Common Car AC Problems and Diagnosis

Updated on March 8, 2020
Dan Ferrell profile image

Dan Ferrell writes about do-it-yourself car maintenance and repair. He has certifications in automation and control technology.

Some common AC system problems can be diagnosed by the average car owner.
Some common AC system problems can be diagnosed by the average car owner. | Source

Common car AC problems, sooner or later, will creep up into your system. For example, when your AC:

  • stops blowing cold air
  • blows only cool air
  • blower works on only one speed
  • blows cool air only when increasing engine speed
  • stops working

When your car's AC stops blowing cold air or is barely cool, you may be dealing with a mechanical, vacuum, electrical or refrigerant-related issue. You may diagnose some of these issues yourself, and possibly fix them as well with the help of the repair manual for your vehicle make and model. Your manual has the specifications, troubleshooting and repairs you may be able to do yourself on your particular car's AC configuration.

If you don't have this manual yet, you can get a relatively inexpensive, aftermarket copy from Amazon. Haynes manuals come with many step-by-step troubleshooting, maintenance and replacement component projects you can do at home. Most of the time, you'll need some common tools and a few diagnostic tools like a multimeter. And soon, you'll recoup your small investment.

The following sections will help you make an initial diagnostic of your AC problem. Each section heading describes a common AC issue. Just head over to the relevant section for your particular case and, with the use of your manual, you can try troubleshooting and fixing the problem.

Index
1. Preliminary Checks
2. My AC is Not Blowing Cold Air
3. The AC Compressor Clutch Is Not Engaging
4. My AC Blower Always Blows at the Same Speed
5. My AC System Is Not Cooling Right
6. AC Cools Air Only When Depressing the Accelerator
7. My AC Compressor is Noisy
8. The AC System is Not Cooling
9. AC System Is Not Blowing Air
10. There Is Frost On the Condenser
11. The Outlet Fitting on the Receiver-Drier Is Too Cold
12. Checking for AC System Restrictions and Refrigerant Level Using Your Hand
13. Checking Refrigerant Through the Sight Glass
Video: How to Recharge Your AC System
14. Getting Help From Your Car's Computer to Fix AC Problems

1. Preliminary Checks

Often, an AC performance issue can be tracked down to a simple and common problem you may check for yourself.

Check the belt:

  • The belt should not be too tight or loose. An over-tightened belt can lead to an accessory or accessory's pulley and bearing fault. A loose or oily belt will slip, overheat and cause the belt to harden and crack, and lead to bearing damage from heat as well.
  • Check a belt for signs of worn edges, tears, and splits.
  • On a serpentine belt, check the belt tensioner pulley mark. If necessary, adjust or replace the belt.

Check the compressor:

  • With the engine off:

    • Look for signs of oil leakage. If so, the compressor seal might be bad.
    • Accumulation of black dust around the clutch plate may indicate belt slippage.
  • With the engine running:

    • Start and idle the engine.

    • Engage the parking brake.

    • Set the transmission to Park (automatic) or Neutral (manual).

    • Set the AC to MAX, fans on High.

    • Open the hood.

    • The compressor clutch should be engaged (center of compressor pulley is rotating).

      • If the system is barely cooling and the compressor clutch is engaged and working, it's possible your system is low on refrigerant.
      • Also, a worn, slipping clutch can reduce cooling efficiency. A hot clutch is a sign of a slipping clutch.
      • If the compressor clutch is not engaged, the clutch may be bad, there could be an electrical problem in the circuit, or the system could have developed unusually low pressure, potentially because of a leak. Your repair manual can tell you how to check for power or voltage to the clutch.

Check for leaks:

A common sign of refrigerant leak is a thin, oily residue around fittings or hoses in the AC system. Check for oily and green spots around hoses and fittings connecting the following components and on the components themselves:

  • condenser
  • compressor
  • receiver/dryer
  • evaporator and drain valve
  • Schrader valve

Check the condenser:

  • check the front of the condenser for buildup like dirt, leaves and debris
  • signs of leaks
  • damaged fins

An evaporator is also a common source of leakage, but it is usually located under the dashboard and may be difficult to inspect. If necessary, your vehicle repair manual may help you gain access to the evaporator.

When Diagnosing or Working on your AC System

The AC system is a closed and pressurized system. If you need to replace a component, have the system discharged first. Consult your vehicle repair manual. Don't loosen or disconnect hoses or lines, or tighten fittings, unless you are following a specific repair procedure outlined in your repair manual. You may introduce air, moisture, or dirt and contaminate the system or damage fittings. The AC reaches temperatures of 200 to 400 F, and a pressure over 200 psi.

An AC control system may also cause problems.
An AC control system may also cause problems. | Source

2. My AC is Not Blowing Cold Air

With the engine at operating temperature, turn the AC to "MAX cooling."

  • If you hear a noise, this other post on AC System noises can help you diagnose the problem.

  • If the air coming out of the vents is not cool, the problem could be:

    • A bad compressor
    • A low level of refrigerant
    • A refrigerant leak
    • A faulty control circuit
  • Also, there could be a problem in the control system or air duct system.

