Auto Service World
News   October 13, 2022   by Tim Miller

From the magazine: Car AC not blowing cold air: Diagnoses and fixes

Having an AC that won’t blow cold air is frustrating in summer’s heat. Find out how to diagnose and fix a car with this issue in a few steps

A car AC not blowing cold air is one of the most common problems a driver will face. Some ACs blow warm air while others blow moderately cool air. Either way, the driver’s dealing with a malfunctioning AC.

A clogged filter, bad AC compressor or refrigerant leaks could be the problem. Hence, instead tolerating an uncomfortable car, it’s best to diagnose the issue and find a fix for your customer. Let’s go through the easiest methods of diagnosing a car AC that’s blowing warm air so you can repair it properly.

You could even save time and their money if you can figure out the proper fix.

Cooling Fan Problems

Vehicles use cooling fans to move cold air into the cabin. If you’ve set the air conditioning to the max and the fans are on the high setting, but the air is moderately cool, the cooling fans could be the culprit.

A vehicle has two cooling fans — one is the AC condenser fan, and the other is the radiator fan.

Here are some of the symptoms of a bad AC condenser fan:

  • LUKEWARM AIR: The first symptom of a failing AC condenser fan is lukewarm air.
  • BURNING SMELL: The vehicle has to release any hot air coming from the system. When it cannot remove the hot air, the AC parts become so hot that they burn and emit an acrid smell. This odor will be coming from the AC vents when you turn on the system. If you notice such a smell, turn the AC off immediately to prevent further damage to the parts.
  • OVERHEATING WHEN IDLING: If the car is idling with the AC on and you notice the vehicle is overheating, the car’s condenser fan is not working correctly. It shows that the condenser fan is generating heat and causing the en­gine’s internal temperature to increase.

How do you diagnose a bad condenser fan? The condenser fan spins immediately after you turn on the AC. Locate this fan under the hood as it sits next to the radiator fan. Then, have someone turn on the AC and observe if it starts to spin.

If it does not begin to spin, you may want to identify the cause, as it could be a failed fan relay, blown-fuse, failed temp sensor, damaged wiring, or the ECU not commanding it to turn on.

To fix, you will need to fix the problem according to the cause. For example, a blown-fuse or wiring issue should be easy to do at home. In addition, you may need to change a faulty temperature sensor as it could be preventing the fan from coming on when it doesn’t relay the turn-on message to the ECU.

An auto mechanic can identify and fix all these problems, and most condenser fan problems don’t cost more than a few hundred dollars to fix.

The radiator fan cycles on and off with the engine warmed up or idling. Some of the symptoms of a failing radiator fan include:

  • Whirring noises
  • Overheating engine
  • Fan won’t come on
  • Blown radiator fuse
  • Broken fan clutch
  • Temperature warning light

Diagnose by locating the radiator fan on the radiator. Next, turn on the vehicle and let it get warm. Then, observe if the radiator fan starts to spin when the vehicle becomes warm. A radiator fan that does not spin could be a problem with the fan itself or its motor.

To fix, it’s always best for a technician to look at the radiator fan to determine the cause of the problem. Replacing a radiator fan costs between $550 to $650, while the radiator fan itself will cost around $400 to $450.

A bad AC compressor

The car’s AC is dependent on the compressor to keep the air circulating. If the compressor is in bad shape, the refrigerant will not move around and the AC will not produce cold air.

