One of the most commonly asked questions regarding A/C systems is what the low- and high-pressure gauge readings should be when the engine is running and the system is on. Unfortunately, there is no single answer to this question, due to variables such as engine speed, airflow across the condenser, system design, blower speed, mode setting, and refrigerant type, all of which can cause variances in the high- and low-side readings.
A good rule of thumb is that pressures should read in the region of 35 psi on the low side and 212 psi on the high side at idle (Image D). The reason approximately 35 psi is the optimal pressure on the low side is that it is equivalent to evaporator temperature, which is what you feel coming out of the vents; this should be close to the freezing point of water.
If you look on the low-side pressure gauge, there is a temperature scale next to your pressure scale. The number on that scale translates into evaporator temperature. Since moisture will collect on the evaporator, you want to try and keep the evaporator temperature just above the freezing point of water; this will prevent the moisture on the evaporator from freezing. If this pressure becomes too low, and the evaporator starts to freeze, the low-pressure switch will cycle the compressor off temporally to allow the evaporator to thaw.
The reason we use 212 psi as a base reading on the high-pressure side is that it is the average reading you will have at 85 degrees if everything is working properly. With systems using R-134a refrigerant, the high-side pressure usually will equate to 2.2 and 2.5 times ambient temperature. That means on an 85-degree day, you should expect to see high-pressure gauge readings between 187 and 212 psi. High-side pressure has a broader operating range than low-side pressure due to heat loads on the evaporator, airflow across the condenser, engine speed, and humidity. All of these factors can cause a higher reading on your high-pressure gauge. We'll see how that plays into problem solving when we use the gauges to diagnose specific issues.
If you're still using R-12 refrigerant, the procedure is the same, but the math is a bit different. On an R-12 system, the high-side pressure usually will equate to 1.8 to 2.1 times ambient temperature. That means on that same 85-degree day, you would expect to see high-pressure gauge readings of between 153 and 178 psi.
Leaks are the most common cause of automotive A/C-system problems. Refrigerant leaks out and reduces the total pressure in the system, causing the low-pressure switch to disengage the compressor. This is a safety feature to protect the compressor from damage if the system becomes low on refrigerant. Prolonged low refrigerant can cause the air conditioning compressor to become damaged internally and spread small metal flakes through the entire system. This can cause components to become clogged with debris and require expensive repairs.
Most of this debris collects in the condenser, which can cause it to become partially blocked. This reduces cooling performance. Condensers changed to a more efficient design when manufacturers switched from R-12 refrigerant to R-134a. Unfortunately, we're seeing more condensers fail on today's vehicles due to the multi-path layout of these condensers.
The multi-path design has much smaller passages than did the earlier R-12 units. These condensers can easily become contaminated with debris from a failed compressor or other component. If you have a failed compressor and find metal particles in your system, it's highly recommended that you replace your condenser.
If the debris passes through the condenser and enters the high-pressure line, it can plug the orifice tube or the expansion valve. Debris can also migrate backwards from the compressor through the suction hose, causing a blockage in the accumulator or receiver dryer.
If you open the system and find blackish sludge, it's most likely the result of moisture contamination. This sludge can damage the compressor and plug the orifice tube or expansion valve. The moisture-absorbing desiccant in the accumulator is designed to prevent this from happening, but the desiccant can only hold so much moisture, and once saturated, sludge will begin to form.