A/C off but clutch engages

Tiny
STEVELEGGIT
  • MEMBER
  • 2009 MITSUBISHI COLT
  • 1.3L
  • 4 CYL
  • 2WD
  • MANUAL
  • 75,500 MILES
At idle if I switch my A/C off the the compressor clutch keeps clicking in and then out every 20 or so seconds. The A/C dash light correctly remains off. Other than that the A/C appears to work fine though I have had difficulty clearing a misty windscreen first thing when cold. A few months back I was getting a small amount of frost on the inside of the screen from using the A/C with heating. Any suggestions gratefully received.
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Wednesday, February 19th, 2020 AT 7:42 AM

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Tiny
KASEKENNY
  • EXPERT
Your vehicle should have a humidity sensor which we need to monitor to see what it is reading. The control module will monitor the humidity of the air that is blowing through the system and if it is high in humidity, it will run the compressor. The A/C compressor not only cools the air but it dehumidifies the air. This means it removes moisture.

So this means one of two things. Either you have a higher amount of humidity in the HVAC system normally from a heater core leaking.

The other possible cause, is the humidity sensor is just wrong and it is cycling the compressor when it should not.

Unfortunately we don't have technical info on your vehicle as we don't offer it here in the USA but Mitsubishi in general applies this strategy in all their products.

I would start with checking for codes and then if you have a scan tool see what the humidity sensor is telling the HVAC control module to see if it is accurate.
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Wednesday, February 19th, 2020 AT 3:26 PM
Tiny
STEVELEGGIT
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Hi, thank you for the quick response, you guys are really helpful.

No codes recorded for anything and AC is showing as N/A.
No humidity sensor but there is an external temperature sensor mounted in the front fender and a thermistor in the cab heater unit. There is a coolant temperature sensor on the AC compressor and just a pressure sensor on the AC high side pipe. I have some info for a very similar variant of the car.
The coolant level in the radiator has never required topping up and appears normal and I have not noticed any antifreeze odor in the cab so I don't think the heater matrix is leaking.
However, as part of these checks I have noticed that I have an engine coolant thermostat that appears reluctant to open. The radiator bottom hose is cold for too long. Should start opening at 82 C but looks like at least 90 C measuring temperature at / adjacent of the thermostat housing. The dash overheat warning light (no gauge just blue and red indicator lights) only comes on when I first start the engine, presumably as a check. The cabin heater churns out hot air when required but this is not so good as it was at middle setting (maybe my memory playing tricks) (I've never had the need to have the heating set to full). This indicates to me the water pump is working okay, pushing coolant through 3/4" heater pipe and not thermosiphoning, but the temperature may be reduced a little by the AC compressor cycling and dumping cold into the heater unit even though AC is turned off. Presumably an internal baffle in the heater unit would prevent most of the cold air mixing with cab air - that does raise a question, what happens to any water that condenses on the evaporator under this circumstance, could this be the source of moisture?

My priority is to change the thermostat this weekend. That should make it easier to see the wood for the trees. Would higher than normal coolant and therefore engine temperature cause cycling of the AC compressor, even with the AC turned off?
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Friday, February 21st, 2020 AT 3:31 AM
Tiny
KASEKENNY
  • EXPERT
Unfortunately the coolant level would not effect this.

If this is cycling all the time and you don't have a humidity sensor then I would suspect the engine control module is cycling it improperly. Basically the ECM commands it on and when it sees the pressure in the AC system reach a limit it shuts it off. The fact that it cycles points more to an input telling the ECM to command it on. I would suggest hooking up an AC gauge set and seeing what the pressure is when it shuts off. I suspect the pressure is reaching close to max pressure and if this is the case then the pressure is what is telling the ECM to shut it off.
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Friday, February 21st, 2020 AT 4:29 PM
Tiny
STEVELEGGIT
  • MEMBER
Many thanks for your time and patience.

Installed a new thermostat this morning. The AC compressor clutch no longer cycles, engaging and disengaging, when the AC switch is off.
However, when the AC is switched on the cycling restarts, at least at idle, not noticeable when driving.

I only have a low DIY filling kit with low pressure gauge so did the best of a bad situation and connected it up. Low side, at 12C (54F) with the AC on 20/25 Psi, and then when the clutch disengaged it went up to 40/45 Psi. This seems normal? I should add I have never recharged the AC on this car.

When test driving with the AC off, the heater seems noticeably cooler, unless turned full on when it is really toasty. The old thermostat was reluctant to open early so maybe a bit of over-temperature.

Got some technical info for the car off the web which seems to support what you say. It states:
- Compressor Energy Conservation Control. Optimum ON/OFF control of the compressor is achieved according to the outside/inside air mode air temperature (air temperature sensor), and insulation.
Also
- Detection Control For Refrigerant Leaks. When it is judged from the air temperature (air temperature sensor) and the refrigerant pressure (A/C pressure sensor) that the refrigerant amount is the specified value or less, or the refrigerant pressure is abnormal, the compressor is forced to be cut off to protect the A/C system.

What next please?
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Sunday, February 23rd, 2020 AT 2:03 PM
Tiny
KASEKENNY
  • EXPERT
That is progress. At this point, we need to find a tool rental store so that we can get a vacuum pump and a real gauge set. I assume you are using the gauge on one of those refill bottles. If that is the case then that is not going to tell you anything worth while.

Most parts stores like AutoZone have tool loaner programs so you can use it free as long as you return it.

We need to start with getting all the refrigerant recovered because you cannot vent it to the atmosphere. So most shops will recover this for you at a low cost and then put the same amount back in. We need to do this for two reasons. First we need it out of the system, so we can pull it into a vacuum to check for a leak. Second they will tell you how much they recovered so we know how much was in the system.

Then pull it into a vacuum using the pump that you rented. It should hold for about an hour with out losing vacuum. If it doesn't then you can put the correct amount of Freon back in. If it does then you have a leak that you need to find.

If you just refill it with those bottles and you don't do this and you have a leak then that will get costly when it all leaks out again.

https://www.2carpros.com/articles/re-charge-an-air-conditioner-system
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Tuesday, February 25th, 2020 AT 4:37 PM

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