Here are some of the symptoms of a bad AC compressor:

  • HOT AIR: If the AC is not well-maintained, hot air could indicate an AC com­pressor failure or low refrigerant. Hot air warns that the AC is about to go bad completely.
  • ODD SOUNDS: An AC that’s working properly produces a clicking sound when you switch it on or off. Nevertheless, when the AC starts failing, parts may begin to grind, causing a whining sound when you turn it on or off. This sound could suggest the bearings or other components are failing.
  • FLUID LEAKS: The internal bearings in an AC com­pressor prevent the pressurized refrigerant from leaking. When the bearings become worn out, they’ll cause the refrigerant to leak; hence you’ll encounter moisture leaks around the AC lines.
  • STUCK COMPRESSOR CLUTCH: A car’s AC compressor has a clutch that connects to the engine to draw power. The com­pressor uses this power to turn with the help of a pulley. When the clutch breaks, the compressor cannot receive power from the engine. Determine whether or not it’s ide­al for replacing the clutch only or the whole compressor when you have a stuck compressor clutch.

After you notice signs of a bad compressor, the next thing will be to diagnose it.

  • Check for temperature fluctuations when the AC is running.
  • Visually inspect the AC compressor for rusts, oil leaks, and physical damage.
  • Inspect the compressor clutch to see that it’s not hard to turn and not making a grinding noise as it turns.
  • Listen for skipping or squealing noises when the engine is on, the AC is at its lowest setting, and the fan is on max.
  • Check if you’re low on refrigerant using an AC refrigerant detector.

After concluding that the problem causing the AC to blow warm air is a bad air compressor, the best thing is to replace it. Consider replacing the O-rings, the accumulator, and the expansion device when replacing it.

Refrigerant Leak

The AC system is filled with refrigerant to work correctly. This refrigerant starts as a gas on the low-pressure side and is converted into a liquid on the high-pressure side. It’s this process that keeps the cabin cool when the AC is on.

Over time, leaks develop in the system, and the refrigerant level drops.

Here are some of the symptoms of refrigerant leaks:

  • Sudden refrigerant loss: An abrupt change from cool to warm air when driving is a sign of a sudden loss of the refrigerant. After the level of Freon drops, you’ll see a white, cloud-like emission, and the smell of Freon fills up the cabin.
  • Visible refrigerant leaks: Refrigerant contains some oil to lubricate the compressor when it’s in a liquid state. A Freon leak is similar to an oil leak, but it’s lighter than oil. You’ll find it on the front shaft, service ports, pressure lines, condens­er, fittings, and the accumulator.
  • A clutch that does not engage: When you turn on the AC, there’s a click sound that indicates the clutch has en­gaged. The absence of this sound means the clutch failed to engage, probably because the refrigerant is too low.

It’s time to recharge the system, especially if you’ve never refilled it in the last six or seven years. Unfortunately, vehicle owners can’t recharge their AC at home because refrigerant needs proper handling by a licensed technician. Also look for leaks if the cause of the low refrigerant is a leak in the system.

Clogged or dirty filter

The AC filter removes contaminants from the air entering the vehicle’s air conditioning system. It removes impurities, allergens, and pollutants that make the cabin uncomfortable.

A cabin filter will become dirty and clogged over time. When it’s excessively dirty, it will show some symptoms such as:

  • REDUCED AIRFLOW FROM THE VENTS: If you experience a reduced airflow from the vents, the cabin filter is likely clogged, and it’s preventing cold air from passing through.
  • NOTICEABLE REDUCTION IN EN­GINE POWER: A clogged air filter plac­es additional strain on the AC blower motor, which causes the blower motor to work harder. The extra stress passes on to the engine, which drives it to op­erate at lower power when the AC is on.
  • INCREASED ALLERGENS AND DUST IN THE CABIN: If the customer notices that their allergies worsens when they’re driving, it’s a good sign that the cabin air filter is clogged.
  • WHISTLING NOISE: Restricted air­flow can produce a whistling noise as it tries to pass through the filter.

To fix, there’s no way around a clogged or dirty air filter except to replace it. A standard particulate air filter needs replacing after every 50,000 km, while an activated charcoal cabin air filter should be replaced after 25,000 km or once a year.

Fixing the problem if a car’s AC not blowing cold air is not always easy. Remember, you can always check the car’s manual.

This article originally appeared in the July/August 2022 issue of CARS magazine

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