Diagnosing HV Leaks

Jan. 1, 2020
Hybrids usually roll into your shop for the same thing: oil change, a CVT fluid change, the very rare brake job, maybe even a transmission or high voltage (HV) battery if the mileage is high enough.
Hybrids usually roll into your shop for the same things: oil change, a CVT fluid change, the very rare brake job, maybe even a transmission or high voltage (HV) battery if the mileage is high enough. It is rare to find other problems, though any competent hybrid technician will take precautions against them. One such problem is an HV leak.

HV leaks sometimes occur internally in the electric motor windings inside the transmission. Others result from dangerous pinhole leaks in the HV wiring and cables. Before you can determine the source of an HV leak, you will need to be acquainted with HV leak testing.

Safety First

Before we can perform any tests on an HV system, we need to be aware of the proper safety precautions. Make sure you have the proper information, safety equipment and hands-on training. Precautions must be taken before attempting to diagnose or repair any component that has orange wires connected to it. You will be working on a vehicle that is just as dangerous as the powerlines outside your shop.

When working on these vehicles, take off any watches, jewelry or any other bling you might be rocking. Remember the essential equipment that is needed: a CAT III meter and scope that is capable of handling 1,000 volts, correct vehicle information, 1,000-volt lineman's gloves with protective liners, an HV safety hook for the shop and a scan tool capable of working on that particular model.

Never fear HV electricity; just respect it and follow all safety procedures.

Before you do any testing or work on a hybrid's HV components, make sure to power down the vehicle by manually shutting off the HV battery power. This can be as simple as pulling out some sort of orange switch from the HV battery. We're focusing on a 2001 Toyota Prius in this article, but this procedure is different from vehicle to vehicle. Be sure to check your service information system before proceeding.

On the Gen 1 Prius, make sure the ignition key is completely off. If there is anyone else around, put the keys in your pocket or toolbox so no else can accidently turn the ignition back on. I've heard horror stories of people getting hurt because someone else turned a key or plugged something in, not knowing what the other guy was doing.

Now it's time to put on your lineman's gloves. With these gloves on, remove the shut-off in the left rear of the trunk. Now, go have a coffee break, tell your boss the G man said it's the thing to do. It will take about 10 minutes for any residual power to leave the wiring.

Real World HV Leak Diagnosis

Like any vehicle, you will start with the basics. That is speaking to the customer and taking a look at the dashboard. On Toyota hybrids a red "Triangle of Doom" will alert you that the Check Engine Light is on for a problem related to the HV system. Now it is time to plug in your scan tool. You will need a capable aftermarket or factory tool capable of communicating with the Battery Control Module and enhanced PCM data.

On a Gen 1 Prius, I was posed with a DTC P3120 (HV Transaxle Assembly Malfunction). So there's something wrong with the transmission, right? Something tells me on a hybrid it's not something as simple as a bad shift solenoid. I notice there is a sub code and it is for a DTC 253 (Generator Resolver Inter-phase Short).

We also paid some attention to the code definition:



The HV ECU checks the energy balance and detects abnormality if the magnetism of the motor or generator greatly decreases

Now, we're getting somewhere! Is there a short internal to the motor-generator (MG) or transmission? It would be nice to be sure it's not associated wiring before we sell the customer on one of those, right? It's time to test, not guess.

So where do you think we should start our diagnosis in order to find a HV leak or short? With your Cat III certified automotive-hybrid specific meter. You will need such a Megaohm meter to perform an insulation test that will confirm that the HV cables and motor are either good or bad.

The test is not rocket science. It tests to see if there is 50 to 1,000 volts running through the wires, is any of it leaking through the insulation. How? The meter will zap a high voltage (but low amperage) burst of electricity out of the power wire on one side of the circuit, and with your black lead on the other side you take your reading. Judging from the reading, you know whether or not there is a high voltage leak in that section of the circuit.

Before we go over specifics on insulation tests, let's first think about what wiring is all about in a simple way. Doing an insulation test is like pressure-testing a pipe. You look for leaks in a pipe by increasing pressure to a point where you see water or some other liquid shooting out somewhere, right? Well, in wires, instead of liquid pressure, there's electrical pressure: voltage. An insulation test uses relatively high voltage to make current leakage detectable for us.

Whether or not wiring passes an insulation test tells us whether there is damage to a wire's insulation and/or conduit, or damage to the motor winding insulation. Hence the term insulation test, right?

This is how you do it. Your first step is to put the leads in the proper jacks: the insulation-specific jacks, not the voltage jacks. Then you will need to put the rotary switch on your Megaohm meter on the insulation (50 volts to 1,000 volts) position and select 1,000 volts to check the HV cables. The reason why you want to select 1,000 volts is to make sure that the wires are stressed about double the normal voltage that travels through the wires. As a reminder, make sure the HV voltage is powered down before performing the insulation test.

When doing an insulation test expect a reading on both HV cables/wires and MG (motor-generator/transmission) to be 2.2 gigaohms or a number very close to this. A bad reading (a number less than 2.2 gigaohms) indicates the cable/wire or MG winding may be compromised.

Figure 1 is an example of good and bad motor winding. We can see the difference between a good and bad AC motor-generator using an insulation test. We will get into greater detail on this particular motor later in this article. Right now we just want to make sure that you fully understand how to test for HV leaks no matter where they are.

Toyota HV Batteries and MGs

Toyota uses two electric motors inside the transmission. MG 1 mainly charges the vehicle's HV battery and provides power to MG 2, while also acting as a starter for the engine and controlling the vehicle's CVT transmission. MG 2 primarily drives the vehicle in electric-mode, but it also charges the HV battery during regenerative braking. The MGs are three-phase AC electrical motors that generate voltage in the 500-plus volt range.

