Mastering Hybrid HVAC Systems, Pt 1 — Toyota Prius

By Dave Hobbs. Oil spills in the Gulf of Mexico and continual conflict in the Middle East always mean the same thing – fuel prices aren’t going down, and will most likely increase in months and years to come. Combine that with increased CAFÉ standards, and you have the ultimate recipe for changing powertrain technology. Alternative fuels, turbocharged small engines, clean diesels, hybrid electric and pure electric are all examples of new technologies to increase efficiency. Whether one of these technologies will win over the others, or there will be a combination of technologies on the road, it is a sure thing that hybrids electric vehicles are gaining popularity. For the service professional, like them or not, they’re here to stay. That means opportunity for the automobile HVAC technician willing to keep up with the changes they present.

While the majority of the electrical systems on today’s hybrid vehicles are the familiar 12-volt ones that most of us are knowledgeable about, the steep learning curve on this new technology is comparable to the learning curve with the first computer controlled emissions systems that appeared over 30 years ago. But the difference with that sudden change in technology was that there were no technician safety concerns. Performing an improper procedure with a new computer controlled vehicle in 1981 meant maybe a bad ECM. On a hybrid electric vehicle, an improper procedure could mean possible injury or even death from electrocution. Do not be dismayed though. We as professional technicians have always had to learn about and deal with new hazards. Flammable liquids, extremely high pressures in liquid and hydraulics, strut and spring assemblies, even the proper setting of a lift, are all hazards we’ve learned to handle with care and respect. Higher voltage in hybrid electric vehicles is just one more hazard that a true professional can handle.

Now before diving into the wild blue yonder of hybrid electric vehicle HVAC systems, an overview on hybrid vehicles in general would be prudent, especially considering what was just stated about potential hazards. This overview by itself is by no means adequate to ready the reader for safe hybrid vehicle service. But it will at least provide a good introduction prior to attending quality leader led hybrid vehicle safety training.

Figure 1: There’s no shortage of warning labels on hybrid vehicles. In general, if it’s the battery pack, an orange cable, or anything with an orange cable connected to it, it’s potentially hazardous.

First off (and especially for our purposes here), with the exception of the first three years of production, all Prius models use electric A/C compressors that operate on high voltage. Technically speaking, in the world of industry, high voltage is generally considered anything over 1000 volts. The conventional thinking with hybrid vehicles, however, is that any voltage level that can hurt you is considered to be “high voltage.” So in the hybrid vehicle world, if the conditions are “right,” with the voltages present (even at low amperage; well under one amp), serious injury or even death can occur (Figure 1). So from here on in this report, I’ll describe all potentially injurious voltage levels on hybrid vehicles as high voltage (HV), just to distinguish them from the lower, non-injurious voltages that we are used to working with.

Hybrid Electric 101

There are many different types of hybrids on the road today, with models ranging from the GM BAS (Belt Alternator Starter) 36-volt stop/start hybrids, to the “mid-voltage” Honda Parallel Hybrids, to the full-feature, parallel/series HV hybrids, such as those from Ford, GM, Toyota/Lexus and others. But we’re going to focus this report on the most popular model – the Toyota Prius. Every HV hybrid has a 12-volt auxiliary battery, and the Prius is no exception. It is important to note that on all of the Toyota and Lexus non-SUV models, a trunk- mounted absorbent glass mat (AGM) battery is standard equipment. This means that you must either trickle charge it, or charge with a battery charger rated for AGM batteries. Otherwise, damage to the battery will result. For keeping the 12-volt battery charged and for handling the 12-volt electrical accessories on the vehicle, there is a DC-DC converter (basically a step down transformer) to handle the job of the traditional 14-volt alternator used for decades on conventional vehicles.

In addition to a 12-volt battery, the vast majority of hybrids on the road have a HV battery. Toyota Prius models are all either 201 or 273 volts, and their HV batteries are mounted in the rear of the vehicle. No power leaves the HV battery pack unless a set of special heavy duty relays (contactors) close, and put the higher DC voltage on-line. With Toyota and Lexus hybrids, this means pressing the “POWER” button on the IP while your foot is on the brake pedal. An orange LED in the power button will illuminate momentarily, then go off, as an icon that says “READY” appears on the instrument cluster. Depending on several factors, the gas engine may or may not start at this time, but it is ready to start, and can do so without notice. So keep your hands clear of things under the hood, and make sure the selector stays in Park. If you just want to run the blower motor or check air distribution without the high voltage system on-line or the gas engine ready to run, you can select the equivalent of key on/engine off (KOEO) by NOT depressing the brake pedal while pushing the POWER button. The fi rst press in this case results in a green LED in the POWER button, indicating that the vehicle’s ignition status is “accessory.” The second press of the POWER button (without foot on the brake pedal) is the KOEO status, indicated by an orange LED in the power button. These last two ignition positions are strictly working off the small auxiliary 12-volt battery, as opposed to the HV battery pack feeding the DC-DC converter. Since DC power isn’t as effi cient as AC power, hybrids also use an inverter to change DC to AC for the motor generators, which use or create HV three-phase AC power.