  • If moving the control from hot to cool changes the air temperature little or not at all, the problem could be with the blend door or air distribution case.

Make sure the compressor clutch is engaging.
Make sure the compressor clutch is engaging. | Source

3. My AC Compressor Clutch Is Not Engaging

When you turn on the AC system, you may notice the clutch doesn't engage. The clutch is the center piece at the pulley, in front of the AC compressor. If the clutch is not working, the system will fail to produce cool air. The problem could be a:

  • Bad clutch
  • Fault in the clutch electrical circuit
  • Bad AC thermostat unit
  • Bad sensor (depending on vehicle model)
  • AC system in need of refrigerant

If necessary, consult your vehicle repair manual to test the clutch, circuit and thermostat.

Check the AC vents if compressor seems to be working but no air is coming out the vents.
Check the AC vents if compressor seems to be working but no air is coming out the vents. | Source

4. My AC Blower Always Blows at the Same Speed

With the engine idling and the AC operating, select different speeds for the AC blower:

  • If the air comes out of the vents at the same speed, regardless of the speed setting

    • There could be a bad switch or resistor block.

If necessary, consult your vehicle repair manual.

Adding Refrigerant to the AC System

If your AC system uses refrigerant R-134a type, you can add refrigerant by following the procedure outlined in your vehicle repair manual. 1993 and previous vehicle models use R-12 type refrigerant in their AC system and needs to be charged by a qualified shop.

Check the condenser core for debris and bent fins that may disrupt AC proper operation.
Check the condenser core for debris and bent fins that may disrupt AC proper operation. | Source

5. My AC System Is Not Cooling Right

Most of the time, cooling problems develop from common issues:

  • Debris clogging the radiator or condenser core
  • Clogged cabin air filter
  • Radiator or condenser fan(s) not running

Also, there can be problems with the evaporator, system restrictions or leaks.

Sometimes, you can check for restrictions yourself. Head over to the next section, Checking the AC System for Restrictions.

Other potential problems that may affect AC performance:

  • Refrigerant level low
  • Problems with the blower motor or electrical circuit
  • Debris blocking the evaporator
  • Drive belt slipping

Keep in mind that insufficient air flow through the condenser can cause damaging high system pressure. So make sure to check the radiator and condenser for leaves, bugs, dirt, and bent fins. You can clean the core and straighten fins using a "fin comb". If necessary, consult your vehicle repair manual.

If cold air comes out only when depressing the accelerator pedal, check for a worn or faulty compressor.
If cold air comes out only when depressing the accelerator pedal, check for a worn or faulty compressor. | Source

6. AC Cools Air Only When Depressing the Accelerator

AC system pressure issues can lead to an AC system that blows cooler air only when you step on the accelerator, increasing engine speed.

This could be a sign of problems with the compressor. Increasing engine speed forces a faulty or worn compressor to work harder and raise pressure to a proper level. If necessary, have the compressor or system pressure diagnosed to find the actual problem.

Check your compressor mounting bolts if it seems to rattle during operation.
Check your compressor mounting bolts if it seems to rattle during operation. | Source

7. My AC Compressor is Noisy

AC compressor problems that may manifest through different types of noises include:

  • Loose compressor mounts
  • Seized compressor
  • Leaking shaft seal
  • Pressure problems

The compressor is not that only source of noises, problems in other parts of the AC system may cause the AC to become noisy:

  • Loose parts
  • Loose fan
  • Belt slipping
  • Faulty blower bearing
  • Too much refrigerant

If necessary, consult your vehicle repair manual.

Check AC lines and hoses for leaks.
Check AC lines and hoses for leaks. | Source

8. The AC System is Not Cooling

A bad compressor clutch may fail to engage when you turn on the AC system, failing to produce cool air. But the problem may also come from a fault in the electrical circuit or thermostat unit.

Other causes of AC not cooling:

  • Bad compressor
  • Refrigerant low level
  • Refrigerant leak
  • Faulty control circuit

If necessary, consult your vehicle repair manual to test the clutch, circuit and thermostat.

If the AC is not blowing air, check the blower motor as well.
If the AC is not blowing air, check the blower motor as well. | Source

9. AC System Is Not Blowing Air

Another common problem may occur when turning the AC system and having no air coming out the vents. Usually, the problem is with the climate control system:

  • Faulty blower motor
  • Electrical system failure
  • Bad electrical component control or module

Check the condenser lines.
Check the condenser lines. | Source

10. There Is Frost on the Condenser

An AC condenser can develop leaks or become restricted, often shown as frost on the outside of the unit. To check for potential restrictions, head over to the section Checking the AC System for Restrictions.

If you need to replace the condenser, you may be able to do it yourself. Consult your vehicle repair manual.

The receiver/dryer is a common source of AC problems.
The receiver/dryer is a common source of AC problems. | Source

11. The Outlet Fitting on the Receiver-Drier Is Too Cold

Receiver-Drier units are a common source of problems. They can cause refrigerant contamination or become restricted, which sometimes shows as a cold outlet fitting. To check for receiver-drier restrictions, head over to the section Checking the AC System for Restrictions.

