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Mike V.
Mike V., Subaru Technician
Category: Subaru
Satisfied Customers: 24797
Experience:  Certified Subaru Technician, factory trained by Subaru.
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2005 forester,,not getting a ground signal from ecm to a/c

Customer Question

2005 forester,,not getting a ground signal from ecm to a/c clutch relay,,can ground relay and a/c works fine
Submitted: 12 months ago.
Category: Subaru
Expert:  Mike V. replied 12 months ago.

Hi there,

Do you know how much freon you have in the system?

Customer: replied 12 months ago.
no, I don't know exactly how much 134 is in system,,as I stated in my question,,when I ground the compressor relay, a/c system works fine,,am not getting a ground signal from ECM from pin 33 on plug b135,,brown wire
Expert:  Mike V. replied 12 months ago.

I understand. The computer isn't going to ground the wire if there system pressure is to high or to low. This would be if there is less then half a pound in the system, then it would be to low and the compressor won't come on. The pressure switch will stop it. That being said, if there were .3 in the system and you force the compressor on, it will blow cold.

I would try jumping the pressure switch. Jump the green/orange stripe and brown/yellow stripped wires at the connector and see if the compressor kicks on. If it does, you either need a pressure switch or you need the freon level adjusted.

I believe this is mounted either in the high pressure line or the receiver dryer.

Try the jumper and let me know.

Customer: replied 12 months ago.
green/orange wire is 12 v into switch,,brown/yellow wire is 12 v out,,which feeds the power to the compressor relay and evaporative thermo switch,,,as I said before ,,I am not getting a ground signal from the ECM,that activates the compressor relay,,my fear is there is a circut in the ECM that generates a ground to pin 33 and it's burned out,,I don't see anything in the wiring that brings a ground wire into the ecm from the a/c system
Expert:  Mike V. replied 12 months ago.


The ecm won't ground the relay if the pressure switch is no good. So, since you don't have the ecm grounding, we have to make sure the other sensors or switches aren't the ones telling it not to ground. Does that make sense? This is why I wanted to check the pressure switch first.

Now, since we know that switch is working as it should, you have to check continuity on the brown wire, from the ecm to the relay. Make sure it's not broken, if the wire is good from one end to the other, then it's likely the ecm is the culprit as you suspect.

Expert:  Mike V. replied 12 months ago.

Also, if the control head isn't telling the a/c to come on you won't get any compressor action either.

Customer: replied 12 months ago.
mike, I got my 12 hours in today, I'll talk with you tomorrow,,Lloyd
Expert:  Mike V. replied 12 months ago.

Sounds good Lloyd, hopefully we can get this worked out.