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 this inverter that you’ll fi nd 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.

Inside the Prius transaxle is where you will find the two AC motor generators mentioned. Motor generator 1 (MG1) is primarily used to start the gasoline engine and recharge the HV battery, while MG2 (the larger of the two) 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. The latter is referred to as regenerative braking, and can supply most of the braking action to the point where it’s not uncommon to see Prius brake pads last over 150,000 miles!

All Toyota and Lexus hybrids use a liquid cooling system for the inverter/converter and transaxle mounted motor generators. This system is separate from the gas engine’s cooling system, but uses the same Toyota longlife antifreeze used in the engine’s cooling system. A dedicated 12-volt electric pump is used to circulate this coolant throughout the system.

Working as Safely as Possible

On hybrid vehicles, cables that contain enough voltage to be lethal are orange in color. Caution must be taken when working with or around HV battery packs, DC-AC inverters, DC-DC converters, motor-generators, orange power cables (AC or DC) and A/C compressors (if electrically driven). Prior to working around any of the aforementioned components, always perform a HV system power down. My recommendation to be extra safe is to follow a four-step process that can apply to virtually any HV hybrid vehicle:

1. 

   Figure 2: This is the HV battery disconnect plug on a Toyota Prius. Removing it prevents the high voltage relays from putting power to the orange cables leading to the engine compartment. To remove it, slip your finger (with a Class 0/1000 volt glove on) into the opening of the plug and extend lengthwise. Then you can pull outward to unlatch and remove it. When reinstalling, don’t forget to press the open end back together. Failure to do so will result in a no start and DTC P0A0D.

   Remove the key. On Toyota/Lexus and Nissan hybrids equipped with “smart key” systems, this step has to include reading the manual to follow the steps to deactivate that system. The smart key fob can be lying on top of the dash, but the HV system can still power up and the gas engine start if someone presses the brake pedal and the dash mounted power button – the fob DOES NOT have to be in its IP (instrument panel) slot for it to work.

2. Disconnect the 12-volt battery. If the vehicle is shut off, and then the 12-volt battery is disconnected, the HV system can’t come on line. Modules that control the relays inside the HV battery pack can’t come alive without a 12-volt supply.
3. 

   Figure 3: These are typical Class 0/1000 volt gloves. On the right is the actual protective glove. At left is the leather protector for the insulator glove. They MUST always be used together.

   Remove the HV battery service disconnect (plug) per service manual instructions (Figure 2). Prior to doing so, put on a pair of Class 0/1000 volt gloves (Figure 3). To ensure the integrity of the gloves, always perform a quick inspection (air leak test) prior to putting them on. Wear the correct leather covers over the Class 0 gloves, to protect them from damage on sharp edges.

4. Wait five minutes, then still wearing the gloves, use a Category III/1000 volt meter (with leads/probes rated to the same spec) to ensure that the HV connection/component you are about to work with (i.e., the A/C compressor) is no longer powered up. Toyota’s HV system should completely power down in less than one second, but the extra time is to be on the safe side. It will assure that the HV capacitors have discharged. Capacitors inside of the DCAC inverter assembly contain enough voltage (450 volts) to be lethal.

You don’t want to be overly dramatic with this new emphasis on safety, but you also don’t want to be ignorant or careless. Much like handling loaded firearms, you simply want to be well trained and respectful of the potential for injury.

2001 – 2003 Generation I Prius HVAC

The Prius line came to the United States in the 2001 model year. It began life as the Generation I model with a 273-volt battery pack and “THS” (Toyota Hybrid System) labels on its trunk and inverter cover. This 4-door sedan model lasted just three years, and was replaced in 2004 with an all-new Generation II model. The Gen 1’s HVAC systems were much like the car’s exterior; on the surface, there is not a lot of radical change from a traditional vehicle. For the HVAC tech, the heater is where this little hybrid car boasts being different.

