Let's Clear the Air: Hybrid Vehicle Testing and Repair: Part 1 – First Generation Prius February 2012

Tuesday, February 28th, 2012 Technical Articles
Let's Clear the Air: Hybrid Vehicle Testing and Repair: Part 1 – First Generation Prius February 2012

Let’s Clear the Air: Hybrid Vehicle Testing and Repair: Part 1 – First Generation Prius February 2012

By Mark Schmidt, NAFTC
Author’s Note: We are now 13 years into the technical evolution of the modern hybrid electric production vehicle sold in this country. The following is part one of a series of columns on the subject of hybrid electric vehicles and their evolution over the years.
  Before working as the national instructor at the National Alternative Fuels Training Consortium (NAFTC), I was an automotive instructor at Ohio Technical College (OTC), a National Training Center member of the NAFTC. In 2002, OTC purchased a 2001 first generation Toyota Prius hybrid. The new Prius was displayed during Earth Day activities the following spring. Thousands of people walked by, looking at the vehicle like it was a spaceship. We displayed pictures, which showed the motor/generators (MGs), and these seemed to make people realize the difference in technology when contrasted to what they drove to the event.    I talked to a lot of people on that day, explaining the technology, which included a conventional internal combustion engine (ICE), coupled with an electric propulsion system. The Prius was equipped with a transmission with two internal MGs and a large nickel metal hydride (NiMH), high-voltage (HV) battery, consisting of 38 modules connected in series, with each being 7.2 volts. If you do the math, the HV battery supplied a minimum voltage of 273.6 volts DC (direct current) to the inverter, which used the stored energy for the MGs. The voltage at the liquid-cooled Inverter (see Figure 1) was as high as 300 volts due to a surface charge from the HV battery. While still relatively unfamiliar with the hybrid technology, attendees at the Earth Day event were genuinely interested in the environmental and economic benefits of driving a Prius.   Figure 1: Toyota Prius liquid-cooled inverter. Credit: NAFTC   With the introduction of the first generation Prius in this country, we needed to examine the differences between mild hybrids and full hybrids. While the Honda Insight – first introduced in the U.S. in 1999 – is a mild hybrid, the Prius is a full hybrid, meaning that it can launch from a stop using only the electric drive. Today all manufacturers that produce a full hybrid model use a liquid-cooled inverter. The inverter receives direct current from the stored energy in the HV battery and changes it to alternating current (AC) for the MGs.   As an automotive instructor teaching future technicians, I pointed out that the battery electronic control unit (ECU) was hard wired to the HV battery, monitoring every other module for voltage and temperature for several specific regions. A technician could view each of the parameter identifications (PIDs) that represented 19 monitored modules. The battery consisted of 38 modules; however, only one module of each set is hard wired into the battery ECU.   Maximum and minimum voltage could be viewed for each monitored module and corresponding temperatures for that region. If voltage did not display a consistent reading across all 19 modules, a determination could be made as to what set of modules were suspect, offering clues to the technician. At that time a battery replacement was the only repair option and in most cases was under the factory warranty. Today that model year vehicle has run out of the eight- to 10-year battery warranty, depending on where the vehicle was purchased. Below is a picture of a battery being tested (see Figure 2). Very few aftermarket repair shops are testing and repairing individual batteries for the general public. (Maybe another NAFTC eNews “Let’s Clear the Air” column will discuss more about advanced battery testing.) Some of our National Training Centers (NTCs) reside in states with large numbers of early generation hybrids. These training centers may be involved in advanced battery diagnoses. (This may be a good time to remind our readers that we welcome our NTCs and Associate Training Centers the opportunity to contribute to this eNews “Let’s Clear the Air” column.)   The picture below (see Figure 2) is a first generation Prius HV battery. Notice the blue wires. Each wire end connects an individual 7.2-volt battery to another individual battery for a total of 14.4 volts. Now look at the top of the picture. Notice the clamps connected to the open terminals. If a volt meter were measuring across the terminals – one positive and one negative – a voltage of more than 14.4 volts should appear on the meter. The preparation to get to this point was performed with all safety consideration carefully executed. At this point what you see are 19 different low-voltage batteries on a bench. To test this battery, the equipment used here would cost you up to $10,000. Data acquisition testers collect discharge and charge curves, while going through a controlled battery test. Is battery rebuilding your day job? Training and common sense practices using PIDs from the battery ECU may be all that is necessary to perform inexpensive battery diagnosis. The money you save in equipment could be used to buy eight to 10 years of subscriptions for Toyota’s Technical Information System (TIS) – $1,000 a year.   Figure 2: Toyota Prius battery being tested. Credit: NAFTC   After making the decision to test the system with a scan tool, the technician could interrogate the system by plugging a scan tool into the data link connector (DLC). Two network wires within the DLC provided access to the battery ECU computer through the controller area network (CAN).    Should scan diagnoses lead to further testing of the hybrid’s ECU, it is located in the passenger side floor. Pinpoint testing of individual circuits of the ECU would require the technician to pull back the carpeting enough to remove a steel plate bolted to the floor. After the plate is removed, the ECU circuits can be accessed.   The first generation Prius came equipped with software that allowed the technician to take over control of the battery management system and how it communicates with the ECU. The OEM scan tool was capable of controlling electric vehicle mode. During this control, the vehicle could be driven on pure electric mode up to a speed of about 37 miles per hour. During normal driving, the algorithm written into the Toyota program forced the internal combustion engine (ICE) to start and run on gasoline after reaching a speed of 14 miles per hour.   If a person were to drive the vehicle slowly, the vehicle would stay in electric vehicle mode until reaching a speed of 24 miles per hour. At that speed, the ICE would start up and assist the electric drive.   If a technician used an OEM scan tool to enter the transmission’s service mode for control of the MGs during a road test, electric drive could be controlled with only the accelerator pedal. Engine load was not part of this service mode because the ICE was disabled during electric drive mode. The electric motor (MG 2) could drive the vehicle at speeds as high as 37 mph for about five minutes. After the time elapsed, the vehicle would quit and a technician would notice a master warning light on the information center. By simply recycling the ignition key, the ICE would start and the Prius would take you home. That diagnostic feature was discontinued with the second generation Prius. It was a great diagnostic tool for the technician, but it must have been hard on the high-voltage battery warranty.   The first generation did not have an electronic air conditioning (AC) compressor.  The AC compressor was driven by a belt operated by the crank shaft. It also did not have a smart key, and the parking pawl was the same as any other conventional vehicle. The first generation was from model year (MY) 2001 through 2004. While I am not discussing the Atkinson Cycle in this part of the series, the engine, including the Atkinson cycle, will be covered in parts two and three.   The success of the first generation Prius helped lead the way for other hybrid electric vehicles, creating popular demand for hybrids all over the country. Hybrid technology has even played a prominent role in the increase of all advanced electric drive vehicles, including plug-in hybrids and all-electric vehicles.