A/C Not Working? Here’s How to Diagnose It

 

Does your car have a broken A/C? On a hot and humid day that’s the worst. No one wants to feel like a fried egg, and sometimes putting the windows down just doesn’t cut it. So here’s a list of symptoms that can help you diagnose what’s wrong with your car’s air conditioning.

Just find your symptom on the list, and we can tell you what to check so you can troubleshoot the problem.

When I turn the A/C on…

Nothing happens when I turn on the A/C

Hot air blows out of the vents

The air flow is weak

The air is not as cool anymore

The air is cool but then gets hot

There are loud noises under my hood

The A/C smells

There are water stains on my floor

 

Nothing Happens When I Turn on the A/C

If you don’t feel air coming out of your vents when you turn on your A/C, the you might need to repair or replace your:

Before moving onto these parts, first check the fuse or relay for the blower motor. Your owners manual can help you locate your fuse box and identify your blower motor fuse. If the fuse is good, you’ll need a test light or voltmeter to see if the switch is getting power from the circuit, the resistor is getting power from the switch, and the blower motor is getting power from the resistor. Start by unplugging the blower motor, which is usually found under your passenger side dash or behind the glove compartment. Ground your test light or voltmeter and put the probe into the plug you disconnected from the blower motor. With the car on, turn the fan speed knob on your dash. If the light comes on or you get 12 volts at all settings, then you likely need to replace the blower motor. If the light comes on or you get 12 volts at some settings, but not others, then you likely have a problem with the resistor. You’ll have to replace the resistor, which is typically connected to the blower motor. Usually if this is the case your fan will run on some speeds but not others.

If the fan won’t run at all and no power comes to the blower motor plug on any setting, you’ll have to check if the problem is with the HVAC controls. Unplug the resistor, and probe the resistor plug with your test light or voltmeter. Check if the resistor plug gets power when the blower motor switch is on. If not, the problem is likely the HVAC control panel. If the light on your A/C switch doesn’t turn on, when you turn on the switch, this can also indicate there may be a problem with the HVAC controls.

Replacing the control panel can be somewhat involved, but is doable. Usually, you’ll have to pry out the dash panel with trim tools. Then unplug the HVAC controls. The HVAC controls are usually held to the back of the dash panel with small bolts or screws. Once you’ve removed the old control panel, you can line up the new one, bolt it to the dash panel, plug it in, and press the dash panel back into the dash.

Hot Air Blows Out the Vents

In this case, the problem could be:

To test HVAC controls, you’ll need a test light or voltmeter to see if the controls are getting power.

There’s an easy way to test the compressor clutch. Once you’ve pressed the A/C switch inside the vehicle on, turn the fan to the highest setting and listen for the compressor clutch clicking on and off under the hood. A consistent cycling of the A/C compressor clutch every few seconds is normal. If it is clicking on and off rapidly or not at all, the compressor clutch may be faulty or there may be low refrigerant from a leak. If there is no clicking at all, check the voltage to the compressor clutch. If there is not any voltage, the compressor clutch can’t work, and you need to determine why your compressor clutch isn’t getting the electrical signal to turn on. Low refrigerant and blown fuses are common causes of this. If there is voltage at the compressor clutch and the clutch isn’t engaging the compressor, the problem may be the clutch itself. If the clutch is engaging but the compressor fails to turn, it is likely a damaged compressor.

Refrigerant will not dissipate or evaporate over time. In order to have low refrigerant, there must be a leak somewhere in the system. This is a very common problem with malfunctioning A/C systems, and there are a few ways to troubleshoot leaks.

First, if you suspect you have low refrigerant, you can use A/C manifold or pressure gauges. If you find that the low pressure side has lower-than-normal psi, then your refrigerant may be low.

Depending on the model, to test for leaks you can use:

  • UV Dye
  • Electronic Leak Detector

To use the UV Dye, you may need to add a little refrigerant to the system, but be careful not to overload it. Once it’s cycled throughout the system, you can use a UV light to find staining on the part of the system that is leaking.