Part of a hybrid’s fuel economy advantage is realized with idle stop – keeping the gas engine off when it’s not needed. In all hybrids (except Honda’s mid-voltage models and GM’s BAS lower voltage models), this means the two AC electric-powered motor generators work with a high voltage battery to allow times of idle stop while the vehicle is stationary, and while driven at lower speeds. So if this is the case, how in the world is the engine going to be able to, one, produce hot coolant for the heater core, and two, continue pumping that hot coolant to the heater core?

To help with the creation of heat, a set of positive temperature coefficient (PTC) elements are embedded into the heater core itself. The PTCs are powered up by a relay protected by a fuse marked “PS HTR 1.” The PTCs are wired in parallel. Another set, in the form of ceramic honeycombs and situated in the duct-work, run off individual relays protected by fuses marked “PTC HTR 1 and PTC HTR 2.” These little heaters work on the same principle as a ceramic heating element in a hair dryer, although they won’t put out hair dryer levels of heat. They are wired to the vehicle’s 12-volt DC power bus, and only amount to about 165 watts. A hair dryer is wired to 115 volts AC and has 10 times the wattage output. Use this comparison whenever a Prius owner asks why his/her PTC electric heater doesn’t feel warm enough. They are just there to take the edge off the cold air, and to help clear the windshield a bit faster when the gas engine is off. The # 2 problem with hybrids and heaters is since the gas engine may or may not run at idle or low vehicle speeds, the issue of a belt driven water pump not moving hot coolant arises. This problem is overcome with a 12-volt DC auxiliary water pump. This pump shuts off if the gas engine starts up, due to the operation of the conventional belt driven water pump.

All of these items are controlled by the Toyota A/C amplifier which communicates with the Body ECU via the BEAN (Body Electric Area Network) serial bus. The Body ECU receives inputs like ambient air temperature and coolant temperature from the ECM, to pass on to the A/C amplifier, so it can know when to turn on the PTC heaters and auxiliary coolant pump. The duct PTC heaters only run at low temps when the HVAC head is set to the hottest setting with the air output on floor or windshield, and no inhibit signal. The heater core PTC has the same criteria, with the addition of the blower having to be on. Again, your customer is not supposed to feel very hot air.

The familiar DENSO scroll-style belt-driven compressor with an electric clutch performs the A/C duties on the Gen 1 Prius. The condenser is a bit unique in that it’s efficiency has been increased with a two-stage setup which utilizes a lower section to “super cool” the refrigerant by incorporating a gas liquid separator (referred to by Toyota as a “modulator”). The condenser is incorporated into the radiator to further reduce size and weight. The evaporator has been coated with a resin containing an antibacterial material to combat smelly evaporator problems.

Obviously, if the compressor is belt driven, idle stop for fuel economy’s sake is sacrificed when an A/C request is selected on the HVAC head. This is a nice to know tidbit for drivability techs. Murphy’s Law will go into play for the tech wanting to clean injectors or current ramp a fuel pump; the engine will want to idle stop. Simply turn the A/C to Max Cold, and the little gas engine will start up and purr away.

2004 – 2009 Generation II Prius – High Voltage AC A/C

Figure 4: The DENSO ES18 Electric Inverter Compressor is driven with variable voltage to control its speed. It bolts to the lower front left of the engine block.

In 2004, the Prius underwent a radical change both in body style and technology. The body went to a four-door hatchback that is distinctively, well, Prius. For the most part, style and technology with the Gen II remained unchanged right up through the 2009 model year, after which it was replaced with the Gen III Prius for 2010. By far, this vehicle is the most prolific hybrid on the road today. If you encounter a hybrid vehicle in your service bay tomorrow, it’s very likely going to be a Gen II Prius.

Changed with this model is the NiMH (Nickel Metal Hydride) battery pack, which actually went down from 273 volts to 201 volts. The difference was made up for when a new component in the inverter/converter assembly, called a boost inverter, was added to raise the voltage used by MG2 to up to 650 volts, for added performance in acceleration.

From an HVAC standpoint, a really radical change came with the switch to a three-phase, high voltage (201 volts of alternating current), brushless, variable-speed A/C compressor (Figure 4). You’ll find three orange cables in a flexible orange plastic conduit leading from the compressor up to the inverter/converter assembly atop the transaxle (Figure 5). There is a smaller inverter inside the main inverter assembly, designed to invert some of the high voltage battery pack’s DC into AC, to run the compressor at the desired speed for the cooling job at hand.

