January’s Featured Course: High Voltage – Hands On with Electric Vehicles

January 4, 2021



This month’s featured course is electricity. This is a very unique course in it’s construction where we started off with basic 12-volt electrical systems and continued on all the way to the High voltage vehicles such as hybrids and full electric vehicles! We even made a dedicated generation 3 Prius that we use as a training aid when teaching the live hands-on courses around the country. We also put a heavy emphasis on building extra content into the program such as manuals, power points, lesson plans and worksheets. NATEF tasks were also covered in this course so if an instructor wanted to use this material to teach electrical content it is already to go with really no extra effort needed on the part of the instructor. Be sure to check out this new style of training available from AVI.


Sometimes called a Megameter.
When Insulation test button is depressed, it will send a high voltage reading 50-1000 volts into wire to test for leakage.
The hold button on the meter will act like a lock button on a cordless tool and keep supplying voltage for test without having to hold the button, good for 1 minute and 10-minute tests. Use this meter on all HV circuits and or components i.e.. Inverter, windings, compressor, battery and wiring.
Notice special lead jack for insulation testing. Often times when an insulation leak occurs you will have a DTC such as a HV cable/circuit insulation leak present.
Meter can be set at varying voltage test ranges example: 50-1000 volts.


The capacitors located inside an inverter assembly can retain high voltage for sever-al minutes after the HV disconnect has been removed on various makes and mod-els of HV vehicles. A safe practice to per-form before beginning work on any HV component or circuit is to perform the LIVE-DEAD-LIVE test. First perform steps on previous slide.
Second set your meter to voltage DC and with gloves on check your approved meter is working by touching the 12-volt battery terminals. You should get about 12 volt reading.
Next while still wearing gloves, go to component being serviced/tested and make sure there is no residual HV present from the capacitors by touching each of the HV wires and measuring voltage. It should be 0!
Finally, go back to 12-volt battery and touch the terminals again, this should prove that not only is your meter good, but the circuit has powered down and it is safe to remove gloves and begin your work.
Remember, the HV battery assembly will still have potentially fatal voltage so be careful when you remove its cover!

Learn Today…. Earn Tomorrow


To safely begin working on a HV vehicle we must first start by locating and removing the high voltage disconnect plug. Although the manufactures use various styles of these plugs as well as locating them in various places found on the vehicle, your information system has very clear locations and disconnect procedures for your safety.
Pictured here is the disconnect from a generation 3 Prius , which happens to be located directly on the HV battery pack. To remove this plug one would simply slide the switch to the left to disconnect the 12-volt circuitry, then lift up on the handle to pull it out of the HV battery pack. This procedure will isolate the HV from the rest of the vehicle after the capacitors have discharged typically located within the inverter on the vehicle.
Be careful and don’t\’ let your guard down with the disconnect removed. Remember, the HV battery still has all the individual cells located within it’s assembly and there is still high voltage present inside this battery pack if you were to remove the tin covers. I al-ways wear my high voltage protection devices when working on the battery pack even though I may have the disconnect unplugged!


It is common for HV vehicles to use two motor generators. Location: Inside the transmission. The stator windings and the rotor are exposed to transmission fluid. Often, coolant passages within the transaxle will help remove excess heat. The stator windings have three big wires which makes it a 3-phase device. The smaller white plug connects to a temperature sensor. Scan data can be used to help diagnose an over heating condition. Often, there are multiple IMG such as IMG 1 and 2.
Typically MG 2 is used to drive the vehicle while MG1 is used primarily to charge the HV battery.
Note: There are special tools that many manufactures have available to remove the MG’s from the transaxle assemblies. Remember, these have a strong magnetism that are wanting to hold them into the transmission, so be careful to use the appropriate tools to help minimize the risk of pinched fingers.
Here is a little trick if you think you have a shorted MG 2 in a Prius.
Turn key on, put in neutral, then remove the PLM (park lock Motor) relay. Then you can turn key off. Try pushing the vehicle, if you hear clicking/snapping sounds and see the engine rocking on its mounts, then you may have a shorted MG 2. Remove three wires to know for sure.
Remember that since there is no torque converters, a misfiring engine can seem like a bad transmission as well.


Milliohm meters are used in hybrid diagnostics to identify shorts between phases, as well as loose cables or terminal connections.
When diagnosing a motor or inverter-related DTC, your service information may require you to measure the resistance between all three phase pairs of the three–phase hybrid motor. Resistance specifications can be found in OEM service information, and are typically less than 200 milliohms. Use the lowest setting of the Amprobe MO-100 Milliohm Meter at 0-200 milliohms. A typical HV DVOM can not read resistance in such small values!
This month’s featured course:
High Voltage
Recommended other courses of interest:
LBT-281 Making money servicing hybrids, ASE L3 Hybrid /EV specialist, LBT-376 Best of Hybrids


LBT-398 High Voltage – Hands On with Electric Vehicles
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LBT-398 High Voltage – Hands On with Electric Vehicles

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AVI’s instructor John Forro walks the student through the practical ins and outs of low- and high-voltage electricity theory through lecture and examination of electric vehicle components. Forro shows the proper testing and servicing procedures involved with traditional 12 volt systems and the newer high voltage components found in electric vehicles.

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