Another option is to use an electronic leak detector. You can scan the detector underneath the lines with the system running, and the detector will alarm once it has found the leak. Sometimes this method is not as reliable as the dye.

 

Air Flow Is Weak

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

Evap cores can be susceptible to mold and mildew, especially if the drain is clogged. If your evap core is filled with mold, moisture, and dirt, this can restrict air flow. Luckily, sometimes it can be fixed with an anti-bacterial spray.

The cabin air filter can restrict airflow by accumulating dust, dirt, and debris. If your vehicle has one and it hasn’t been changed in a while, it may be the reason for the dip in airflow.

If the brushes inside the blower motor have worn, the power of the motor can reduce, and this may lead to less air pushing through the vents. Sometimes little critters like mice will nest inside the blower motor since it’s insulated, or air flow can be restricted from debris clogging it.

The blend door actuator controls the direction of airflow, and if malfunctioning, it could be responsible for the unpredictable temperatures.

The air ducts that carry the air to each vent in the vehicle may be compromised. While not a common issue, if you’ve removed the dash to work underneath, a loose screw or hole/cracking in the duct may result in a loss of air.

 

The Air Is Not As Cool

If you’ve noticed an increase in temperature on the highest setting, there may be a problem with your A/C system. It’s important to know that some vehicles adjust the cabin temperature depending on the ambient temperature, so make sure this has been a pattern regardless of the temperature outside.

The problem could be:

  • Leaking
  • Expansion Valve or Orifice Tube
  • Compressor or Compressor Clutch
  • Condenser
  • Cooling Fan
  • Evaporator or Evap Core
  • Control Module
  • Blend Door Actuator
  • Cabin Air Filter
  • Blown Fuse

The most common problem with failing A/C systems is leaking. Sometimes you can find the leak by inspecting all of the fittings, hose clamps, and lines. It’s important to know that the evaporator core will drain excess water after it dehumidifies your cabin, so a leak on the ground does not always mean there is an apparent leak or problem in the system.

You can check the amount of refrigerant in the A/C system by hooking up a pressure gauge to the high and low pressure valves. Readings can vary depending on the temperature outside, and you can always check your manual to see if it lists your system’s recommended psi. The low pressure and high pressure readings should have a significant difference. Low pressure readings can range from 30-50 psi, and high pressure readings can range from 170-250 psi. The type of reading you get can help you diagnose what’s may be causing the issue. If the low pressure psi is low, you most likely have low refrigerant, which means there is a leak.

There are two common ways you can test to find a leak

  • UV Dye
  • Electronic Leak Detector

You may need to add refrigerant to the system so that the UV dye can circulate, and once this is done and you’ve let the A/C cool the cabin with the setting on high, you can shut the system off and scan the lines and fittings with a UV light. Staining will indicate where the leak is.

Another method you can troubleshoot with is an electronic leak detector. You can place the scanner underneath the lines and wait until it detects where the leak is. This method is not always as accurate as the dye.

A/C Manifold gauges or pressure gauges can also help you detect other possibilities, such as the expansion valve. If the reading is high for the high pressure and low for the low pressure, you may have a clogged expansion valve, which can lead to a warmer temperature.

If the reading is low for high pressure and high for low pressure, the problem may lie with the compressor. Turn on the A/C system and see if the compressor clutch engages the compressor. If it doesn’t, check the compressor’s electrical connections and see if there is voltage to the compressor. If there is no voltage, there may be a blown fuse or low refrigerant. If there is voltage, the problem may be the clutch or compressor.

Since the condenser is located near the front of the vehicle, it can be susceptible to dents from rocks and other debris. This can lead to leaking and other problems. Inspect the condenser thoroughly for leaks. Also, turn the A/C on and see if the cooling fan is engaging. If the fan is not cooling the condenser, your refrigerant is going to have a hard time cooling, giving you warmer air.

The same can go for the evaporator core. If the evaporator core is leaking or is letting in moisture, you can wind up with a loss of refrigerant and warmer air.

