Since this column's inception, we've received a substantial number of questions concerning air-conditioning troubleshooting. Answering each one would take quite some time, but we will attempt to address most of them over the next few issues.
Questions like "I added some refrigerant to my Corvette, but it won't get cold—what do you think is wrong?" are almost impossible to answer without further detailed information. In an effort to help take away the mystery of A/C repair, we'll cover how you can diagnose these problems yourself.
Diagnostics are the essential first step in A/C-system servicing. To get started you'll need an A/C manifold-gauge set, a thermometer, and a leak detector. The manifold gauges will allow you to obtain more information about what is taking place inside of the system.
The basic manifold-gauge set should have three hoses. The hoses are color coded to help indicate which service ports they're intended for. The hose intended for the low-pressure service port is blue, and the hose intended for the high-pressure port is red. The yellow middle hose can be attached to a refrigerant can or cylinder for charging. It can also be used when a vacuum pump is needed to evacuate air and moisture from the system. Your manifold-gauge set will have corresponding gauges and control knobs for both low and high pressure. The gauges and knobs are usually the same color as the hoses, to indicate which is which.
The low-side pressure gauge should be on the left side of the gauge set, just above where the blue hose attaches. The low-side gauge serves a dual purpose: It can be used to measure pressure or vacuum. The numbers around the outside of this gauge indicate pressure in pounds per square inch (psi), and the numbers near the bottom of the gauge (in red) indicate vacuum in inches of mercury (in Hg). The smaller scales near the center of the gauge list the temperature relationship of different refrigerants. Regardless of which refrigerant is being used, the scale designated as pounds per square inch is the one used to read system pressure when charging and diagnosing an A/C system.
The high-side gauge should be on the right side of the gauge set, just above where the red hose attaches. The high-pressure gauge is used to measure the high pressure in the system. Remember, never open the high-side control knob when the vehicle's A/C compressor is running. This could result in the pressure from the high side causing the can of refrigerant to explode. When attaching or disconnecting the A/C gauges, the vehicle should always be off.
The initial reading we'll take is the static-pressure reading. With the vehicle off, attach the gauge hoses to the corresponding service ports. The gauges should read the same (or be extremely close) on both the low- and high-pressure sides (Image A). This reading indicates that both sides are equalized. Static pressure will not show you if your system is fully charged with refrigerant. Static pressure is only an indicator that there is some refrigerant in the system, and that you can proceed with your A/C-performance test.
The purpose of checking the static pressure is to ensure that there is sufficient pressure in the system to engage the low-pressure switch and operate the compressor. There also must be enough refrigerant in the system to perform leak testing if necessary. Temperature plays a big role in static-pressure readings. The higher the ambient temperature, the higher the static pressure will read. If static pressures are under 50 psi, you should consider that the system is low. The system will need to be recharged before proceeding with any accurate performance testing or diagnostics.
The best way to ensure exactly how much refrigerant is in the system is to recover all of the refrigerant, introduce a vacuum, install A/C-leak dye, and then recharge the system to the factory-specified amount of refrigerant. Most vehicles will have a label under the hood that specifies what type and how much refrigerant that particular vehicle's A/C system should have installed (Image B).
With the gauges installed, start the vehicle, then turn the A/C on at the maximum recirculation position with the blower motor on high and the vehicle's doors closed. (Open the driver-side window just enough to allow you to reach into the car if the vehicle becomes locked.) Using these settings will help keep the engine and exhaust heat from causing abnormal heat load on the evaporator, and should give you a more accurate temperature reading. A thermometer should be positioned in the center vent, so temperature can be checked periodically during testing (Image C).