A/C compressor will come on but shut off right away

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This is not about my car, but someone elses A4.

The owner says when he turns the A/C on, the compressor will start running but stops right away. He was supecting that a temperature sensor was defective, but the sensor that tells the computer ambient temps works fine (based on the temp display). The A/C is charged. What could cause this issue?
 
Im thinking the opposite... If its overcharged, the compressor will do a little work on it and then have to stop. Wont cool very well like that.

JMH
 
You could try and jump the connections of the pressure switch, and see if the compressor will keep running. Only a minute or so.
Probably just low refrigerant, though.
 
my vote is for either the cycling switch or low on refrigerant. an overcharged system is far less likely. it's probably only got about 25psig in the system (compressor off) and when the compressor starts, it very quickly drops below the low-cutoff of the cycling switch.

-Bret
 
That's how my system reacted while I was charging it from below the threshold of the cut off switch. The compressor would turn on ..suck the pressure down below the threshold ..compressor on ...off..on.. as the refrigerent charge built up ..the cycles got longer.
 
Yup most likely it's very low on refrigerant. As already stated when it's low the suction side where the low pressure switch is will get low enough pressure or even go into a vacuum and the low pressure will trip the switch.

Once the switch turns the compressor off the pressures wil begin to equalize. once the pressure comes back up to around 40 psi the compresor will come back on untill the pressure goes down again.

This is a safety feature designed to protect the compressor since low refrigerant = low or no oil delivered to the compressor.

Another possibility is that the cooling fan(s) are not coming on and the head pressure is too high. If the head pressure gets around 425 psi the high pressure switch will turn the compressor off.

Once the compressor is off the pressures will begin to equalize and when the pressur is low enough the switch will turn the compressor back on again.
 
I relayed all those suggestions and he checked again. This time he found a code that indicated a defective high pressure switch. Thanks for all your help!
 
on my accord mine turns on and off ever since we bought it, its like a energy saver thing, the fans do the same in relation to the compressor, it still blows cool air even after i got it recharged.

1991 accord.

..same as my 99 villager
 
It cycles to prevent the evaporator coil from icing up, which will start happening if the coil gets below the freezing point due to the moisture in the air condensing and freezing on the coil.

When I worked for a company that decided to use household-grade AC units to cool their data center, when fall rolled around they started having problems with the indoor coils icing up due to the low outdoor temperatures. (Let me tell you it made a mess when they iced up...water everywhere when the ice started melting).

The fix was to add a low-pressure cutoff switch wired in series with the outdoor condensing unit's fan. That way, as the pressure dropped, the fan would turn off for a few seconds, and then turn back on as the pressure rose.

The system in a car works similarly, except that instead of cycling the fan, it cycles the compressor (and the fan too, on some models). Additionally, as mentioned in a previous post on this thread, the switch provides a safety-cutoff function by not allowing the compressor to engage if the pressure is too low as a result of a loss of the refrigerant charge.
 
How are you determining that it isn't cycling?

If the outside temperature is cool enough, it should cycle. It will also be more likely to cycle with the fan control set at low in recirculate mode.
 
quote:

Originally posted by brianl703:
How are you determining that it isn't cycling?

If the outside temperature is cool enough, it should cycle. It will also be more likely to cycle with the fan control set at low in recirculate mode.


Could also be that the 300ZX he has uses one of those "variable displacement" compressors which DO stay running all the time. IIRC, valves inside the compressor vary the amount of refrigerant that run through it based on some sort of ambient temp. sensor.
 
hey guys im having an even weirder problam after putting in a can and a half of freon for some reason whenever it is sunny out the ac comes on and shuts right off if it,s cloudy or nitetime it blows cold and compressor stays i can,t figure out why it all started after i put freon in this car is a 98 pontaic grand am
 
JC1990,

I'd look towards your high side cutoff.. do you know what your high side pressures are? you've probably over filled (or there is a prob in the system) it to the point that the additional heat load of the sun pushes it over the high cutoff limit.

-Bret
 
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