Rick's Free Auto Repair Advice

Car AC blows warm at idle

AC blows warm at idle — 4 reasons

If your car AC blows cold when you’re moving but the AC blows warm at idle, slowdowns or stop lights, it can be caused by any of the items listed below. Start by checking the radiator fans

#1 cause of AC blows warm at idle

Radiator or condenser fans that aren’t running or aren’t running the proper speed can cause an AC blows warm at idle condition

If your radiator fans aren’t working or aren’t working at the proper speed, the condenser can’t properly cool the refrigerant. Lack of cooling means the refrigerant can’t condense and turn back into a high pressure liquid. Pressures rise in the condenser and the high pressure switch turns off the compressor clutch or signals the compressor control valve to stop pumping refrigerant.

However, as soon as you star moving, vehicle speed forces air across the condenser coil which reduces pressure and your car’s AC starts cooling again.

Carmakers use three different setups for cooling the radiator and AC condenser; two separate fans running at a single speed, two fans running at two speeds or variable speed, or one fan that has multiple or variable speeds.

In a two-fan setup, carmakers often use one fan is used to cool the radiator and the second fan only when you turn on the AC. That pulls more air across the condenser and radiator. The condenser and radiator. In those setups, both fans are on usually mounted on the engine side of the radiator. But in others, one fan is on the engine side of the radiator and the AC condenser fan is mounted in front of the condenser (behind the grille).

GM and others use either a three-relay set up to run the two fans at low speed for normal driving and high speed when the AC is on or the low speed can adequately cool the radiator. Other carmakers use a solid-state speed relay that varies the speed of the fans based on engine cooling and AC needs.

What’s the point of this? If your AC is on and the fans aren’t running at high speed or one fan isn’t running at all, AC condenser temperature and pressure will rise and the high-pressure switch will turn off the compressor. If the fans aren’t running at high speed or one fan isn’t running, your AC may blow cold air when you’re moving because enough “ram” air is coming across the condenser coil to keep temperature and pressures low. But when you stop moving, the temps rise and the compressor shuts off.

So check both fans and make sure they’re both running a high speed if your AC blows warm at idle.

#2 cause of AC blows warm at idle

Your AC system is low on refrigerant. Since the AC compressor is driven by the engine, it doesn’t pump as much refrigerant when you’re at a stop or driving slowly. If the system is low on refrigerant, it will show up as AC blows warm at idle and blows colder when the engine is running at higher RPMs.

Get your system refrigerant level checked.

#3 cause of AC blows warm at idle

Your engine is overheating

Your engine computer constantly monitors engine temperature and commands the radiator fans to speed up if it senses coolant temps rising. However, if the computer has the fans running at full tilt but the engine temp is still too high, the computer will shed heating load by turning off your AC. That automatically reduces air temp coming into the radiator by about 20° and helps the computer maintain safe coolant temps.

#4 cause of AC blows warm at idle

Clogged condenser

If you recharge your AC system regularly without actually fixing the leak, the air in the AC system can react with the oil and refrigerant, causing sludge buildup that settles in the condenser and evaporator. Your AC compressor may be able to circulate more refrigerant through the narrow passages when you’re driving at highway speeds, but may not be able to pass enough through at idle speeds.

Conduct a superheat test to determine condenser efficient.

©, 2020 Rick Muscoplat




Posted on by Rick Muscoplat

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