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Guest Message by DevFuse

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A/C no power to relay, which wires to bypass pressure switch?

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6 replies to this topic

#1 uniberp



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Posted 16 October 2017 - 09:47 AM

2008 Forester Auto n/a. A/c compressor doesn't come on, in cool or defronst mode. The switch light is on.


The a/c compressor doesn't switch on. Static pressure is about 30lbs, and the pressure seems to cycle correctly when the relay is bypassed, down to (about 20?). 


Pull the relay and cross the power directly, it works.

Swap the relay with one of the headlight relays, still nothing.


The pressure switch connector has 4 wires. I tried crossing every combination. Apparently one combination blew the fuse under the dash panel.

Nothing switched on the compressor.


Any thoughts other than low freon?








#2 1 Lucky Texan

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Posted 16 October 2017 - 10:29 AM

9 year old car. Has the system ever been serviced? you could easily have lost refrigerant in 9 years.

#3 john in KY

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Posted 16 October 2017 - 12:18 PM

Pull the relay and cross the power directly, it works.

Swap the relay with one of the headlight relays, still nothing



Sounds like the relay isn't getting its "switch" voltage. The relay should have 4 terminals. One always hot and another only hot when the A/C is turned on from inside the car. Could use a test light to check the 2 hot sides. Also on the "switch" side, one terminal is a ground. 


With the engine running and the A/C on, 2 of the 4 wires at the relay will be hot. If so, then problem is no ground. 

#4 Rooster2


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Posted 16 October 2017 - 05:55 PM

If it were me, I would add a can of 134A freon to the low port of your A/C system. It is possible that the compressor will not turn on because you are low on freon. I have seen this happen on other cars, that were not Subarus.

#5 Fairtax4me



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Posted 16 October 2017 - 11:05 PM

Static pressure measured with what? Can guage?
30 is around normal low side operating pressure.
Static pressure should be ~75 psi depending on temp.

Pressure switch is a dual switch, has two sets of contacts inside. One set is relay ground through the ECU, the other set tells the ECU if the system pressure is too low or too high.

Some newer cars actually have a transducer so the ECU can actually read system pressure, not sure if the 08 has that or not, but if it does jumping it won't help.
You can jump a dual switch but have to jump both sides correctly or the ECU won't close the ground for the relay.

#6 golucky66


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Posted 17 October 2017 - 06:49 AM

A/C is one of those systems that its hard to diagnose and work on without the proper equipment. And 95% of DIY's aren't going to have it.

You might get it to work, you might not.

I'd recommend getting it to a A/C certified shop. But that's just me.

#7 golucky66


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Posted 17 October 2017 - 06:51 AM

And if you do use a can of r134a.

For the Love of god, find a can that DOESNT have sealer in it.

It can mess up 5,000$ a/c machines and cause the shop to need to have to call someone out to service it. And your car isn't fixed. The right way to deal with sealer in the system is to by special filters for the ac machine and filter all of the sealer out before proceeding.

Edited by golucky66, 17 October 2017 - 06:52 AM.

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