Tech Stuff

Hybrid vehicle air conditioning service

Gen II Prius HVAC

In 2004-later models, a three-phase, AC voltage dropped to 201 volts, with a brushless, variable-speed A/C compressor. An orange sheath containing three orange cables runs from the compressor to the transaxle-mounted inverter/converter assembly. A smaller inverter is located inside the main inverter assembly. This inverts a portion of the HV battery DC power into AC, which operates the A/C compressor. Since the engine is not always running, the A/C compressor must still be able to operate. Through the use of an electric inverter compressor (as opposed to a belt-driven unit), the compressor is driven by an electric motor that’s built into the compressor housing and powered by AC voltage from the vehicle’s power supply system. Except for the portion that is actuated by the electric motor, the basic construction and operation of this type of compressor is the same as the scroll compressors used in other Toyota vehicles.

The Gen II Prius features an evaporator temperature sensor as well as a humidity sensor. The heater core is located in front of the evaporator. Small ceramic (PTC) heaters also are featured in the heater core and ducting (similar to those in Gen 1) which are located directly behind the heater core. These heaters run off of 12 volts and aid in generating a bit of cabin heat when the gas engine isn’t running (and therefore not producing coolant heat). The expansion valve is located to the right of the evaporator (accessed under dash).

The Gen II Prius evaporator is notably smaller and lighter than that found in Gen 1 models.

The A/C amplifier (HVAC ECU) is located below the center of the dash, near the floor, and sends and receives messages on the BEAN bus. The blower motor is located under the right side of the instrument panel. The blower is controlled by a pulse width modulated controller (variable duty cycle), which can be checked with a quality DVM. A “room temperature sensor” (which incorporates a humidity sensor) is located to the left of the selector lever. As humidity level changes (for example from dry to humid conditions), the voltage changes from approximately 1 volt to approximately 3 volts. When humidity is low, the compressor spins slower. When humidity is high, the compressor spins faster.

The A/C amplifier (an ECU) utilizes a logic program that takes into consideration multiple factors in order to control the A/C operation. These factors, or inputs, include a sun load sensor, grille-mounted ambient temperature sensor, windshield wiper operation and actuator door position. According to experts, the information from the ambient temperature sensor is multiplexed through a gateway module that translates the information into a BEAN bus message to the amplifier (since the A/C amp isn’t on the CAN bus).

Since the AC produced by the motor generators can’t be stored in a DC battery, the inverter’s second job is to rectify AC to DC, to recharge the HV battery. It is inside the inverter that you’ll find an additional three-phase AC inverter for the electric A/C compressor used on some hybrid models. On these models, in most circumstances, the HV battery pack has enough power to supply energy to the A/C compressor to keep the cabin cool (even with the gas engine off) and still have enough power to propel the vehicle down the road at low speeds for a limited distance.

Cutaway view of a Toyota high voltage electric inverter A/C compressor. This is a scroll type, driven by an electric motor. Compressor speed is controlled by an ECU.
<p>Cutaway view of a Toyota high voltage electric inverter A/C compressor. This is a scroll type, driven by an electric motor. Compressor speed is controlled by an ECU.</p>

The Prius transaxle houses two AC motor generators. Motor generator 1 (MG1) is primarily used to start the gas engine and recharge the HV battery, while the larger MG2 is connected to the transmission output, and primarily functions as a traction motor to power the vehicle at low speeds (and assist the gasoline engine at higher speeds) and create electricity to recharge the HV battery under braking and deceleration. In addition to the gas engine’s cooling system, the Prius hybrid also features a separate liquid cooling system for the inverter/converter and for the two motor generators that are housed inside the transaxle (along with a dedicated 12-volt electric pump). Both the engine and additional inverter/converter/generator cooling systems are designed to use the same Toyota-approved antifreeze.

Compressor oil

The biggest difference (beside the obvious electric versus belt-driven design in Gen II/III) is the type of lubricating oil used in the system. While PAG or Ester oil is primarily used in traditional A/C systems, POE (polyolester) type oil is used in hybrid vehicles. The specific type used and mandated by Toyota is ND-11.

ND-11 oil provides high dielectric properties, which helps to maintain the integrity of the compressor’s electrical windings.

According to Robinair, with just 1% of PAG oil in the system, this can lower the insulation resistance of a compressor from over 10 mega ohms to under 1 mega ohm. If PAG oil is used to completely fill an electric compressor system, the insulation resistance can essentially be reduced to zero. Based on recommendations from Honda, Toyota, Ford and GM (again, according to Robinair) it is unacceptable to allow even the slightest amount of PAG (or other oil) into the system. Honda even goes so far as to recommend replacing all A/C system components if the system is cross-contaminated with the wrong oil.

In June 2010, the SAE published a revised version of the SAE J2788 standard that covers the operation of an A/C recover, recycle and recharging (RRR) machine. A machine suitable for servicing both vehicles with a high voltage electric compressor that uses POE oil and traditional PAG oil systems must:

• not have an on-board automatic/manual oil or dye injection, and

• be capable of charging refrigerant into a system with less than 0.1% by weight of any residual oil.

A way to tell if a machine is certified as suitable for servicing vehicles that use high voltage electric compressors is to look for a marking that features a yellow triangle with a lightening bolt, accompanied by the words “Certified for High Voltage Compressor Service.” This alert should appear somewhere on the machine either as a separate decal or on the decal that indicates SAE J2788 certification. An example of such a machine is Robinair’s 34788-H machine (which does not feature on-board oil injection). ●

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