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Discussion Starter #1
Hi there, my A/C compressor is kicking in and I'm at a loss for how to fix it.

I figured it just needed to be regassed so I took it to the shop and they said it was fine. They did some poking around and said that it was an electrical issue and it'd be a $90 minimum diagnostic fee in addition to the $40 for the AC check. For that kinda money, and for how common A/C issues are in Florida, I'd rather learn how to do it myself.

To that end I did a deep dive into common problems and troubleshooting and after chasing down the likely suspects I still don't have a fix.

System: 2016 Ford Fiesta SE 1.4 4-Cyl, manually controlled A/C

Symptoms: When I turn the A/C on the A/C light on the dash comes on but the compressor doesn't kick in. The blower just blows whatever the current air temp is. The failure was sudden not gradual.

Trouble Shooting So Far:
  • Freon gas is fine (according the A/C shop).
  • When the bridge the compressor it kicks on.
  • I checked the engine bay and and driver side fuses and they look fine.
    The A/C relay under the hood has a twin that operates another system, so I swapped them to see if that would fix the A/C (and therefore indicate a bad relay) but no change.
  • I read someone had a problem with the high pressure side sensor and mine showed some wonky resistance readings so I bought a new one but no fix.
  • I traced the wires and couldn't see any obvious breaks.
  • I ran checked the DTC and it's flashing 2-2 (blender door circuit) but I don't really see how the blender door would cause the compressor not to kick in, so I think it's a red herring - i.e. the system sees that the temp isn't matching the selected range and thinks that the blender door isn't working.
The Chilton DIY online manual has a pretty extensive set of docs on the A/C system and looking through them, there are a few sensors I can check.

When an A/C request is received by the PCM , the PCM engages the A/C clutch relay when:
  • the refrigerant pressure is not excessively high or low.
  • the engine coolant temperature is not excessively high.
  • the ambient air temperature is above approximately -6.6°C (20°F).
  • a Wide Open Throttle (WOT) condition is not present.
  • the evaporator discharge air temperature is above approximately 1.5ºC (35ºF).
I don't think the engine coolant temp is too high - there would be other indicators of that.
I don't think it's the WOT condition either but I'll hold that in reserve.

Looks like I need to check the ambient air temp sensor - that should be easy though, just gotta press the dash button until it changes to show the outside air temp. I figure if my dash is showing an accurate temp then that sensor is fine.

It could also be the evap temp sensor. From the description of that: "The sensor varies its resistance with the temperature. As the temperature rises, the resistance falls. As the temperature falls, the resistance rises." I.e. if something inside the sensor broke and caused an airgap then the resistance could be super high and the computer thinks that the evap coils are hella cold.

I think the easiest way to diagnose things right now would be start bridging:
  • Does the compressor switch on if you bridge the relay?
  • Does the compressor switch on if you bridge the evap temp sensor?
  • Does the compressor switch on if you bridge the pressure sensor?
I'm hesitant to start jumping thought since I'm not super familiar with the pins and wiring directions and I recall hearing that you shouln't start bridging things unless you know exactly what you're bridging. Any one want to explain how to bridge these (i.e. which pin ports should I stick my paper clip into)?

Once I bridge things I think I'll have a better understanding of which circuit has the problem.
 

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Discussion Starter #2
Well, as is normal in these kinds of things, the last thing on the list was the problem. The evap temp sensor was jacked up. It's on the passenger side around where the passengers feet might be. I imagine someone managed to kick the **** out the thin plastic cover over the sensor and bust it out of position. I put it back as best I could and the compressor kicked on first try. I've got a new sensor on order to replace the twisted one but it's an easy job and a cheap sensor. I also suspect that the pressure transducer was rotten - it was showing resistance well above the limits noted in the Chilton guide, so one or both sensors were the problem.

Would have saved my self a month without AC if I had just bit the bullet and read the darn shop manual for a description of the compressor dependencies. Admittedly most of that month was under lock-down in mild weather so I wasn't particularly pressured to pour over documentation. One more problem bites the dust and a good lesson get's reinforced - study the docs first.
 
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