Hybrid A/C service

April 1, 2014
Have you looked under the hood of a hybrid vehicle lately? If you have, then you already are familiar with the fact that there no longer is a serpentine-style drive belt to turn the accessories. Perhaps you have seen the advertisements that on some models of hybrid vehicles you can control the climate control system from your smartphone. Things definitely are changing. 

Toyota Techstream enables you to perform active tests on the HV A/C compressor.

Have you looked under the hood of a hybrid vehicle lately? If you have, then you already are familiar with the fact that there no longer is a serpentine-style drive belt to turn the accessories. Perhaps you have seen the advertisements that on some models of hybrid vehicles you can control the climate control system from your smartphone. Things definitely are changing.

Hybrid vehicles have pushed engineers to rethink the way they design A/C systems. While they maintain many of the same components that we are familiar with, quite a few new components and diagnostic procedures have emerged. Traditional testing methods might not apply to hybrid A/C system diagnosis, and we as technicians need to get up to speed to keep pace with technology. In this article, we will take a look at hybrid A/C technology — and discuss repair and diagnostic strategies so you are well prepared when a hybrid A/C job comes through your bay doors.

A look at the differences

A high voltage A/C compressor is identifiable by the orange wire loom. Image courtesy of Denso

One of the major component differences in hybrid A/C systems is the design and control of the compressor. Early hybrids, such as the first-generation Toyota Prius, had a belt-driven A/C compressor. The strategy here was simple: If you turned the A/C on, then the engine had to run.

The compressor turned at a rate of speed determined by the engine speed and compressor clutch engagement, no different from traditional A/C systems. This worked well, but the mpg rating suffered considerably anytime the A/C was on.

With high demands on manufacturers to raise the mpg rate and improve battery life, unnecessary loads on the engine had to be removed. The problem this presents is that a belt-driven compressor cannot meet the need for A/C, especially if the engine is turned off.

This is an Ejector Cycle Evaporator. Image courtesy of Denso

Honda’s early hybrid A/C compressor choice was a dual scroll-style compressor that used two separate drive methods: a belt and an alternating current (AC) electric motor, each with its own scroll. The strategy here was that anytime the vehicle went into idle stop mode (engine turned off), the electric side would take over to continue to provide climate control for the passengers.

In partnership with Denso, Toyota took the technology to a new level and developed a fully electric three-phase A/C compressor. Early models were fed high voltage A/C from the inverter, but in current-generation Toyota hybrids, the compressor contains its own DC-to-AC converter. Other manufacturers have adopted this style of compressor for use on their own hybrid platforms.

The use of an electric compressor has enabled engineers to design an efficient strategy for compressor control, with the ability to vary compressor rpm based on commands from the HVAC electronic control unit (ECU). With the use of a high-voltage (HV) compressor, however, safety must not be overlooked. In many cases, as much as 200 to 300 volts may be present. We will talk more about service and safety shortly.

HV compressor with integral DC to A/C inverter. Image courtesy of Denso

The other features

To further improve fuel economy, manufacturers have developed several other ancillary technologies that are used in hybrid A/C systems. These ancillary devices have made for quite an improvement in overall air conditioning system efficiency — and you can expect to be seeing them in non-hybrid applications as well. To improve efficiency, the engineers had to find ways to ease the workload on the compressor, as it is the main consumer of energy.

One component they looked to change was the condenser. As with all A/C systems, the gaseous refrigerant leaves the compressor and enters the condenser, where a heat exchange occurs to transfer heat from inside the cabin to the outside air. In doing so, the refrigerant cools and condenses, or turns to a liquid. This works well for the most part, but not all of the gas content is condensed in traditional condensers.

A cutaway look at the ejector. Image courtesy of Denso

New condenser designs re-circulate the gaseous refrigerant before it leaves the outlet of the condenser so that it can be turned to a liquid. The result here is a much more efficient condenser. You will see these referred to as “high-efficiency” or “sub-cool” condensers. Once the liquid refrigerant leaves the condenser and passes through the receiver drier, we need to drop the pressure and temperature of the liquid refrigerant.

