We're off-roaders and we don't mind being hot, cold, wet, or dirty when we're out running trails or tackling our favorite dirt. However, at some point, we all crave some creature comforts. When the mercury is rising and you're starting to sweat, sometimes you'd like to punch that little button that soon brings waves of cool air across your face and starts to decrease the sweltering temperature in you cab. Thank you, auto air conditioner.
Basically, an engine-driven compressor with an electrically actuated clutch compresses refrigerant gas, which is then routed and cooled by the evaporator in front of the radiator and piped into the cab. Here, an expansion valve controls the flow release to the condenser sitting in the fan box. The gas cools as it expands, and a fan blows across the condenser to deliver the cool air to us.
Auto air conditioning systems can fail due to system leaks at any point in the system. The compressor can leak, seize, or fail to engage or develop sufficient pressure. In addition to performing leak checks if needed, home mechanics can do some simple troubleshooting to determine how the system has failed.
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Here’s a simple diagram showing the main components of an auto air conditioning system.
An inline switch such as this may be used to monitor line pressure. Should the system pressure drop below some point where there is insufficient refrigerant for safe operation of the compressor, this switch will signal the A/C controller to shut down compressor operation to avoid compressor damage. On occasion, these switches can fail and prevent the compressor from turning on, even though the system is adequately charged. The system has to be purged and recharged for switch replacement. However, you can jumper a failed switch so it is ignored. This will allow you to continue to operate the A/C system, but you won’t have the safety of the low-pressure switch to protect the compressor should you have a refrigerant leak.
On the front of the compressor is a belt-driven clutch assembly. The A/C control unit in the dash turns the electric clutch on and off based on whether the A/C is switched on by the driver and based on pressure build in the system. You can check the clutch function briefly by jumping 12 volts to the compressor clutch. If the clutch engages with the jumper but not during normal operation, the A/C control unit is not sending a signal to the compressor or you may have a wiring issue.
To service an A/C system properly, it’s best to use a quality set of gauges to allow you to monitor low- and high-pressure conditions. This can help you determine if the system has the right amount of refrigerant, if there is moisture in the system, and if the compressor and expansion valve are maintaining proper pressures for optimal cooling performance. Filling the system blindly can lead to over-charging and compressor damage.