Some cars also come with a control module that sends signals to the A/C system. When this malfunctions, it can render your system inoperable.

Sometimes a malfunctioning blend door actuator can send cool air out of one vent and hot air out the other. Since its duty is to regulate the direction of airflow, this may be the problem.

If your vehicle has cabin air filters, the dirty and clogged filters can restrict cool airflow, sending you warmer air through the vents.

 

The Air Is Cool and Then Becomes Hot

The problem could be:

  • Expansion Valve or Orifice Tube
  • Compressor or Compressor Clutch
  • Cooling Fan
  • Blown Fuse
  • Leaking

Too much refrigerant can lead to a clogged expansion valve or orifice tube, and too little can lead to an overheated evap core. To test the expansion valve, you can hook up a pressure gauge to the high and low pressure valves. If the reading is high for the high pressure and low for the low pressure, your expansion valve may be causing this issue.

Low refrigerant can also lead to the compressor clutch clicking rapidly on and off, which will keep the compressor from engaging, thus keeping the refrigerant from flowing consistently. A failed compressor will also keep the refrigerant from circulating. Sometimes a blown fuse can keep the compressor from receiving the needed voltage to work consistently.

Lastly, the most common issue with failing A/C systems is a loss in refrigerant. If you suspect low refrigerant, you can test the system’s pressure with A/C pressure gauges. While a “normal” low pressure psi can vary from vehicle to A/C system, a common measure is 30-50 psi depending on the temperature outside. If there is lower low psi than your vehicle recommends, you may have a leak.

There are two useful methods to test for leaks

  • UV Dye
  • Electronic Leak Detector

If you are low on refrigerant and the compressor is not engaging, you can add a little refrigerant before adding the UV dye. Then, in a dark area and after cycling the A/C until the cabin is cold on the highest fan setting, use a UV light to detect where the refrigerant is leaking.

You can also use an electronic leak detector by scanning the probe along the lines of the A/C system, and the detector will react to the leaking area.

 

There Are Loud Noises Under My Hood

This could indicate an issue with:

Noises underneath the hood are likely from a worn belt or compressor bearing, which will result in a squeaking or squealing sound. The compressor bearings may also be seized, which will prevent the belt from freely moving over the pulley and resulting in a grinding or squealing sound.

 

Bad Smell From the Vents

You may need to replace:

While not a common problem, a bad smell from the vents similar to a gym locker room or dirty sock can be one of two things: you may have a clogged and dirty cabin air filter, or you may have a clogged evaporator core drain that is letting moisture accumulate in the evap core, and pushing the smell into your cabin.

Sometimes the evaporator core can accumulate mold and mildew from moisture in its cool dark setting, making it a haven for fungi. And sometimes you can remove this mold with a special anti-bacterial spray.

 

Water Stains on the Floor

The problem could be:

  • Evaporator Core

Water stains under your car are different from water stains under your dash. After using the A/C, water stains under your car are a good thing. This means the excess condensation from the humidity in your cabin is draining out. The problem is when that water ends up on the floor. This may mean your evaporator core’s drain is clogged, which can eventually lead to an A/C failure, like the air starting out cool, and then becoming hot or having an unpleasant smell from the vents.

 

Recharging the System

If you removed the compressor, vacuum the lines to remove any potential metal shavings that can ruin your new compressor. Once you’ve resolved the issue, you’ll need to recharge the system. Many shops offer recharging services, but you may also be able to do it at home with a kit.

An A/C recharge kit will typically contain one or more canisters of refrigerants and a dispenser with a hose and pressure gauge. Start by turning on your car and turning the A/C to high. Then attach the dispenser to a refrigerant canister. You’ll find a cap on your A/C’s low pressure line. Remove the cap and attach the dispenser hose. Then pull the trigger on the dispenser and hold it until the gauge reaches the correct pressure reading. This will vary from car to car, so read any instructions that come with the recharge kit or in your owner’s manual. Once the A/C is recharged, disconnect the hose and twist on the A/C line cap.

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