A/C Air Condition Aircon compressor power

[unibrook]

2001 Forester 100k miles.

A/C switch light comes on when pressed, but compressor does not engage.

Multimeter test to compressor shows no power.

I switched compressor relay, but still no power to compressor.

Power to Low pressure switch is ok at 14.2v. And I jumped the two wires in it to see if that would cause power to flow to compressor, but no.

 

What next? Are there other fuses or relays in the system that I haven't found yet, that I should test?

 

Should I jump a wire straight from the battery pos terminal to the compressor connector to see if it will engage that way? and to see if refrigerant is still up to good pressure?

[Fairtax4me]

Use a manifold gauge to read system pressures, trying to check it by tricking the compressor into turning on tells you nothing.

 

There are several relays for the AC system IIRC. I don't remember of hand exactly what each one does, but you may be able to find out by searching the boards here for an online copy of the service manual for your car.

 

Where was the switch that you jumped? The pressure switch is mounted on top of the accumulator/drier on the passenger strut tower.

 

Be sure to check any fuses related to the AC system in the main fuse/relay panel.

[OB99W]

Let's start with a simple test. With the ignition switch OFF, disconnect the harness from the pressure switch (top of the receiver/drier). Using an ohmmeter, check continuity between the terminals of the pressure switch -- it should be very close to zero ohms. If there's no continuity (open circuit, infinite resistance), the refrigerant charge is likely too low -- it requires at least 31 PSI to close the switch, and considerably more will be there with the system off (static pressure) under normal conditions.

 

If the pressure switch is closed (indicating refrigerant charge level isn't the problem), let us know.

[unibrook]

Use a manifold gauge to read system pressures, trying to check it by tricking the compressor into turning on tells you nothing.

 

There are several relays for the AC system IIRC. I don't remember of hand exactly what each one does, but you may be able to find out by searching the boards here for an online copy of the service manual for your car.

 

Where was the switch that you jumped? The pressure switch is mounted on top of the accumulator/drier on the passenger strut tower.

 

Be sure to check any fuses related to the AC system in the main fuse/relay panel.

 

Yes. The low pressure switch is atop the dryer canister. That is the one I jumped. Will check the fuse in the cabin also.

[unibrook]

Here are a couple of good clips showing multimeter testing of AC electronics:

 

 

[unibrook]

ok, so I hooked up the pressure gauge to the low pressure port and it read zero, so I figured I would inject a 3 ounce can of R134a with red dye into the system to see if compressor would at least turn on. And it did. The pressure gauge read 39 lbs when finished injecting refrigerant into the system. The high pressure line felt nice and hot. But the return low pressure line never got cold.

 

After several minutes of running the AC on High, still no cold air coming out of the vents. And I could not see any red dye leaks anywhere in the system in the engine compartment. I suppose it could be leaking near the evaporator inside the dash. Any ideas? Should a 3 ounce can of R134a have bumped the system pressure from 0 to 39 lbs? Should I go ahead and inject another can of refrigerant into the system to try to get cold air?

What else could be the culprit? Some sort of valve/switch near the evap? Line clogged?

[OB99W]

[...] Should a 3 ounce can of R134a have bumped the system pressure from 0 to 39 lbs? [...]

Was that a static or dynamic pressure reading? What was the ambient temperature? Would you like links to some info that might help you better understand a/c?

[unibrook]

That is the pressure at the low pressure port with the system turned on.

 

Ambient temp was 65F.

 

Yes, plz, on links or ideas.

[OB99W]

[...]Yes, plz, on links or ideas.

Okay -- I prefer "teaching to fish" as opposed to "giving a fish".

 

http://www.motor.com/magazine/pdfs/041999_03.pdf (theory/diagnosis)

http://www.motor.com/magazine/pdfs/041999_07.pdf (inspection)

 

http://www.aircondition.com/tech/questions/82/

http://www.autoacrepairs.com/gauge_reading_and_diagnosis.htm

http://www.chillers.com/PT%20charts%20for%20refrigerants.htm

http://www.aa1car.com/library/ac98.htm

http://www.aa1car.com/library/ac_recharging.htm

 

Last but not least, lots of articles here:

http://www.endwrench.com/archive/sysheat.html

[unibrook]

I took it to an inde AC specialist shop in Medford, MA.

They found a leak in the "discharge line" that runs from the compressor to the condensor. They replaced it and recharged it, and now it chills like new.

I asked if they would replace the receiver/dryer at this age, they said no...unless system was wide open for a long time, and this one was not.

 

Funny how the magic price around here is always $514.00 to fix an AC leak.

Cost me the same 2 yrs ago to have a diff shop replace the condensor.

 

Ridonkulous $$. But I don't have room to store lots of tools to do it myself.