It's common to replace the receiver/dryer when:

  • AC system has been leaking
  • Moisture enters the system
  • The unit becomes restricted

Match line numbers in the following section to the ones in the image to identify AC system lines.
Match line numbers in the following section to the ones in the image to identify AC system lines. | Source

12. Checking for AC System Restrictions and Refrigerant Level Using Your Hand

Common AC problems are system restrictions and low refrigerant, which may indicate a potential leak. The following series of checks will help you spot possible system restricted areas, if any, and refrigerant level.

These checks require you to touch hoses and components with the AC system running. Keep your hands away from moving components and hot spots like engine surfaces and the exhaust manifold.

Match the numbers in parentheses below to the numbers in the diagram above to help you identify components and lines in your actual AC system.

  • Bring your engine up to operating temperature. You can idle the engine for about 20 minutes, or drive your car for 20 minutes.

  • With the engine at idle, set your transmission to Park (automatic) or Neutral (manual).

  • Engage the parking brake.

  • Open the hood.

  • Carefully feel along the line connecting the compressor to the condenser (1).

    • The line temperature should feel hot.
    • If there's a difference in temperature along the line, it is restricted. Flush or replace the line.
  • Feel along the condenser (2) from top to bottom or the return bends.

    • Temperature should gradually change from hot (top) to warm (bottom).
    • A sharp change in temperature along the condenser's surface or bends indicates a restriction. Flush or replace the condenser.
  • If your system has a receiver/drier (consult your repair manual), feel the inlet and outlet lines (3):

    • Both inlet and outlet side lines should have about the same temperature, close to the temperature at the bottom of the condenser.
    • If there's a difference in temperature, or you see signs of frost on either line, there's a restriction. Replace the receiver/drier.
  • Feel along the line connecting the receiver/drier to the expansion valve (4).

    • The line should feel warm to the touch.
    • Signs of frost on the line means a restriction. Flush or replace the line.
  • Check the expansion valve (5).

    • There should not be any frost on the expansion valve.
    • The inlet line should feel warm and the outlet line should feel cool.
  • Instead of a receiver/dryer, some models come with an orifice tube between the condenser and evaporator.

    • Feel the line (6). If there's a difference in temperature in the line before the orifice tube, as compared with the line to the evaporator inlet, you may need to flush the line or replace the orifice tube if it is restricted.
  • Feel the inlet (7) and outlet (8) lines at the evaporator. Both should feel colder than ambient temperature, and have about the same temperature.

    • If the inlet is colder than the outlet, the system is low in refrigerant.
    • If both lines are not colder than ambient temperature, have the system checked.
  • The line going from the evaporator to the compressor (9) should feel cool to the touch. The accumulator, if your system has one, should feel cool to the touch as well.

    • However, if you notice frost on this line, probably the expansion valve is flooding the evaporator.

Modern AC system require the use of special gauges to properly check AC system pressure.
Modern AC system require the use of special gauges to properly check AC system pressure. | Source

13. Checking Refrigerant Through the Sight Glass

A sight glass can be useful to check the refrigerant level and contamination in the system. A sight glass is most commonly found on older systems (1993 and earlier models) using refrigerant R-12 and a thermal expansion valve. You may also find a sight glass on some R-134a systems, but it is not as reliable, since refrigerant tend to have a more milky appearance. So the next descriptions apply to the R-12 system.

  1. Park your car in a well-ventilated area.
  2. Set the parking brake.
  3. Set your transmission to Park (automatic) or Neutral (manual).
  4. Start and let the engine idle for about 5 minutes.
  5. Set the AC control to MAX cooling and the fan to the highest speed.
  6. Open the hood and locate the AC system sight glass. If necessary, consult your vehicle repair manual.

Look how the refrigerant appears though the sight glass:

  • If it appears like clear water flowing, the system is OK.
  • If you see streaks of oil, the system is empty of refrigerant.
  • If it appears foamy or with bubbles, refrigerant level is low.
  • If it appears cloudy, most likely the refrigerant is contaminated.

On newer R-134a systems, a pressure and performance test is a better indicator of refrigerant level.

If you need to add R-134a refrigerant to your system, you may be able to do it yourself. The following video walks you through the process of adding refrigerant to the system.

How to Recharge Your AC System

14. Getting Help From Your Car's Computer to Fix AC Problems

Most car owner's can diagnose some common AC problems without the use of special equipment or tools. However, if you have a late model vehicle with automatic climate control system, you may have access to a self-diagnostic test procedure form your AC system.

On some models, you can access trouble codes by pressing buttons in a certain sequence on the control panel or through a scan tool to access diagnostic trouble codes in the body control module (BCM).

Trouble codes can make diagnosing and repairing the AC system much easier and faster.

Consult your vehicle repair manual to gain access to the self-diagnostic test or trouble codes.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2020 Dan Ferrell

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