Car A/C question

[pjcalla]

I know we have some mechanics on here, so I have a quick question. My A/C in my car runs well (really cold) when I am driving down the road, but as soon as I stop (idle) at a light/traffic, it gets warm. It gets cold again after I start going down the road again. Is this the compressor going out? If not, what else could it be? Thanks.

 

[Bill B]

check to make sure the fan is running and that the radiators aren't plugged. If those are ok you might need to give the freon (ok, it isn't freon but you get the idea) a charge. DO NOT get the coolant with the "stop leak" stuff in it, it will mess up your system. The stuff with the oil in it should be ok. If you are the least bit unsure, have a shop do it.

 

[tyler34]

cheapest way is to get an ac gauge and see if your low on coolant.

 

[femurphy77]

The ac system is most efficient with high amounts of air flowing across the evaporator. At stoplights the only air that is flowing thru the evaporator is the air that is being drawn across it by the fans mounted to it or the engine mounted fan. These can't compete with the airflow across it at around 30mph and higher. Air blowing out warmer at a stop vs traveling down the road is normal. This is based on most automotive ac systems. YMMV of course. Newer cars have even more efficient evaporators so the temp increase at stop may not be as noticeable.

 

[danielocean03]

check to make sure the fan is running and that the radiators aren't plugged. If those are ok you might need to give the freon (ok, it isn't freon but you get the idea) a charge. DO NOT get the coolant with the "stop leak" stuff in it, it will mess up your system. The stuff with the oil in it should be ok. If you are the least bit unsure, have a shop do it.

The ac system is most efficient with high amounts of air flowing across the evaporator. At stoplights the only air that is flowing thru the evaporator is the air that is being drawn across it by the fans mounted to it or the engine mounted fan. These can't compete with the airflow across it at around 30mph and higher. Air blowing out warmer at a stop vs traveling down the road is normal. This is based on most automotive ac systems. YMMV of course. Newer cars have even more efficient evaporators so the temp increase at stop may not be as noticeable.

Great responses. Check to make sure your fan is running, and have a qualified person check your refrigerant level.

 

[ReSSurrected]

If this is a new problem, I'd start with the suggestions of checking for fan function (should run anytime the A/C or defrost is on) and debris clogging the fins of the condenser. If those check out, THEN I'd look at system pressure.

R134a is a bigger molecule than R12 was, which makes it harder for the device that controls line pressure (either a metering block or an orifice tube) to provide adequate pressure at low speed, and still not overpressure the system at high compressor speeds. They make variable valving orifice tubes and metering blocks for some vehicles.

 

[sgreen3]

The ac system is most efficient with high amounts of air flowing across the evaporator. At stoplights the only air that is flowing thru the evaporator is the air that is being drawn across it by the fans mounted to it or the engine mounted fan. These can't compete with the airflow across it at around 30mph and higher. Air blowing out warmer at a stop vs traveling down the road is normal. This is based on most automotive ac systems. YMMV of course. Newer cars have even more efficient evaporators so the temp increase at stop may not be as noticeable.

The evaporator is located in the dash area of the vehicle, you mean the condensor. Other than that I agree with what you said

 

[Militarypol21]

Simple Resolution - Don't stop anymore.. KEEP GOING!

 

[femurphy77]

The evaporator is located in the dash area of the vehicle, you mean the condensor. Other than that I agree with what you said

DOH, I always screw that part up!

 

[sgreen3]

DOH, I always screw that part up!

Its easy to do

 

[DHolder]

Make sure the condensor is clean and not full of bugs, and that the cooling fan is comming on if so equipped. It would not hurt to check the drive belt to the compressor.

 

[Ogre]

Car A/C is overrated

 

[jfed85]

I too have this problem. Although I can hear a fan of some sort slapping around and getting stuck. Just been too darn hot to mess with it so MY a/c is windows down and 80MPH till there is a decent day t omess with it.

 

[jsunjones]

check to make sure the fan is running and that the radiators aren't plugged. If those are ok you might need to give the freon (ok, it isn't freon but you get the idea) a charge. DO NOT get the coolant with the "stop leak" stuff in it, it will mess up your system. The stuff with the oil in it should be ok. If you are the least bit unsure, have a shop do it.

I would check and make sure your fans are kicking on like Bill suggests. Had the same problem and that was it. Plus keep an eye on your engine temp, mine would overheat if I was in a lot of stop and go traffic under these conditions.

 

[danielocean03]

Good catch on the evaporator/condensor mix-up, I just blew right through that post assuming you had written that correctly.

 

[Scutter01]

The ac system is most efficient with high amounts of air flowing across the evaporator. At stoplights the only air that is flowing thru the evaporator is the air that is being drawn across it by the fans mounted to it or the engine mounted fan. These can't compete with the airflow across it at around 30mph and higher. Air blowing out warmer at a stop vs traveling down the road is normal. This is based on most automotive ac systems. YMMV of course. Newer cars have even more efficient evaporators so the temp increase at stop may not be as noticeable.

I once knew a woman who though the engine worked the same way. When her oil pump blew on the expressway, she thought she could keep the engine cool by driving faster. It worked out about exactly as you're thinking in that she had to go find at least one rod after it shot through the hood.

 

[madeuce50]

A plugged cab air filter will limit a/c cooling. The more outside air going into the evap. unit the more will come out of the vents. Plugged condenser fins will cause high head pressure and severly limit cooling. Freon should not be added into a system without first checking pressures and ambient temps. Too much freon is as bad as not enough.

 

[danielocean03]

If this is a new problem, I'd start with the suggestions of checking for fan function (should run anytime the A/C or defrost is on) and debris clogging the fins of the condenser. If those check out, THEN I'd look at system pressure.

R134a is a bigger molecule than R12 was, which makes it harder for the device that controls line pressure (either a metering block or an orifice tube) to provide adequate pressure at low speed, and still not overpressure the system at high compressor speeds. They make variable valving orifice tubes and metering blocks for some vehicles.

I've always been taught the exact opposite, R12 is a larger molecule than R134a.

AllChem Company |*| Alternative one-component refrigerants

R12: dichlorodifluoromethane CCl2F2

Dichlorodifluoromethane (R12), CCl2F2, Physical properties, safety, MSDS, enthalpy, material compatibility, gas liquid equilibrium, density, viscosity, flammability, transport properties


R134a: tetraflurorethane H2FC-CF3


Tetrafluoroethane-1,1,1,2 (R134A), C2H2F4, Physical properties, safety, MSDS, enthalpy, material compatibility, gas liquid equilibrium, density, viscosity, flammability, transport properties