A/C Low Pressure Switch
 

04 Lancer Sportback LS - Is there a (secondary) low pressure switch inside the compressor preventing the clutch from engaging? If so, can it be jumped and how?

It's probably time for a compressor but would love reassurance. I just purchased an 04 Lancer Sportback LS knowing it was blowing warm air. Per the salesman a schrader valve was recently replaced but a turn of the dust cap confirmed it was still stuck open. Figured it was a cheap & easy fix. I don't recall the compressor cycling before evacuating but wish now I had paid more attention.

After evacuating the system it will not take any freon because the clutch will not engage. The compressor is getting voltage when the A/C dash switch is on but none when it is off. The same is true with the fan. There is a switch along the right fender on the high pressure line. I have to assume this is the low (and high) pressure switch. Disconnecting this switch kills power to the compressor and the fan. Static pressure is 20 lbs on both sides so I assume the orifice is OK. I switched around a few relays to confirm and the fuse is fine.

A new compressor then? Am I missing anything else obvious like a fuse on the compressor or a reset switch? Thanks for the help.

Chris
95 Montero SR (replaced the ac compressor 10 years ago and still cold)
04 Lancer Sportback
10 Outlander LS 4WD

20lbs on the high side is too low.

if there isnt enough pressure the compressor will not turn on.

so my next question is how are you testing for voltage at the compressor? becasue if there is not enough pressure in the line the pressure sensor will not allow the compressor to turn on.

is the compressor block grounded or not? if not check the ground for power, as some pressure sensors actually control the - not the hot side

Thanks for the response. I was checking voltage at the single (hot) lead into the compressor. I wasn't able to safely check it at the clutch pulley. In hindsight I should have checked resistance from the compressor block to the engine block since there is no (-) wire out. It a great tip for future diagnosing.

I was puzzled about the low pressure and why the compressor would not turn on. I could not get the pressure any higher, though, so I had to assume something electrical was going on (or not going "on") inside the compressor.

Desperate for a/c and for lack of being able to get more than 20 lbs into the system I chanced a $140 junkyard compressor. After swapping, it charged perfectly and blows super cold. I'm still curious if the compressor has an internal pressure sensor but I'm pretty sure now that it was just the clutch. Maybe a bench test will confirm.

Lesson learned: Never assume the obvious problem is the ONLY or most serious problem. So take lots of measurements BEFORE starting, even if doesn't seem relevant to the repair.

Thanks again.