Q: What Sensors in the Air Conditioning Tell the Car if the System Is Working or Not?

Modern automotive air conditioning systems come in two basic types:

1. Manually-controlled AC systems
2. Auto AC systems

The manual AC systems require the driver to adjust the desired amount of cooling by changing the fan speed and heat / cool knob or lever to maintain the comfort level they desire. The Auto AC systems use a computer monitored system which allows the driver to select a temperature to maintain, and then allows the system to adjust the interior comfort (much like turning on your AC at home and depending on the thermostat input to cycle its output operation to maintain that requested temperature.

Each of these systems use sensors to transmit data to one of several modules or computers during operation. This information helps the engine adjust idle to compensate for the AC’s compressor, which puts a load on the engine when it operates. The sensors also control the time of compressor operation to match the requested cooling effect.

Depending on the type of system the information will be sent to, the PCM (Powertrain Control Module), the BCM (Body Control Module), AC amplifier (modules) or AC ECU (Electronic Control Unit)

Sensors include but may not be limited to:

• ambient temperature sensor (mounted external to the vehicle)
• interior temperature sensor
• high and low pressure sensors
• humidity sensors.

The ambient temperature sensor is usually placed in the grill or on a bracket in front of the AC condenser allowing it to communicate the ambient or actual external temperature. Just to clarify, ambient temperature refers to the temperature that is surrounding the object, which in this case, is the vehicle’s external temperature. The computer will activate the AC based on the customer’s input signal, indicating the AC request from the switch or control panel on the dash.

There are several pressure sensors or switches that can be used or monitored as well, which will report pressures to indicate operations, and when the cooling fans should be run to dissipate heat from the AC condenser. These sensors will allow the computer monitoring the system to shut off the compressor if the system refrigerant charge is too low, or if the system pressure increases too high.

While the sensors indicate the operation condition of the AC system, an integral part of some systems is observed by the comfort experienced by the driver and passengers in the vehicle. Since humidity plays a role in AC comfort, it is monitored to help ensure the proper cooling comfort of the vehicle’s interior in some AC systems, especially those with the Auto AC. Think about how humidity changes your comfort level and your body’s ability to shed heat, which is, in effect, our own personal AC system. Less humidity equals more comfort, but too little can dry the eyes and make one uncomfortable.

Some vehicles have heating and cooling systems for the passenger seats and many hybrid vehicles actually use AC to cool the or High Voltage (HV) Traction batteries when the temperature of the charging batteries cannot be controlled or corrected through the use of the available ambient air normally used to cool the batteries. Remember, these sensors only report an electrical input to the computer, so any connection issues or faulty sensors will result in less or possibly no AC system operation.