After 20 years in the automotive repair business, I somewhat accidentally found my way into a high school automotive technology classroom as an instructor. Over 10 years, I have risen through the ranks and now hold the position of Assistant Dean with a community college automotive program, and I have enjoyed every second of it. In addition to my day job, you can also find me traveling the continental U.S. and Canada presenting classes for working technicians through WorldPac. There is truly nothing better than seeing the “lightbulb turn on” and having a student find success as a technician.
Over these last 10 years, it has become increasingly evident that many of the technicians that I instruct struggle with the concepts of electricity and electronics that are essential in the repair of modern automobiles. My most recent class dealt with the subject of a control module that used duty cycle to control a component. I realized that, in my attempt to describe how duty cycle control happened within a given frequency, my voice had apparently turned into the wonky trombone voice of Charlie Brown’s mother. It was clear that I was talking and that the class was hearing me, but the words that were coming out of my mouth were not connecting.
I would say that in every class I ever teach, regardless of skill level, this phenomenon occurs with at least a portion of the attendees. The core of this problem stems from an industry issue of no required certification level for techs, no required training and an overall failure of our industry to correct these underlying problems. This is a story for another article, but I’m always concerned with what we can do now to help technicians who struggle with electricity and electronics. This industry issue is compounded in light of the rapid increase in electronically controlled systems, such as ADAS, and the evolution of the Electric Vehicle market. In this article, we will look at some concepts that are need-to-know for today’s technicians. Some of this may sound familiar, and if so, should be a good refresher for those who are more advanced. I hope to point out how advances in technology are compounding the need for advanced electricity and electronics knowledge and to shed some light on how to apply it.
AC – Alternating current – Let’s start with AC. AC is typically defined by current flow that is both positive and negative in polarity. AC voltage and current flow are a familiar concept to technicians. AC is created by the vehicle generator/alternator as well as devices such as two-wire ABS and Crankshaft Position sensors. While most are familiar with the concepts of AC voltage and current, it may have been quite some time since you have “ohmed out” an alternator stator. In fact, it would not be unusual if you have never had an alternator apart in your lifetime.
AC applied
Fast forward to the electric and hybrid electric vehicles. The motor-generator (MG) unit on most EVs and HEVs is very similar in function to the alternator in terms of the AC concept. The main difference is that the rotor on an HEV or EV is typically a permanent magnet with no field winding. These MG units are capable of hundreds of volts during regeneration, so if you were not comfortable with diagnosing AC from an alternator, the voltage provides a “shocking” realization (pun intended). You will need to understand how AC is generated to understand how to diagnose and fix one of these issues. You will also need to know that if we send voltage/current to the windings of the stator that the MG unit will propel the vehicle.
A hypothetical case study supports why this knowledge is essential. Imagine a customer with a high-mileage HEV or EV comes in with the complaint of a slight shudder during initial acceleration. There are no codes present in the engine control module or the high-voltage ECU. After confirming the complaint and ruling out other driveline vibrations, the technician thinks that this may be coming from a problem with the vehicle transaxle which, in this case, contains a motor-generator unit.
For the motor-generator to propel the vehicle, a high-voltage ECU supplies the sequencing of the positive and negative polarity through the three windings (U-V-W) of the stator. The smooth operation of the MG during acceleration is dependent upon the equal application of voltage through each phase during each polarity change. This is controlled by the high current-rated insulated gate bipolar transistors (IGBT). The IGBTs are essentially creating north and south magnetic field polarity in each of the phases of the stator and that acts as the “push-pull” force needed to turn the motor.
Providing the shudder is coming from an MG unit, the failure of an IGBT is one possibility for this vibration. But the more likely cause is a difference in the resistance of the three windings themselves. Each of the windings of the MG unit must be the same overall resistance to carry out the push-pull to propel the vehicle. To further explain this, the stator is allowing the voltage/current applied to each winding to provide the push and pull required to turn the rotor and essentially move the wheels. If the resistance of each winding is not identical, this could lead to a shudder phenomenon such as described by the driver of this problem vehicle.
Now that we understand how the AC concept is occurring, we have to determine which diagnostic practice to utilize. Factory service information would indicate the use of a megohmmeter or milliohmeter to properly diagnose one of these faults. For first-time HEV/EV repairers, this means a cost in tooling of about $3,000 for both tools. If you read Motor Age every month, you will know we have covered these tools on several occasions. If you missed those features, be sure to search the searchautoparts.com archives.
If you are not familiar with these tools, here is the Cliff’s Notes version:
A megohmmeter will utilize a high voltage (typically 500-1,000) to provide an electrical pressure test (think cooling system) of the three phases of the stator or other HV components. This test will pinpoint large variances in resistance or will help to identify shorts between the three phases or shorts to chassis ground. The megohmmeter test would provide a good starting point in diagnosis at a price tag of around $600. However, further testing may be required.
