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Auto Air Conditioning Maintenance & Repair - Cool Breezes

Posted in How To on August 1, 2008 Comment (0)
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It's hot enough to fry an egg on the sidewalk, and you jump in your truck with sweat dripping off your brow. Fire up your engine and push the magic button, and in a matter of minutes, you're bathed in a cool wind pouring from your dash. Sure, there are plenty of us that run sans tops or doors, but when summer hits, either our trail rigs or our tow rigs often have air conditioning.

Basic diagram showing the major components of an auto air-conditioning system.

So, how does that chill-making machine work? It all starts with the basis of a compressed gas in a closed system and the fact that an expanding gas drops in temperature. By utilizing these simple physics laws, we can build a system to cool us when outside temperatures soar.

The heart of the A/C system starts with an engine-driven compressor that utilizes an electrically actuated clutch to turn on and off. The compressor takes low-pressure refrigerant (Freon gas for older vehicles or R-134 for newer models) and pumps it to a high pressure at its outlet. This act drastically raises the temperature of the refrigerant. It's then pumped through lines to the condenser that sits in front of the radiator, where some cooling occurs, and the refrigerant starts to liquefy.

At this point, the refrigerant usually passes through a nearby filter/dryer (receiver) canister where desiccant is used to extract any moisture that may be present. Then the refrigerant enters lines running under the dash in the cab. By this time, the refrigerant has turned to a liquid state, and it passes through an expansion valve (small restrictor tube) that serves to meter the refrigerant flow into the evaporator at a controlled rate.

As the refrigerant enters the evaporator, it has the chance to expand back into a gaseous state, and when this happens, the temperature of the gas drops rapidly. This is the point where your blower fan pumps air across the fins of the evaporator, heat from the air is absorbed by the evaporator, and cool air is pumped into your cab.

From here, the refrigerant moves back toward the compressor and should be gaseous again before entering the low-pressure inlet, and the whole cycle continues. Additionally, light oil is moved through the system and used to lubricate the compressor internals. There are a few variants of how the compressor cycles and how the metering function at the evaporator inlet is performed, but this is generally how all vehicle A/C systems work.

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The clutch assembly consists of the pressure plate (left) and rotor. These steel components clamp together when the clutch is engaged to send pulley drive to the compressor unit. When the clutch is disengaged, the pressure plate springs forward, and the rotor freewheels with the belt system.

Usually there's not a lot involved in the maintenance of an automotive A/C system. Best efficiency can be maintained if the condenser fins are kept clean and clear of debris, and the compressor-belt tension (if adjustable) is proper.

As the system ages, it may develop the smallest of leaks. Should the system be cooling but not at full chill, you may simply be low on refrigerant due to a slow leak. Such very small leaks may be hard to trace, and it may be necessary to add a bit of refrigerant charge about once a year.

You'll want to use caution if you add refrigerant to the system. Follow the specific fill instructions for your setup, always add refrigerant on the low-pressure side, and do not overfill. Careless filling can result in over-pressure and damage to the compressor.

Here you can see the stator that lies behind the clutch rotor. An electrical connector plugs into this to actuate the clutch plates and engage/disengage the compressor operation. The applied voltage creates a magnetic field that pulls the pressure plate and rotor together.

Leaks can occur at compressor seals, O-rings at the various hoses and components, or the rubber hoses themselves. Gross leaks may be identified by telltale oil seepage at a hose or fitting. Also, fluorescent dies can often be added to the refrigerant and then can be observed later at the point where they leak from the system. Air-conditioning technicians typically also have electronic or ultrasonic leak detectors that can find very small leak sources.

Should any part of the closed system be taken apart, it is necessary to vacuum pump it after reassembly and before recharging with refrigerant.

If the clutch is not engaging to turn on the compressor, the culprit could be due to a number of problems. There could be an electrical harness break, the A/C controller may have failed, or the operation has been disabled by the low-pressure-detection switch in the system. The clutch operation can be quickly checked by using a wire to jump battery voltage to the clutch connector for a few seconds to test if the clutch engages. If it does, then the problem lies somewhere in the wiring or control.

One other thing we should mention involves parts replacement. Should the expensive compressor need to be replaced, the filter/dryer should always be replaced at the same time. One reason is that the dryer loses its desiccant efficiency over time, and compressor failures often leave debris caught in the dryer.

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Hot, compressed refrigerant from the compressor is pumped forward in the vehicle to the condenser that lies in front of the radiator. Here, the high-pressure gas is cooled via airflow. Next, the refrigerant temperature starts to drop, and the gas turns back to a liquid state.

Another common point of failure over time is the blower fan in the dash. If you find you have the problem that some fan speeds do not work, then you may have a vehicle that uses a resistor module and one or more of the resistors have failed. These modules are often located somewhere on or near the evaporator/fan housing.

Should you have an older R12 A/C system, there are various means by which to update to the newer refrigerants. Some conversions can be done with a simple gas replacement. Others require changing hoses or other components. In any case, do some careful research as to which may work best for your application. There is a wide variance in system survivability and their ability to cool well.

Note also that it is illegal to vent refrigerants to the atmosphere, which is why you see service shops capture it in a recovery tank.

This isn't meant to give you a comprehensive guide to A/C systems, but to explain the basics and share some information that you may find helpful when doing some of your own work or evaluating proposed work from a shop. Hope you stay cool!

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