To accomplish this, we historically have used thermal expansion valves, or TXVs. The expansion valve was designed to reduce the pressure and temperature of the liquid refrigerant before it entered the evaporator.

The expansion valve worked well, but from a performance perspective it was highly inefficient. What takes place in the expansion valve is a sudden pressure drop where the diameter of the valve goes from small to large, which significantly lowers the pressure and temperature of the refrigerant as it enters the evaporator. One downfall of the TXV, then, is the development of turbulence in the area of the pressure drop. To overcome this turbulence, the compressor simply has to work harder. To eliminate this inefficiency, Denso has developed a new evaporator called an ejector cycle evaporator.

Denso first developed ejector technology for the refrigeration and truck market in 2003. The initial version of the ejector was quite large, but after six years of research, Denso was able to make it small enough so that it could be manufactured as part of the evaporator for the 2010 Prius.

The ejector cycle evaporator eliminates the TXV and improves overall system efficiency by recovering the energy that is created as a result of expansion. The ejector is similar to the TXV, but re-circulates refrigerant cooled in the evaporator back into the inlet area of the evaporator through a venturi — much in the same manner in which old, small engine venturi carburetors functioned. This venturi effect increases the velocity of the refrigerant, and eliminates energy losses while achieving the same cooling effect as the TXV.

An insulation tester such as the Fluke 1587 may be necessary to diagnose HV insulation faults. Image courtesy of Fluke

The velocity that is developed by the ejector also raises the inlet pressure at the compressor, which means its compression ratio can be lowered to improve overall system efficiency. Denso claims that the system performance improvement is 50 percent at 18 degrees Celsius, a considerable gain.

Coordination of these new technologies is typically done by a dedicated ECU. This is not a new concept, as manufacturers have been using dedicated HVAC ECUs for some time now. The main difference in control of hybrid A/C systems is that the hybrid battery and inverter, not the engine, is powering the compressor. This means that the A/C ECU and the power management or inverter ECU will have to communicate.

Knowledge of CAN and LIN communication systems is a must for diagnosis should a problem on the communication network, or bus, occur. The benefit to having these ECUs is system access via the scan tool. The downfall is that if your scan tool doesn’t support the Parameter Identifiers (PIDS), you might not be able to diagnose the vehicle.

The repair itself

Now that we have an overview of hybrid A/C system operation, we can dive into the service and repair issues you might face. You might need to invest in some tooling to get the job done, and as with any technology, hands-on training is highly recommended.

Toyota does not allow its service techs to work on hybrid A/C systems unless they have passed six days of hybrid hands-on training. You might want to consider taking similar measures at your facility. Mistakes on a hybrid A/C system can be rather costly, so proceed with caution.

The second-generation Prius inverters have a built-in inverter for the compressor, while the third generation has the inverter in the compressor. Notice the three-phase cable on the early inverter.

Safety — As with any other hybrid-related repair, you need to proceed with caution and adhere to the manufacturer’s recommended safety procedures at all times. HV components typically are easy to identify by their bright safety orange wire looms.

Don’t be fooled by a vehicle that has a belt on the compressor. Vehicles such as the Honda Civic hybrid might have a belt on the front of the compressor, but an HV wire harness routed so that you might not see it at first. Be careful.

Whenever working on an HV component, you will have to shut down the HV system. Follow service information instructions to the letter. If you should decide to unplug an HV compressor while the system is powered up, you not only will destroy the connector on the compressor, but you also will run the risk of serious injury, and even death. The connectors on electric compressors are not serviceable and come as part of the compressor assembly. Keep in mind, however, that even though this is a HV system, you most likely won’t be doing much high-voltage work unless a compressor fails or there is an insulation fault somewhere.

The orange HV cable to the A/C compressor on this current-generation Toyota Hybrid has high voltage DC sent to the compressor where it is inverted to A/C to drive the motor.

