Tech Stuff

Electronic air conditioning -- An overview of systems found in today's hybrid vehicles


Evaporator sandwich. The ejector tube is located inside one of the evaporator tanks (note cutaway).
<p>Evaporator sandwich. The ejector tube is located inside one of the evaporator tanks (note cutaway).</p>

Electronic A/C systems are now appearing in hybrid vehicle applications. You’re more familiar with these systems than you might first realize. You grew up with them. You may even have one in your bedroom window. They are air conditioners — refrigeration units — powered by electricity.

The basics of refrigeration apply whether it is an automotive air conditioner, a window air conditioner or a kitchen refrigerator. Let’s use the Toyota Prius system for our example, since the Prius is the most popular of the hybrids and typical of what you will see in the shop.

Hybrid vehicles have special needs when it comes to heating, ventilation and air conditioning (HVAC) operation. Since an electric motor propels the car, normal belt drives won’t cut it when the engine isn’t running. We need another way to power the compressor.

The 2004 to present Prius compressor is powered by a 200-volt AC (alternating current) electric motor built into the compressor assembly. The electric motor takes the place of the belt-driven pulley found on typical vehicles. Other than the compressor’s AC power source, the A/C operation is the same as Toyota’s other vehicles. NOTE: If there is a problem such as a short circuit or broken wire in the compressor wiring, AC power is turned off.

Incidentally, the heater core in the HVAC system doesn’t get heated coolant soon enough or consistently enough because the engine is not always running. Cabin heat is backed up with an electric heater.

Of course, any power used for heating and air conditioning comes from the car’s high-voltage battery system. That can have a negative effect on fuel economy. To minimize the electrical power draw, the air conditioner compressor’s motor is controlled by an electronic control module (ECM).

The A/C ECM receives inputs from several sensors such as the cabin temperature sensor, the sun load sensor, a humidity sensor (integral with the cabin temperature sensor), a temperature sensor at the evaporator and a position sensor on the out door of the HVAC case. The A/C ECM also receives reports as to whether the wipers are running.

The information is processed and a signal is sent to the car’s high voltage electronic control unit which tells the AC inverter how much power to send to the compressor’s electric motor.

Since the 2004 model year, even the blower motor uses less power by running on a pulsed, duty cycle electrical input.

The duty cycle is controlled by the HVAC control module.

Troubleshooting the Prius air conditioning system is similar to other vehicles’ A/C systems. However, there are some trouble codes that are vehicle specific to hybrids. Always consult a shop manual to be sure. As you know, the preferred way to obtain diagnostic trouble codes (DTCs) is by using a scan tool. Connect yours to the diagnostic link under the dash and follow the instructions.

(NOTE: Check the appropriate Service Manuals for DTC charts.)

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