The milliohmmeter will certainly help to identify the cause of a shudder. Utilizing a milliohmmeter allows a technician to obtain precise resistance readings of each phase. Should one of the phases be different than published specifications, you have identified the culprit of the shudder. A variance in resistance of one winding will affect the pull-push of the rotor, especially at low motor speeds, but will not be as evident as the motor increases in speed. To obtain this information, a tech will need a good milliohmmeter, which retails for approximately $2,000.
The third way of testing the MG unit is through the use of a differential probe in conjunction with a PC-based lab scope. For the most part, this type of testing is not factory specified as there are still a great number of dealerships operating without lab scopes. Think of a differential probe as an attenuator for very high-voltage applications. A differential probe retails for about $400 and you will need to utilize three — one for each phase. In addition, you will need to be familiar with lab scope operation. The beauty of the differential probe is that you can easily view the signals coming from the MG unit by putting the vehicle on the lift and turning the vehicle wheels while observing the waveforms acquired. A bad phase should be noticeable from the output amplitude. As noted earlier, there is not typically factory-supported diagnostic information for this procedure, and you may want to obtain some hands-on training before utilizing this tool for diagnostics.
More AC challenges
To charge plug-in HEVs and EVs, the vehicle owner will have to utilize the vehicle charging plug as well as the wiring connected to the electrical service in their home. The issue this brings to light is that you have an infrastructure side; a customer interface scenario, in which the customer has to connect the charger; and a vehicle side that determines if the connection to infrastructure has been properly established to begin the charging event. The challenge in these repairs will naturally come down to identifying which of these three parts of the charging equation are the cause of the concern. Is the infrastructure/charging unit in the home operating correctly? Does the customer understand how to operate the charger and how to properly connect to the vehicle? Or is it a vehicle charging system problem? These are certainly the issues you will run into once these vehicles make their way to high-level adoption.
The charging systems for PHEVs and EVs are designed and classified by ISO and SAE under standard J1772. The connector that interfaces with the vehicle is commonly known as a “J plug” for this reason. The two voltages available for chargers are known as “level 1” or “level 2.” Level 1 chargers are rated at 120V AC while level 2 is rated at up to 240V AC. As automotive technicians, AC voltage at this level is not something we are traditionally trained to repair. The answer to diagnosing these types of concerns may come in the form of a tool that is less intrusive then you might suspect — the vehicle scan tool.
Toyota PHV Charge
Cancellation History – 2012+ Toyota Prius PHEV
If you have read enough of my articles over the years you may have noticed my affinity for the Toyota nameplate. In fact, in recent Motor Age issues, you may have seen articles on new diagnostics and utilities available on the Toyota Techstream scan tool platform. Toyota offers a Utility function on the Techstream particularly for this application; if the customer has issues charging the vehicle, the Techstream may store a DTC. In the event it does not, it puts the technician in a position of determining where the trouble lies. With the Techstream charge cancellation history data, a technician can now determine where the problem in the EV charging system is located. If the problem is due to infrastructure, the technician knows that they may need to contact an electrician to service the EV charger side attached to the home’s electrical network.
Toyota makes this information available to the independent repair market via www.techinfo.toyota.com in the form of a paid subscription that provides technicians access to Toyota repair manual information, electronic wiring diagrams, Quick Training Guides, Tech Tips and Service Bulletins.
Batteries
Batteries may be the single technology that is keeping the EV market from taking off. As of today, most batteries are using some form of lithium-ion or nickel-metal hydride. Both lithium and nickel are natural limited resources, which makes it difficult to mass produce batteries. Additionally, the size and weight of the batteries required to operate EVs are astounding. If you have never seen the battery on a Tesla it is worth a look —it is basically the entire undercarriage of the vehicle and at one point, costs nearly $40,000 to replace.
As technicians, we will have to have a good understanding of battery chemistries. As these vehicles begin to age, we will need to know if the battery itself can be repaired or if it needs an entire replacement. When replacing, we will also need to know where to acquire the replacement battery. Another additional complication is moving the battery once it is delivered to your shop, as it may require a fork-lift or, at the very least, a pallet jack. Once it is near the car, you will also need to know how to install it. All this, plus you will also need to understand how to safely handle and install it, as it is potentially very dangerous without the proper knowledge of safety protocols.
Planning for the future
While EVs and HEVs are still a small part of the overall vehicle market, the technology they bring with them is leading us into a new time in automotive repair — a time to get serious about electrical diagnostics. Take every opportunity to immerse yourself in good literature, high quality training and, really, any opportunity to hone your skills (if you are still reading this you are on the right track). Remember that no one is going to invest more in you than yourself!
