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Old 05-12-2017, 07:47 PM   #1
SimplyMarkB
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Angry Random on and off A/C

Hey- I have a 2009 Mustang GT and when I turn my A/C on it runs for a little while and then turns off on it's own. About 10 seconds later (or longer) it will come back on. Sometimes it won't come on at all and then when I drive my car the next day it will work.

I am guessing that this is due to a faulty sensor somewhere? Does anyone have any insight?

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Old 05-13-2017, 06:55 PM   #2
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It could be a faulty pressure sensor, or it could just be low on refrigerant.
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Old 05-15-2017, 01:25 PM   #3
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Refrigerant was just filled at a mechanic. Will a faulty pressure sensor show up as a OBDII fault? I was thinking maybe the relay or the blower motor itself. I just don't want to arbitrarily replace components.
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Old 05-17-2017, 01:32 PM   #4
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Refrigerant was just filled at a mechanic. Will a faulty pressure sensor show up as a OBDII fault? I was thinking maybe the relay or the blower motor itself. I just don't want to arbitrarily replace components.
Get a set of manifold gauges to attach to your AC and see what's actually going on. Otherwise you will be just throwing parts at it.

No, AC system won't throw an OBDII code. Codes are typically drivetrain, emissions, or safety equipment related. Not auxiliary systems.

If a mechanic just checked out the system, have them recheck it. They should have some type of warranty for their work.

AC pressure sensors rarely go bad, it's usually a leak (which causes low refrigerant), a clog (like the orifice tube), or a worn compressor or clutch switch which sees way more heat and dirt than the pressure switch tucked in with the receiver/drier.
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Old 05-22-2017, 08:53 PM   #5
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If they just had to fill the system, that would probably mean tere is a leak somewhere. When they put the new refrigerant in, did they put in the dye to help locate leaks?
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Old 05-24-2017, 02:21 PM   #6
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If they just had to fill the system, that would probably mean tere is a leak somewhere. When they put the new refrigerant in, did they put in the dye to help locate leaks?
That is a good point. You can use a black light and orange/yellow tinted glasses to find a leak if there is one. Thats only if they used dye and only if you have a leak. You can pick up a black light and glasses at a parts store for about $15-20 or order it on amazon for about half that.
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Old 05-27-2017, 06:21 AM   #7
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Refrigerant was just filled at a mechanic. Will a faulty pressure sensor show up as a OBDII fault? I was thinking maybe the relay or the blower motor itself. I just don't want to arbitrarily replace components.
Wait are you saying that the blower inside the car is what is shutting off?
I guess I was assuming you meant the AC compressor was kicking on and off.

If its the blower that is randomly stopping and starting then you have a problem in the either the control panel or a loose connection somplace in the blower motor circuit.


The AC compressor will kick on and off when refrigerant starts to get low, ususally when the pressure switch goes out it cant read how much pressure in in the line so the compressor wont start at all.
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Old 05-29-2017, 05:51 PM   #8
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It could be the engine fan control module. Is the car overheating? Mine would shut off the A/C for no reason until one day it started overheating and it turned out to be the fan control module. I just replaced the whole fan with a GT500 one.
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Old 07-15-2017, 07:25 PM   #9
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Default Relay? Nope!!

I replace the relay and it still doesn't turn on. It's strange because every now and then out of the blue it'll turn on run for about 5 to 10 seconds then immediately turn back off. Sometimes it'll turn on and run for 10 to 20 minutes and then turn off other times they won't run for days at a time. But at least I know it's not the relay.
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Old 07-16-2017, 06:09 PM   #10
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It could be the engine fan control module. Is the car overheating? Mine would shut off the A/C for no reason until one day it started overheating and it turned out to be the fan control module. I just replaced the whole fan with a GT500 one.
I did notice at times that the hotter it was outside, the less it seemed to work. For example, the times it DID were usually after sundown. Coincidence?
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