Figure 5: Gen II Prius A/C compressor three-phase AC voltage connection. These 201 volt cables leave the inverter and go through this connector on their way to the electric scroll compressor. Always use proper safety equipment and practice OEM-prescribed safety protocol when working with orange cables on a hybrid vehicle.

The compressor is a DENSO scroll-type that Toyota calls an ES18 inverter compressor. It contains a built-in oil separator that helps to separate oil intermixed with refrigerant that circulates in the system, thereby reducing drag on the compressor.

In this compressor, Toyota requires the use of ND – Oil 11 lubricant, a special ester with high dielectric properties. This is the oil they have researched and validated to provide proper lubrication, and to maintain the integrity of the compressor’s electric motor windings’ insulation.

I know some of those reading this may be able to recite success stories from using their favorite brand and type of oil or flushing method for various makes and models of A/C systems, much to the dismay of the OEM engineers who partner with MACS to provide training and information. I won’t beat that dead horse on non hybrids. If you “roll the dice” with conventional systems, a worst case scenario is maybe the compressor fails because you didn’t heed the factory’s recommendations. In the case of a high-voltage A/C compressor on a hybrid vehicle however, worst case scenarios can range from a shorted high-voltage compressor that effectively shuts down the vehicle (spell that “no start”) up to the remote possibility of an electric shock (spell that “possible death or injury”). I wouldn’t trust a GFI (Ground Fault Interrupter) in my bathroom to such a point where I’d deliberately throw a running hair dryer into a sink full of water, so why should I trust the electronic safety circuits on a hybrid to shut down the high voltage system?

Figure 6: The AirSept Charge Guard ensures that PAG oil from other A/C jobs doesn’t migrate from your equipment into a hybrid electric compressor.

I don’t want to paint a melodramatic picture here. Most likely, the OEM’s are thinking long term decay of winding insulation. Have you already put the wrong oil (such as PAG) in a Toyota or Lexus hybrid? Replace all the components? Leave it in? Flush it? Toyota and Lexus are on the long list of OEMs that don’t advise flushing. In the MACS Service Reports from June 2009 and May 2010, Paul Weissler thoroughly covered these subjects, including the issue of cross- contamination of recovery machines which can arise from the mixing of various oils from hybrid-electric vehicles and non-hybrid vehicles. Read Paul’s MACS Service Reports, along with OEM information, to come up with the answer that’s right for your situation.

As for problem prevention, AirSept has come to the rescue with its Charge Guard for hybrids (Figure 6) to prevent cross contamination of PAG oil into hybrids with electric compressors. The aftermarket has also stepped up to the plate, with lubricants that are the equivalent of ND-Oil 11. So there’s no problem obtaining the correct type oil. As far as replacement compressors are concerned, Toyota ships theirs with an inert gas in them (helium), so they advise bleeding this gas off gradually from the service valve of the replacement compressor. They also advise looking at the build date on the OE compressor to determine how much oil you should drain out of the replacement compressor prior to installing. As with all late model vehicles with low refrigerant capacity, a couple of ounces of extra oil may reduce cooling performance. You can read the complete scoop on this in Toyota bulletin T-SB-0048-08 or TSB3001.

Generation II Prius – In & Under the Dash

Figure 7: With much of the IP removed, you can see the cover (circled) for the PTC heaters. The evaporator and heater core are directly behind the PTC heaters, and the A/C amplifier is underneath them.

The Gen II Prius uses a humidity sensor (as do many makes and models of vehicles these days) along with an evaporator temperature sensor. The same PTC heaters are carried over from the Gen I Prius and are mounted directly behind the heater core, which sits in front of the evaporator (Figure 7). Just to the right of the evaporator is the expansion valve, which is accessed from under the IP as well.

The Gen II Prius evaporator features the same mildew prevention coating as the Gen 1. But to reduce weight and leave more room under the IP for the huge electronic content of the vehicle (Figure 8), the Gen II evaporator is smaller. Its length and width is less than a standard 8 ½ by 11 inch sheet of paper, and it’s only 1 ½ inches thick. Now that’s a thin evaporator!

Figure 8: Shown here is a 2004 Prius with the IP covers removed. As you would expect from such a highly engineered vehicle, the electronic content is a bit intimidating. However, everything under the IP is a 12 volt component. Lots of electronic modules, but none that can hurt you!

