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Guest Message by DevFuse

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Understanding A/C comp and ECU wiring

wiring harness A/C

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5 replies to this topic

#1 coclimber


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Posted 05 May 2014 - 11:50 AM

Can somebody please confirm/comment on (e.g., re-fine) my understanding of how the Subaru A/C compressor and ECU work.  As a background, I'm in the middle of re-wiring a Subaru harness to work with a VW vanagon, so I'm trying to figure out exactly how the Subaru wires and Subaru components will integrate with my Vanagon A/C wire and dash controls.


Here's my understanding of Subaru >>


When the driver pushes the A/C ON button:

1. A signal is sent to the A/C compressor to turn ON.

2. The A/C compressor get its power from the belt system (not a wire).

3. A signal is also sent to the ECU to tell it that A/C is ON.  This allows the brains to control idling differently while AC is on vs. off.


The process is the opposite when the driver pushes the OFF button on the dash.


So, there are two wires.

One goes to the ECU

One goes to the A/C compressor


Is this a correct understanding of how the Subaru A/C system works, in a nutshell?

So, I'm thinking I don't need to wire any pwr wires to the A/C comp.  But, I do need to wire the signal wires up.

Are the two wires the same gauge?  Seems like a simple signal could be carried on a 18 AWG wire easy enough.

Edited by coclimber, 05 May 2014 - 11:53 AM.

#2 Cougar


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Posted 05 May 2014 - 01:15 PM

The fan belt does drive the compressor but there is an electric clutch used on it that lets the pulley free wheel when the AC system is turned off. That clutch needs a fairly high current to operate it and uses a relay circuit. There is also wiring used to run the extra cooling fan needed when using the AC. I suggest you get a factory service manual for the model year of the system you are working with and study it.

#3 Fairtax4me



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Posted 05 May 2014 - 08:13 PM

There are a few wires that go to the ECU on the older models. One grounds the control side of the compressor clutch relay. Another comes from the HVAC control head to tell the ECU the AC is on.

You'll definitely want a wire diagram of the system you're using as there are multiple relays used to control the clutch and the fans.

#4 Legacy777



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Posted 06 May 2014 - 12:07 PM

This diagram is rather busy and may not be of much help:




However, what Fairtax4me posted is correct for the older (early to mid 90's Subaru's).  The HVAC panel AC output "request" actually goes to the ECU, and then the ECU has an AC clutch output that goes to the AC clutch on the compressor.  This is done so the ECU can adjust idle speed when the AC is on and so it can cut the AC when throttle reaches 90-100%.


Do you know what year/model the motor & wiring is from?

#5 coclimber


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Posted 06 May 2014 - 12:16 PM

Hi!  Yes, the Subaru is 2001 Legacy 2.5L.  After researching the van wiring schematics, it appears that relays, etc. are all on the VW Vanagon side of the wiring.  The van can handle things like fans, low pressure in the AC and adjust too.  So, I think all I need to do is splice together the ECU AC Switch wire (green/red) with the AC comp (clutch?) wire and then finally connect a single wire to the van's 12v, A/C wire.  If that is the case, it is going to be easier than I thought!

#6 Gloyale


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Posted 06 May 2014 - 08:30 PM

You just need to route the "on" iwre for the AC stystem to the A/C wire of the ECU. 


There is also an AC cut wire that ussually controls the gorund side of the A/C relay.....so when you stomp the gas.....the ECU can cut the A/C momentarily to allow full engine power.

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