Auto Air Conditioning: Why Does It Blow Hot and Cold?
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Auto Air Conditioning: Why Does It Blow Hot and Cold?

The most common reason that a cars air conditioners blows hot and cold is moisture in the system. Replacing the dryer, evacuating the system, and recharging with R134a usually solves the problem.

"The Dog Days of Summer" will soon be upon us, and your car's air conditioner is starting to blow hot and cold. You are grateful for your auto AC and the icy-cold air that flows from the vents. Then, one morning, without warning, all that changed. You cranked up the AC. You relax as icy-cold air washes over your body. Then, a few minutes later the air from the vents starts to get warm. Soon the air from your AC is as warm as the air outside your car. Then slowly it starts getting cold again, then icy cold. What is going on? What is wrong with the AC? If you are driving an older vehicle, chances are that the AC is freezing up. Air and moisture usually cause this problem in the system by turning into ice and blocking the orifice tube.

Things That You Will Need

  1. Basic mechanic's hand tools
  2. Air conditioning duct thermometer
  3. Air conditioning manifold gauge set
  4. Air conditioning vacuum pump
  5. Refrigerant 134a (R134a)
  6. Electronic refrigerant leak detector

By now, if you have been following my Auto Air Conditioning series, you have probably added manifold gauges, set, a vacuum pump, and an electronic leak detector to your tool kit. You need to add a duct thermometer.

Jerry's Basic Air Conditioning Diagnostic Chart

I developed this chart as a handout for an Adult ED class I taught on auto air conditioning a couple of years ago. I will probably do a whole article on using manifold gauges, but this is an opportune time to introduce you to this diagnostic chart. It is straightforward and easy to understand.

LS PRESSURE HS PRESSURE DUCT TEMP PROBABLE CAUSE OF PROBLEM
LOW LOW WARM LOW REFRIGERANT CHARGE
HIGH HIGH WARM OVERCHARGE OF REFRIGERANT
HIGH HIGH SOME COOL OVERCHARGE OR AIR IN SYSTEM
NORMAL NORMAL WARM MOISTURE IN SYSTEM
LOW LOW WARM EXPANSION VALVE STUCK CLOSED
LOW LOW WARM ORIFICE TUBE PLUGGED
LOW LOW WARM HIDE SIDE RESTRICTION
HIGH LOW WARM COMPRESSOR OR CONTROL VALVE FAILURE

Verify That There Is Moisture In The System

Connect the manifold gauges to the system.

  1. Connect the "Red" hose to the high-side service fitting.
  2. Connect the "Blue" hose to the low-side fitting.
  3. Connect the "Yellow" hose to a "Capping" stud on the gauge set.

Start the car's engine and switch the AC to its "Maximum" setting. Open both the high and low side valves on the manifold gauge set. A low side reading between 25 and 40 psi and a high-side reading between 225 and 250 psi confirms the presence of moisture in the system. These readings are for a R134a system, if you have a system that uses R12, you should not be working on it.

How Does Moisture Enter the System?

A certain amount of moisture is sucked into the system between the compressor shaft and shaft seals. This is normal, and the engineers included an Accumulator Dryer" or a "Receiver Dryer" in the system to remove this moisture. The accumulator dryer is used in systems that use an orifice tube to meter liquid refrigerant into the evaporator coil. The accumulator dryer attaches to the evaporator's outlet and stores excess liquid refrigerant. The Receiver Dryer is used on a system that employ an expansion valve to meter liquid refrigerant into the evaporator coil. The Receiver Dryer stores excess liquid refrigerant. The Receiver Dryer and the Accumulator Dryer both serve a second purpose; they remove moisture from the refrigerant. They employ a desiccant that absorbs the moisture. Newer accumulator dryers and receiver dryers use XH-7, a desiccant that is compatible with both R12 and R134a.

Replacing The Accumulator Dryer or Receiver Dryer

The accumulator dryer or receiver dryer should be replaced anytime the system is opened up for a major repair. You should replace the dryer if the system is blowing hot and cold and the manifold gauges indicate normal low-side and high-side pressures. Desiccants can only absorb so much moisture before they become saturated and stop absorbing moisture.

Pull A Deep Vacuum After Replacing The Dryer

  1. Close the high side valve on the manifold gauge set.
  2. Attach the "Yellow" hose to the vacuum pump.
  3. Start the vacuum pump and pull 29 inches of Mercury (Hg.)
  4. Shut the vacuum pump down. Check the compound gauge after 30 to 45 minutes has passed. If the vacuum has decreased, you have a leak in the system that you need to find and repair. Read my article on How to Find A Refrigerant Leak for detailed information on the process.

Recharge the System With R134a

Once you have completed all the repairs and evacuated the system, recharge the system with R134a. Refer to my article How to Recharge Your Air Conditioning System for detailed instructions on charging your auto air conditioner.

Next time we will discuss a few other possible causes for the system to blow hot and cold.

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Comments (2)

great write. I don't have in in my house only my car! but I learnt something today

Very helpful info. Stumble, T'd, dugg!

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