Maintenance Technology

Are You Ready for 800 Volts?

Education Is Key When Servicing EVs

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Fig. 1 The move from analog to digital meters is essential.

| Photo Credit: All Photos by ACDC

Editor’s note: This is the second in a series of articles about servicing electrified motor vehicles (EVMs.) The author’s company, Automotive Career Development Center (ACDC) coined that term to describe a whole range of electric vehicles, including hybrids, plug-in hybrids and more.


This article will compare the low voltage system to the high voltage system and explain how a 12-volt brain can get confused. It is essential that you master the principles of electronic controls, reprogramming, oscilloscope usage, reading wiring diagrams and all aspects of electronic systems used in today’s modern vehicles in order to transition into high voltage systems. Do not work on high voltage systems (over 60 volts DC) if you do not fully understand the 12-volt world. If you are red / green color blind, or have any disability that interferes with mainstream learning, you can do this, if you seek out extra help. I am a color blind technician. 

Electrical work on any HEV or EV sets in place a potentially dangerous environment. Fear is not the point of this statement. Think of the word “respect.” Anyone who rides and/or races motorcycles knows all too well what happens when you do not respect some piece of technology that can hurt or kill you. I am one of them. Recently I bought a new Zero SR/F electric sport bike. Zero to 60 mph in 3.6 seconds. Top speed of 124 mph. Over 100 volts powering the rear wheel. Torque on demand. Respect.   

What you have learned so far about any electrical system will pay off if you stay in the field of motorized transportation, be it two, four or more wheels. Fleets are moving faster to adopt EVs than the private motorist, as the economic savings are there. The fuel and maintenance savings of pure electric power are huge. Get to as many electrical training classes as you can. ACDC can help with our online training and live classes, but there are many good training providers.

Is Ohm’s law useful? 

Ohm’s law is a formula used to calculate the relationship between voltage, current and resistance in an electrical circuit. From the ‘50s to today, most conventional motor vehicles used a 12.6-volt battery and a 14- to 15-volt charging system. The technician could remember basic electrical norms because one part of Ohm’s law did not change much. That part was voltage. 

All was well until the early ‘90s when California required zero emission vehicles, and in the mid-’90s they started to appear on the roads there. It was a California law designed to clear the air. In 1997, the Toyota Prius was sold to Japanese drivers and the hard work of getting to zero emissions was finally underway worldwide. Technicians all over Mother Earth saw vehicles of all types with higher and higher voltages. Now, the voltage was no longer 12 or 24, but 100, 144, 158, 244, 275, 288, 300, 330, 400, 800 and any number in between. To technicians, Ohm’s law is as fundamentally important as Einstein’s Relativity equation (E = mc²) is to physicists. No need to review Ohm’s law here, as all you need to do is look it up and understand it. It explains why voltages increased.

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Fig. 2 Fox Valley meters were popular back in the 1960s.  When an analog meter was measuring voltage, it put a load on the circuit.


DVOM and ghost voltage

This is an issue that relates to using a (Fig. 1) high impedance meter. Some ancient history to set the context. Before computers were used in motor vehicles, a typical volt-ohm meter used a mechanical needle that would swing across the face of the meter. If it zeroed in the middle of the meter face, it could read negative voltage. To get the needle to move it took more current than a digital meter. Fox Valley (Fig. 2) meters were a popular brand in the ‘60s. Most mechanics back then received their updated training from a new vehicle dealer, if they worked there, or a parts company if they did not. The tool man drove a truck and often explained the new technology to their customers when we were buying tools. It was about 1980 when the “Fluke 88” hit the tool trucks. The main reason why we moved away from the old school analog meter, to a digital meter, was to prevent damage to a circuit board. When an analog meter was measuring voltage, it put a load on the circuit it was testing. If that extra load the meter was connected to was a computer circuit, smoke would come out of the computer. Not good.

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Fig. 3 With metal covers removed from high voltage components, ghost/stray voltage may appear on the DVOM.


