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I have a 2000 Honda Civic 1.6L iSR MTX that I bought recently and replaced the engine in. Haven't driven it much, but got the impression the A/C was not working so well. However, I took it on a long trip recently, and on the first 230 kilometers the A/C worked beautifully. Then I stopped someplace, got back in the car to drive again and saw the A/C wasn't working at all. As in the A/C indicator light comes on, air blows, the A/C radiator fan comes on, but the air's not being cooled at all.

What I noticed was that when I engaged the A/C, I didn't feel any additional load being put on the engine. Normally, I would expect to see and feel a slight dip in rpms as the compressor comes on, but before the IAC opens up to compensate.

This lack of any noticeable additional load and the fact that the A/C operated beautifully for the first 230km of the trip makes me suspect the compressor clutch isn't engaging for some reason.

Does this seem like a reasonable guess, how can I verify this, and what are my repair options? Is this a diy kind of job?

EDIT

I just took the car for a drive and decided I'd try the A/C again just for the heck of it and it started up and gave me nice, ice cold air. When I stopped to park, I did a little test where I turned on the fan full blast, waited a few seconds for the rpms to settle, then turned the A/C on. This time I could feel and hear the compressor engage with a kind of clink. Yesterday when it wasn't working I had the hood open and was standing right over the compressor when I had my wife turn on the A/C, and I didn't hear anything.

So it seems like I've got some kind of intermittent compressor issue. Maybe a loose wire preventing the clutch from engaging? What should I check?

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    The air con controller wont engage the clutch if the refrigerant is low on pressure. That would be the first thing I would check. – HandyHowie Sep 7 '16 at 9:46
  • @HandyHowie But wouldn't I have to have a massive, catastrophic leak in order to go from it working so well to it not working at all in such a short period of time? Also, it kinda went from not working so well, to working great, to not working at all. That doesn't sound like a leak to me, even though I have no real expertise in this area. – Robert S. Barnes Sep 7 '16 at 11:02
  • It wouldn't take long for the gas to escape even from a tiny perforation. It may be a different fault that caused it not to work very well before, but once it started working properly, a weakness somewhere could have turned into a leak. A quick press on one of the gas ports will indicate whether you have much pressure in the system (with the engine stopped). If you just get a feeble hiss, then you are low on pressure, – HandyHowie Sep 7 '16 at 11:45
  • @HandyHowie I just took it for a drive and the A/C is working beautifully again. So I think it must be something other than a leak. – Robert S. Barnes Sep 7 '16 at 12:23
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    I agree. Maybe you have a bad connection on the low pressure sensor connector or the compressor clutch actuator connector. – HandyHowie Sep 7 '16 at 12:25
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The most likely cause is intermittent failure of the compressor relay. Find it in the underhood fusebox, remove it. It will be labeled Mag Clutch or have a picture of a snowflake. Then swap it with one of the same part number in the same box. Then test to see if the compressor engages. Your "I feel the load" test is fine but you can see the clutch come on by watching the clutch end plate on the end of the compressor.

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OK, so the problem turned out to be a torn wire on the compressors' thermal protector:

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The two ends where being held together by the red cloth casing and making intermittent contact. I had to finish tearing it to get it out.

I just stripped the ends of the wires, twisted them together, added some solder, taped it up with electrical tape and reinstalled the thermal protector. Now the A/C is working fine.

I followed the "clutch not engaging" flowchart in the Civic WSM which is basically:

  1. Check the dash and under hood fuses
  2. Check the relay
  3. Check for battery voltage on the relay plug
  4. Short B+ to the clutch wire to see if the clutch closes
  5. Check for continuity on the thermal protector and resistance of 3.05 to 3.35 Ohms on the field coil

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