Refrigerant and refrigerant oils — Hybrid A/C systems call for R-134a refrigerant and require POE oil, not PAG oil. There are several reasons for this. Nippon-Denso’s POE oil, known as ND-11, is used in many of the hybrid A/C compressors and has a high insulation value, just more than 10 megaohms (10 million ohms). ND-8, which is a PAG oil, has an insulation value of less than 1 megaohm — a considerable difference.

Hybrid manufacturers state that it is unacceptable to use even the slightest amount of PAG oil in these systems. Denso has claimed that just 1 percent of PAG oil introduced into the system can lower the insulation value of the overall system charge to less than 1 megaohm.

The issue at hand here is that if a short from the HV windings in the electric compressor should occur, it might cause an unsafe condition. Another scenario that has happened to a few Toyota dealers was when the dealer had replaced a component and charged the system with PAG oil. The result was HV insulation diagnostic trouble codes.

Manufacturers run a voltage detection circuit on all of their HV components to monitor HV insulation leaks, and the slightest change in voltage can set codes and disable the hybrid system. It was determined that the use of the wrong oil caused these codes to occur. This has prompted Toyota — as well as other manufacturers — to recommend the use of a dedicated A/C machine for hybrid A/C service.

GM has its own service bulletin that recommends using a hose flush between charges to eliminate any traces of PAG from the lines on their A/C machines. It claims that PAG oil contains some moisture that could possibly degrade the insulation of the HV windings, resulting in a dangerous situation and diagnostic trouble codes (DTCs).

If you are using the Robinair ACR-2000, you can pick up the hose flush adapter kit from Kent Moore (Part No. J-43600-50) pretty inexpensively. If you are planning on doing some hybrid A/C work, look into what the equipment manufacturers are recommending for the machines you own and get yourself a good filter and flushing mechanism. Remember, when it comes to refrigerant service on hybrids, cross-contamination is the enemy.

Some hybrid HVA/C controls look identical to their non-hybrid counterparts.

System diagnosis — Since the beginning of A/C system service, one preferred method of diagnosis was to hook up a set of manifold gauges to the high- and low-side service ports and see what you’ve got. That pretty much has gone the way of the dodo on hybrid vehicles. Not only can pressure readings be unreliable, but you might run the risk of cross-contamination by hooking up your old R-134 gauges.

The majority of diagnosis on hybrid vehicles is going to be done with a scan tool. It might be a good idea to have a look at the PIDs and active tests available on your scan tool platform on a known good car before you attempt to use your tool on a broken car.

If your aftermarket scan tool isn’t up to the challenge, it might be time to think about factory tooling. The factory tools give you all the PIDS you need, as well as active tests for the compressor, which can make the difference between fixing the car and losing it to the dealer.

Play with your scan tool on a known good hybrid the next time one is in your shop. You might be surprised to see the available active tests that can help you get a diagnosis quickly.

Factory scan tools allow you to command the compressor to a desired speed and watch the rpm to determine that it has achieved the command. This might come in handy before condemning an expensive compressor.

Electrification of the A/C compressor allows for the use of new HVA/C control strategies.

Insulation fault diagnosis — You might already have been in a situation where the hybrid system warning light is illuminated and the vehicle doesn’t ready on. As we mentioned earlier, this could be something as simple as someone installing the wrong refrigerant oil. The manufacturers have gone to great lengths to provide DTCs, as well as information sub codes, to point you in the right diagnostic direction.

Should the problem lie in the HV compressor, you will need to perform a test of the windings and the wiring from the inverter to the compressor. To do this, you will need to have a good insulation tester. Take a look at the January 2014 Motor Age article by Jerry “G” Truglia for some great tips on using a “Megger.”

Now that you have some information on hybrid A/C, it’s time to do your homework. Attend some training on hybrid A/C, and explore the capabilities of your scan tool the next time a hybrid vehicle comes in for service. With an open mind and some new knowledge, you can now offer hybrid A/C service to your hybrid customers with confidence.

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