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Discussion Starter #1
2010 MKS Ecoboost;
I've notice my car is not properly cooling the cabin and narrowed it down to the A/C compressor not staying on. Sometimes it works more or less okay, and when the compressor is on, the air is nice and cold, but other times the compressor comes on for about 1 second, and then stays off for 9 seconds, and repeats. Obviously this is basically worthless for cooling the car, so I'm trying to figure out what the deal is. The system has a good charge and runs around 45 psi at 80 degrees ambient on the low side when the compressor is engaged, so I think I'm looking at a bad sensor.

After digging around, I found some threads on a Flex forum about bad evaporator temp sensors and a TSB applying to 2009 MKS' built before 4/09, but my car was built 3/10. I'm still inclined to think this temp sensor or one of the pressure sensors is bad and will probably replace the evap temp sensor as it's low hanging fruit.

What sensors does this car have that control when the compressor is engaged? High, low, and evap temp? Are there any common issues I should be looking for? Are there any diagnostic HVAC modes I can access or test procedures available? Like I said, if I could get the compressor to stay on, it would cool the car just fine, but as it is, the <10% duty cycle is pretty worthless.
 

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A good set of gauges and a scan tool (or FORSCAN) could help you validate pressure sensors. Compare actual readings with scan data. If the evap temp sensor is white: replace. If it's black, it might be faulty.
 

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Thanks for the info! I didn't think about using forscan. Can it read fault codes in individual modules? I did scan with torque, but unsurprisingly, nothing came up.

The temp sensor is black, but for 10 bucks I went ahead and ordered a new one just in case. I'm hopeful, but will start digging through forscan after work to see what I can find.
 

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Discussion Starter #5 (Edited)
Well i ran forscan and monitored as many A/C related PIDs as I could find, and it seems to me that the data confirms the compressor clutch is working when requested and the temperature sensor seems to be reading about what I would expect. The temperature was only around 65 today, but the compressor was again only staying on for very short bursts as shown. I need to get a hold of some pro gauges and compare their readings to what the data is showing, but I otherwise have no idea what I'm looking at pressure wise. Can anyone comment on what this is showing? I'm assuming this is high side pressure, and assuming that it's cutting off the way it should when it exceeds a certain pressure and I should therefore be looking at a possible faulty pressure sensor?

Also, the temperature I was monitoring was "Ambient Air Temperature." Is that the Evaporator temperature sensor or is that the interior temp sensor?
 

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Ambient air senses outside air temp. Your data shows high side pressure of 3336 kPa = 483 psi, too high! Either sensor is wrong, system is overcharged, or there is a blockage in the system from sealer. Put gauges on and compare data to gauges.
 

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Besides checking what has already been suggested check your condenser for debris and check to see if the fans are operating.
 

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Discussion Starter #9
I replaced the temp sensor for the evaporator and hooked up some gauges to the car to see what was happening. It's only around 60 degrees today, so I'm not really sure how the AC should be acting in these temps, but it seemed to be working about how I would expect. The compressor was on more than it had been, and the vents were putting out very cold air. The pressure read out on the computer seemed to match that of the high pressure side gauge, and the low pressure dropped to around 25psi when the compressor was running and up to 40psi after it cut out. It seems to recover quickly enough and doesn't match the symptoms of a clog from what I've been able to find, but maybe the low temperature outside right now is throwing me off.

Oh, and I did verify that the fans are working correctly and that my condenser is clean and unrestricted airflow-wise.
 

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Most recent data looks pretty normal, are you running blower on high and full cold both sessions? Your system uses a TXV (thermostatic expansion valve) that monitors evap-out temp and modulates evap-in flow. If you have low blower speed it will restrict flow and run high side pressures higher. While it's uncommon for these to fail, it's possible this is your problem.
 

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I had my blower manually cranked to the highest setting this time around. While I don't remember exactly, it's entirely possible that my blower speed was on the low to medium settings during the first data log, but both times the climate system was set to "MAX A/C." It seems to me that MAX A/C should crank the blower to max, but my climate control system never wants to bring the blower speed above 50% automatically, and anything above that has to be manually adjusted. Does this sound like a failure of some sort?
 

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Well, after all this troubleshooting effort, it's turns out the system was just overcharged. I drove about 90 minutes away in 90 degrees stuck in traffic, and when I got there I decided to grab a screwdriver and just engage the Schrader valve in the chance that I could just get it to engage the compressor at all. Two good purges and the compressor engaged and stayed engaged. It was acting up again on the way home from work, with temperatures getting up to 97 degrees, so I let out a bit more pressure and the compressor kicked back on and stayed on to blow icy cold air in the sun. I'm now running 30ish psi on the low and 250psi on the high in 95 degrees, which seems a lot more reasonable. I also noticed that the radiator fans kicked down in speed after letting some pressure off, and the "auto" fan actually went all the way up to full blast in "max a/c mode". Apparently it's all connected!

Thanks for the help, guys!
 

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How did it get overcharged?
When I first popped the cap off, I noticed that it appeared the system had UV dye added to it at some point, so I suppose it was overcharged when I purchased the car second hand.

And don't worry guys, I'll capture a few cow farts to make up for my horrible travesty of venting an ounce or two, and take all my old vehicles that are at risk for leaks to preemptively have all their refrigerant evacuated and tell all my friends to do the same. 👍
 
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