A/C Compressor doesn't turn on in cold weather

[mclasser]

I know the compressor doesn't kick on when it's really cold outside as a protective mechanism. It's 15 deg today and when I turned on the defroster, the A/C light illuminated like it usually does but the compressor didn't turn on. What is usually the cut off temperature for the A/C, or does it vary from car-to-car?

 

[DriveHard]

I am pretty sure it is 32 degrees F (Zero C) that is usually the cutoff point.

 

[Rand]

somewhere around freezing usually. As there isnt much moisture in the air under 32f.

By the time you heat up 30F air to 80f+ its going to be very dry already (relative humidity).

 

[MarkM66]

It might say in the owners manual.

I have a Chevy Traverse, and it doesn't kick on in colder weather. I hate it, makes it really hard to clear the windows on a cold rainy day.

 

[rslifkin]

That's perfectly normal. In most systems, it's not a cutoff based on temperature, but as the temperature drops, the resting pressure of the system falls and eventually gets below the minimum threshold for compressor engagement and it just won't turn on. This is also why the compressor will run in shorter cycles (for a cycling-clutch type system) as the weather gets cooler.

 

[Chris142]

The pressure in the system is very close to ambient.if its 15f and the switch is a 22psi switch which most are it wont let it come on.

 

[eljefino]

Around 40'F is the cutoff. It has to do with the physics of A/C and isn't a conscious choice by the automaker, per se.

 

[IndyIan]

Is the evaporator downstream of the heater core? The AC compressor in the Focus seems to come on well below freezing, but this morning at -18C it may not of.

 

[rslifkin]

Originally Posted By: IndyIan
Is the evaporator downstream of the heater core? The AC compressor in the Focus seems to come on well below freezing, but this morning at -18C it may not of.


Depends on the car. Mine has the evap upstream of the heater core. But mine will also kick the A/C on, at least in short bursts, until it gets down to 25*F or so. And even below that, it'll kick on occasionally for a second or so due to the engine bay heating up the system a bit.

 

[440Magnum]

Originally Posted By: mclasser
I know the compressor doesn't kick on when it's really cold outside as a protective mechanism. It's 15 deg today and when I turned on the defroster, the A/C light illuminated like it usually does but the compressor didn't turn on. What is usually the cut off temperature for the A/C, or does it vary from car-to-car?


Its always right around freezing, maybe a few degrees above. The temp sensor is located different places on different cars- some have it in the evaporator, some a little downstream in the suction line. In some cases I think its all done on low-side pressure. In any of the above if the car is really solidly cold-soaked, I wouldn't EXPECT the compressor to run at 15 degrees ambient, probably not even the low-side pressure switching type. At that temperature, there's not much positive pressure in the system.

Depending on the system, you might be able to get it to run by putting it in 'recirc' and using the heater to warm the cabin air. The evaporator core is always UPstream of the heater core in the HVAC airflow so that you get maximum dehumidification performance. Putting the evap downstream of the heater core wouldn't work, since the air would never drop below the ambient temperature and therefore below the dewpoint.

 

[JHZR2]

Originally Posted By: rslifkin
That's perfectly normal. In most systems, it's not a cutoff based on temperature, but as the temperature drops, the resting pressure of the system falls and eventually gets below the minimum threshold for compressor engagement and it just won't turn on. This is also why the compressor will run in shorter cycles (for a cycling-clutch type system) as the weather gets cooler.


+1. As the underhood systems warm up, the AC will often come on just due to rising pressures. There is value to running AC in the winter below freezing, such as in a packed car with cold glass...

 

[Vern_in_IL]

So how do you use defog when it's below zero?

 

[KrisZ]

Originally Posted By: Vern_in_IL
So how do you use defog when it's below zero?


Hot air is more than enough. AC works great when it's warm and humid or raining outside.

 

[440Magnum]

Originally Posted By: Vern_in_IL
So how do you use defog when it's below zero?



The air is already depleted of water vapor when the temp is that low. Just warming the air a few degrees (with the heater only) lowers the relative humidity so that the fog goes away readily. You really only benefit from condensing water from the air on the evaporator coil when the ambient temp is above freezing.