After we moved to high impedance digital meters, all was going along just fine, until aftermarket technicians started experimenting with high voltage circuits using their DVOMs. Some testing was done with the protective metal covers removed (Fig. 3) from high voltage components to gain access to the connectors, bus-bars, relays and other such components. The OEMs would rather have us leave the HV parts alone, but that is not what aftermarket technicians do. One such test was underway at a class I was teaching in 2006. We measured over 80 volts of DC from the high voltage battery positive relay to the metal case that was bolted to chassis ground. How could that be?  It was a Toyota Prius M/Y 2004 owned by ACDC and it had never been modified. It ran great, had no codes and therefore no warning lights on the display. After some debate in class, Al Playter, a college instructor from Canada, told us we were reading “ghost” or “stray” voltage on the meter. Why?

Ghost voltage in a Prius  

“Ghost” or “stray” voltage is a term used by technicians to explain why the measured voltage on their DVOM is showing a voltage reading they didn’t expect to see. Let’s explore this topic. After we did a “Safe Down” we tested the HV battery and HV capacitors to make sure they were at a safe level (Fig. 4). On the capacitor side of the HV battery the reading was 0.004 volts. The system worked as designed. Safe here.

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Fig. 4 After a “safe down”, we tested the HV battery and HV capacitors to make sure they were at a safe level. In this case, we show 0.004 volts.


Then we re-installed the Orange Master Service Disconnect (MSD) and measured the voltage on the battery side and read 226.0-volts DC. (Fig. 5) Good numbers so far, but this live voltage reading was deadly. Remember this is NOT what you would do at a dealership. 

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Fig. 5 With the orange master service disconnect (MSD) was re-installed, voltage on the battery side showed 226.0 volts to DC. This level of voltage can be deadly.


Then we wanted to see what the voltage was from the positive contactor to chassis ground, expecting to see 0.00-volts. We powered on the car to “ready to drive” mode and then powered it down. This reading was taken just as it was powering down. We expected to see 0.00 volts, but saw 90.4-volts DC (Fig. 6) instead. In other words, the driver was asked to shut off the Prius while a fully protected technician was handling the meter and leads. This was part of an ACDC “Safe Down” procedure we were demonstrating at a hands-on class. We were experimenting, so we went looking for voltage from the positive contactor (battery side) to chassis ground. That led us to believe that there was a HV leak to chassis ground, but unknown to us we were chasing a “ghost”. 
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Fig. 6 With the vehicle powered to the “ready to drive” mode, then powered down, we expected to see 0.00 volts as it was powering down, but saw 90.4 volts DC.


We ordered an electrical accessory called a Ghost Voltage Eliminator (GVE) sold by Fluke (Fig. 7) and had it shipped overnight. The next day we installed this small adapter. It plugs into the same ports on your DVOM that the positive and negative leads fit into.

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Fig. 7 The Fluke ghost voltage eliminator (GVE) adapter plugs into the same ports on the DVOM and connects the leads to each other with a 3,000-ohm resistor.


The Ghost Voltage Eliminator (Fig 8) connects the leads to each other with a 3,000-ohm resistor. This small load (the resistor inside the GVE) removes the “stray” or ‘ghost” voltage. We did the same experiment as we did the day before. We powered on the car to “ready” mode and then powered it down. This reading was taken just as it was powering down. We expected to see 0.00 volts. We did. These “induced” voltages appear as “ghost voltage” due to conductors running in parallel with other “current carrying” conductors. This is common in EMVs when the metal covers are removed to access the HV cables. These “ghost” voltage readings “disappear” under load and that is what a Fluke SV225 does. It loads the circuit and makes it work — sort of the same way an old analog (Fox Valley) did. The DVOM will only show a voltage reading if it is connected to the same circuit that has energy in it.

0822_ASP_EV_8_web.jpgFig. 8 The GVE removes the ghost/stray voltage, eliminating this concern if the metal battery cover is removed.

 These magnetic fields are getting induced into the leads of your DVOM when the metal covers are off. This “electromagnetic energy” (when not shielded) is small but will confuse your DVOM, and ultimately you. The next article is this series will cover the orange cables and how the EMV computer finds high voltage leaks. Stay tuned.

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