = 75; A/C didn't work; low side = 78 psi with system off; after driving car, warming it up, turning it off and back on, A/C started working normally.

- Ambient = 74; A/C didn't work; driving car/warming up/restarting = A/C began working normally.

I've been through the service manual but am not an expert at A/C systems. I've not yet done all the diagnostic procedures. I've not been able to find a spec on what the low side pressure should be with a cold/off system.

TL/DR: A/C compressor sometimes does not run until engine has been warmed up and car started/restarted. Other times system works normally right away.

Any thoughts on why my system might be acting this way?
Thanks,
John
 

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From Subaru TSB:

H6 A/C COMPRESSORS CUTTING OUT

If you come across an H6 A/C system where the compressor is cutting out when the A/C is engaged, it might be the revolution sensor on the compressor that is causing the problem. To determine if this is the cause of the concern, you can find diagnostic information in the 09/01 edition of the Tech TIPS Newsletter.

Once you have determined that the revolution sensor is the problem, be advised that the sensor is now available as a separate part. It is no longer necessary to replace the entire compressor as in the past. Refer to Service Bulletin 10-74-02 for more information.

+++++++++++

DIAGNOSTIC PROCEDURE

1) Confirm customer's concern.
2) Disconnect the three (3) pole connector on top of the compressor.
3) Check the electrical continuity between (B) and (C) as shown in figure 1. (below)
4) If no continuity exists, the circuit for the revolution sensor is suspected to be open and the revolution sensor assembly should be replaced.
5) Standard resistance can be confirmed by utilizing the table.


REPAIR PROCEDURE

The repair procedure is outlined as follows:

REMOVAL

1) Conduct the following oil return operation to return the compressor oil in circulation with the refrigerant to the compressor.
a) Increase engine RPM to 1,500
b) Turn ON the climate control
c) Turn the temperature control switch to the lowest temperature setting
d) Put in RECIRC position
e) Turn the blower control switch to HIGH
f) Leave in this condition (at 1,500 RPM's) for ten (10) minutes then turn ignition switch off

2) Disconnect the GND (-) cable from the battery after noting the radio presets and their order.

3) Recover the air conditioner refrigerant using the proper recycling equipment.

4) Remove the air conditioner compressor from the vehicle after installing the compressor plugs provided in the kit and applying electrical tape over the ends of the high and low side hoses. This will prevent dirt, dust, moisture, and other foreign material from contaminating the system.

5) Clean the area around the revolution sensor.

6) Disconnect the grounding wire for the revolution sensor from its bracket and retain the screw for reuse.

7) Remove the revolution sensor connector from its bracket.

8) Remove the rear retaining clip (brown) from the connector and remove the black wire leading to the air conditioner compressor clutch from the connector. Note : perform this operation utilizing the appropriate electrical pin terminal removal tool.

9) Remove the rear retaining clip (brown) from the connector and remove the yellow and black terminal pins (leading to the revolution sensor) from the revolution sensor connector after taking note of their original positions.

10) Remove the revolution sensor from the compressor being careful not to damage the sealing surface of the compressor during removal.
 

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