The HV battery is also relatively simple. On a Gen 1 Prius, it is a 273.6 V battery pack made up of 38 smaller batteries. Each battery should be over 7 volts, with each pair (which you can read as a Parameter ID on your scan tool) between 14 to 15 volts. Gen 2 Prius 2004 and newer vehicles have 201.6 volts with 28 battery modules. They are also divided by two as reflected by scan tool PIDs. These battery modules have nearly identical voltage readings and are diagnosed in the same way. Generally, the batteries just run out of power like your laptop battery, and when individual battery cells dip below the specification, the HV battery is bad (most of the time). These batteries can also be taken apart and load tested if necessary. Usually, not all the batteries go bad at once, just a few do.

Back to diagnosing the P3120. After retrieving the DTCs, we had to figure out where to begin. Our plan of attack was to record the DTCs, clear them, and then take the vehicle for a test drive. The code quickly came back, so it was a persistent fault.

Being that transmissions fail a lot more than HV cables, we went right to testing the MG. Take the red meter end and connect it to one of the three wires (U, V or W) while taking the black lead and connecting it to the transaxle case ground. If the wires are shorted to ground, the reading on the meter will be under 2.2 gigaohms. The next step is to take the red lead and connect it to one of the three wires while connecting the black lead to one of the remaining two wires. Check all the wire pairings to see if there is a short in the windings. This is similar to the procedure used on testing the vehicle wiring.

Take the red lead on one end of the wire and connect the black lead to vehicle body ground. The reading should be 2.2 gigaohms. Your next step is to connect the red lead to one of the wires while connecting the other lead to the shielding on the wire to make sure that the copper wire is not shorting to the shielding. The final step is to connect the red lead to one of the wires and the black lead to the other wire until all wires are tested.

We confirmed that the MG had a shorted winding. Remember that the vehicle has to be powered down or the cables have to be disconnected to the battery.

Another Example of a P3000

In the Toyota hybrid repair world, a P3000 usually means a bad battery — at least according to the dealership. This vehicle has low mileage on it for a 2002 Toyota Prius, and the customer could not believe it needed a battery. She had done a little research of her own online and was insistent that everything that can theoretically cause the problem be tested first.

I worked directly with this shop on this vehicle and knew that they were going to do some leak testing. But before they got that far, it was necessary to scan for DTCs. This vehicle had a P3000 (Battery Control System fault) and a P3009 (HV Power Short Circuit) stored.

We decided to chase down the P3009 first. The code description informed us that everything from the orange power plug not being plugged in properly (this is very common) to a bad inverter or HV battery can be to blame.

The vehicle had a subcode 123, so we can see why the dealer thought that "HV Battery Malfunction" meant the HV battery was bad. However, when you look at the subcode in a little more detail, it's not so cut and dry.

A bad power cable was one of several possible causes according to the chart. Before selling the customer on a battery, we knew that a voltage leak test was necessary.

Now, OEM repair procedures are not always dependable. They miss glaring details from time to time. So, we checked the basics first. We properly powered down the HV system, checking to make sure the plug was fully plugged in. Also, make sure the vehicle has a Toyota glassmat battery in the trunk. A regular aftermarket 12V battery can create real trouble. Neither of these common problems were the case, so we proceeded to do some insulation tests on the high voltage cables, inverter/converter, and the MGs. Everything tested out fine and the flowchart was telling us to replace the Battery Control Module.

Well, not so fast! This is a 2002 Toyota Prius. Battery cells go bad on these all the time. Corrosion around the HV battery packs also is a major problem. Before we condemn the computer, it's time to test the HV battery.

With everything powered down and wearing lineman's gloves, remove the HV battery from the vehicle. Now, take it apart. Crazy things make these batteries go bad. We once saw dog hair make one go bad on a Gen 2 Prius. However, on this Prius it had a relatively standard corrosion problem around the battery terminals.

I had the shop simply put these in a Launch X-Sonic Clean ultrasonic cleaner and they came out like brand new.

So, time to put it all back together and see if it works right? No! Don't work hard, work smart. We have all the HV battery cells in front of us and we know this is a Gen 1 Prius. There is a very good chance the cells are bad. You might as well test and not guess if they are good or bad.

Now, there is no official procedure to how to do this, but there's an easy one you can do with a $20 load tester and a meter.

Just remember that each pack has 14 to 15 volts each, so each individual cell should be about 7 to 7.5 volts. This is the same on both the Gen 1 and Gen 2 Toyota Prius.

All the cells tested fine. So, with a simple load tester, we tested each cell in order to make sure none of these cells were carrying a surface charge. They should hold a load for about 5 seconds, but when in doubt play the comparison game. All the cells have identical characteristics, so if there is one that holds a load for a much shorter time than all the others, it's obvious that the cell is bad.

But that was not the case with this Prius. The wiring was good, there was no corrosion on the battery terminals now, and the individual cells were good. Now it's time to put everything back together and see if the battery ECU will be happy with what we did.

We drove the car 127 miles over the next week. The repair took! Savings to the customer was well over $3,000.

Now in this article, we only discussed the Toyota Prius. Everything from the Honda to Mercedes hybrid works differently and has differing specifications. However, the fundamentals of voltage leak testing are the same. High Voltage cables, transmission windings, and inverters/converters need to be in tip-top shape to make sure that HV stays contained and safe. The integrity of high voltage components is vital on hybrid vehicles.

G. Jerry Truglia, owner of ATTS and founder of TST, is an ASE Master Auto, Truck, School Bus, L1, CNG technician. He is the author / co-author / technical adviser on 25-plus books, including Understanding and Diagnosing Hybrid Vehicles. He is also a noted automotive trainer.

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