A/C Repair: The Big Picture

We all know when our A/C stops working: no cold air. Pretty simple. And usually it’s easy to identify the one item in the system that’s causing a problem, like the compressor, a leak in a line, a failed belt, or a leaking condenser. However, there may be other items that cause your A/C to not perform as well as it should. We’ll investigate some of those here.

How it works

First, a very brief overview of how air conditioning systems work. There are five major components to the system, which runs in a closed loop (nothing from outside). There is refrigerant in the system (typically R134a). The components and what they do are:

  1. Compressor: This is a pump, which both compresses and pushes the refrigerant through the loop. When it compresses the refrigerant, it gets very hot.
  2. Condenser: This basically is the same a radiator, it cools the hot refrigerant, which then becomes a liquid (which is why it’s called a condenser).
  3. Expansion valve: usually located on or near the firewall, and it turns the now liquid refrigerant into a low pressure gas. When you reduce the pressure of the gas in the system it becomes very cold.
  4. Evaporator: Located in your dash (usually in the HVAC box), fills with the very cold gas, and the car’s blower motor (same one as used with heat) pushes the cold air through evaporator, and then directs the cool air into the cabin.
  5. Receiver/drier: filters the refrigerant and absorbs any moisture from the system.

There are some electrical components in the system, too. Most important is the high/low pressure switch. This switch causes the compressor to cycle during normal operation to maintain temperature.

What goes wrong?

The A/C runs in a closed loop, or continuous circle. In a perfect world, nothing gets in or out. But sometimes that’s not the case.

The most common problem with A/C systems is refrigerant loss. Common leak sources are corroded lines or a damaged condenser. A shop can put dye in the system that will show under infrared light to locate the leak.

A failed compressor is probably the second most common reason for no cold air. The compressor has several components (clutch, the actual compressor, valves, switches) but most replace the complete unit.

But there are other direct and indirect elements of the system that can cause the A/C to still not work, or to work poorly. Let’s look at those.

What else goes wrong?

Aside from the condenser or lines leaking refrigerant, other parts of the A/C system itself can cause problems. These include:

  • Expansion valve: The valve can get sticky with age, preventing it from opening and closing properly. This can cause issues with pressures in the system. If it’s working partially, you may get cool, but not cold air. If it’s not working at all, the compressor may shut off to prevent system damage. This can also happen intermittently, causing the system to run cool, then warm.
  • Receiver/drier: Even though the system is closed, the receiver/drier can accumulate moisture over time. If it has too much moisture in it, A/C performance can suffer. Typically a well performed system purge and refill will take the moisture out, but many shops don’t do this because it’s time-consuming. If you find you need to replace the condenser, it’s a good idea to replace the receiver/drier along with it.
  • Evaporator drain: It’s common for the evaporator drain to get plugged, causing water that’s extracted from the cabin air to accumulate in the heater box. Typically it’ll overflow and fill the front passenger footwell if neglected. Clearing the drain is easy on most cars.

Non-A/C Components that affect A/C operation

Finally, some HVAC components can cause poor A/C operation.

  • Blower Motor: It’s obvious that if your blower motor isn’t working properly, you’re not going to get cool air. The A/C system often also looks for blower motor operation before it turns on, which brings us to the next item.
  • Blower Motor Resistor: The resistor controls blower motor speed. Often it fails when the blower motor is worn and drawing too much current. If your resistor has failed, it’s usually a good idea to replace the blower motor at the same time, as if it’s drawing too much current it will cause the new resistor to fail as well.
  • Cooling fans: Like the blower motor, many A/C systems won’t run if the cooling fans aren’t working. Check yours and replace if they’ve failed. This is a good idea in summer, regardless.
  • Radiator: Although a leaking or clogged radiator won’t keep your A/C from working, the A/C puts additional stress on the cooling system and may cause your engine to overheat. Make sure your radiator and coolant levels are in good order.

In summary, there’s more to your system than refrigerant and a compressor. Optimizing all parts of the system will improve performance and extend component life.

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