The HVAC programmer/control module is referred to as an A/C ECU or amplifier. It sends and receives messages on the BEAN bus. This module is mounted near the floor in the center of the IP. The blower motor, which is accessible from under the RH side of the IP below the glove box, is controlled by a pulse controller which utilizes a PWM (pulse width modulated) input and a PWM output. While not necessarily new or specific to hybrids, a PWM-driven blower motor must be understood before diagnosing its power feed. A higher duty cycle means more average voltage to the blower motor, so it runs faster. Toyota factory techs use lab scopes, so the factory service manual shows scope patterns for various circuits for which it’s appropriate to use a scope for diagnosis. But when you know the circuit is a variable duty cycle, you can make do with a duty cycle and/or frequency counter found on most good multimeters.

Figure 9: With the glove box open and extended downward, the Prius cabin air filter slides right out. Toyota recommends replacement every 30,000 miles.

Directly behind the glove box is the cabin air filter (Figure 9). Built into an NCT (Negative Co-Efficient Thermistor) in-car temp sensor is the humidity sensor. This unit is mounted to the left of the “selector lever” (Figure 10). When humidity goes from desert-like conditions to a hot rainy day, the voltage goes from around one volt up to just over three volts. Toyota calls their in-car sensors “room temp sensors,” and this one has a traditional aspirator tube running to it. There is a four wire connector to accommodate both the temperature and the humidity elements. The rational for the sensor is simple – when humidity is lower, the compressor can be spun slower. This lowers electrical demand, thereby limiting the effect on overall fuel economy, while still allowing a comfortable level of humidity inside the passenger compartment.

Figure 10: At first glance, the aspirator tube connected to this sensor next to the gear shifter “joystick” makes it look like a conventional in-car temp sensor. But it also has a humidity sensor built into it.

The A/C amplifier uses something called Fuzzy Logic, which basically means it attempts to “think” more like a human’s mind, with more shades of grey and less black and white decisions. Included in this logic process is even an input to the A/C amplifier regarding windshield wiper status, to aid in keeping fog off the windshield. Other inputs include a grill mounted ambient temp sensor, steering wheel switch inputs, a sun load sensor, and actuator door position signals. In the case of the ambient sensor, the information is multiplexed via the CAN bus from the ECM. Since the A/C amplifier isn’t on the CAN bus, a gateway module translates that info into a BEAN bus message, so the amplifier can receive this input.

The control head for HVAC and other driver activation functions is part of a DIC (Driver Information Center)-looking unit Toyota calls the Multi Display. Climate controls and audio, along with operations such as trip information and various hybrid graphics, are controlled and displayed by this unit. Toyota’s term for the vehicle’s instrument cluster is the Combination Meter ECU. The combination meter communicates with the A/C amplifier on the BEAN bus. The Multi Display communicates via the ACT LAN bus, meaning once again, some translations of bus messages to the A/C amplifier are required. You should be getting the idea by now how important this gateway module is to proper communications on the vehicle (Figure 11).

Figure 11: Mounted under the IP pad on the right side is the serial gateway module. This module is responsible for keeping the A/C ECU and a variety of serial buses communicating with each other.

OK, got all that? Good, because in the next MACS Service Reports (January 2011), we’ll take a deeper look under the hood of a Prius, to study more hardware that is equally radical to what we’ve covered in this issue. We’ll also take a look at diagnostic PIDs (Parameter IDs) like compressor speed, and other measurable readings, such as compressor current draw. Also included will be a peek at the HVAC system on the 2010 MY-up Gen III Prius.

When it comes to hybrids, all said and done, I would have to say that I’ve not seen such a major revolution in technology since computers for engine controls appeared in the 1980’s. I’m sure you’ll agree that even the HVAC systems, in and of themselves, are a real mind bender, with all their differences in technology.

Hybrid cars are polarizing – you either love them or hate them. Without proper training and a little bit of safety equipment, these vehicles can even be dangerous to work on. You’re probably going to walk away from this report feeling excited and challenged. Or, it may cause you to make up your mind that you’ll never work on a hybrid HVAC system. Remember however, that these ultra fuel economy vehicles are not a fad – they are definitely here to stay. Hopefully, you’re excited and challenged, because that’s what it takes to stay in business in this ever-changing profession!

I hope to see you at the MACS convention next month in Orlando, where I’m delivering training classes on “Working on Hybrid Vehicle A/C Systems.”