Highlander Hybrid Inverter

Highlander Hybrid Inverter

I don't know how to say this, but...

“I hate to be the bearer of bad news, but your car requires a $13,000 repair.”

I’ve had this conversation twice now with customers who own a 2006-2008 Toyota Highlander Hybrid. One customer has about 97,800 miles, the other has 100,500 miles. Can you guess what the warranty coverage period is? 8 years or 100,000 miles — whichever occurs first!

The master warning indicator and VSC indicators illuminated after the customer accelerated from a stop. They felt a lurching / hesitation that was alarming enough to cause them to rush to the shop right away. We started diagnosis by using the Toyota Techstream and pulling out the following information from the ECU:

P0A7A = Generator Inverter Performance Malfunction

On the factory scan tool, there is also an information code associated with this particular fault. Information codes are Manufacturer-specific, and often they can be only retrieved by using their proprietary scan tool. In other words, if you didn’t have the Techstream, you may not be able to get an information code to pinpoint diagnosis or provide accurate information about the problem in question. For this example, the information code was:

#325 = Generator inverter fail signal detection (overcurrent by inverter assembly malfunction)

Briefly, the possible problems could be a wiring harness or connector, the HV transaxle, converter inverter assembly (which includes the MG [motor-generator] ECU), or the engine itself. Interestingly enough, when you research the enabling criteria for the fault, you will find a censored document. The logical method for determining the malfunction is protected Toyota corporation Intellectual Property. OK, then, let’s try something else.

Following the diagnostic tree, Toyota wants to eliminate the “easy stuff” first — and so do we:


Check to make sure everything is plugged in properly, like the HV ECU, then the MG ECU on the inverter.
Check to make sure that HV power is getting to what needs it; make sure the three-phase AC cables into the inverter are OK.
Check inter-phase resistance on the transaxle AC cable at inverter. Nothing should be greater than 3 milli ohm between the cables, spec is 36-43 when cold.
Check resistance between chassis ground and each phase (using megaohm meter). Should be 10,000,000,000 Ohm or more to verify no inter-phase short within the transaxle
at this point, if the car starts and sets the code, the Intellectual Property of diagnostic information means you replace the inverter/converter. Nice short cut!

In other words, the short is “somewhere” on the circuit side of the converter/inverter and it’s not necessary to tear it apart to find out specifically what’s gone wrong. This sort of reminds me of doing ABS diagnosis on mid-90s Honda products where brake fluid pressure is being lost, but you don’t really care to determine whether it’s the modulator or the accumulator side. You just replace both components.

Long story short, at time of writing, the inverter/converter assembly is an $8000 part (locally available, I might add — common problem?) that requires at least 4 hours of labor. The TSB outlines a production run of 2006-2008 Highlanders that had inverter/converter assemblies that are known to be defective. There is an upgraded part available, so this should presumably never happen again once it’s fixed. To save you some time, the Toyota warranty process MUST HAVE a symptom present before proceeding with replacement. They are NOT going to give you a free inverter “just because” Art’s Automotive’s website says it’s a problem. Sorry.


For those who are curious about what these components are actually doing on your hybrid, i’ll describe them briefly below. This section is likely to be too basic for anyone who is working on these cars.

First of all, let’s clarify some terminology that will always pop up when talking about electricity and the way it flows. There’s what’s called “alternating current (AC)” and there’s “direct current (DC)”, just like the 80s hair metal band. You can conceptualize the flow of electricity like the flow of water. When direct current is present, the water is flowing from the source of current (the hose faucet) in one direction toward your roses. Alternating current is a bit harder to describe with the water analogy, but if you conceptualize a sand castle built too near to the waves it might work. As the tide crashes in, it hits the castle from the ocean’s direction; when the tide recedes, it hits the castle from the opposite direction. In electricity, the “power” at which it hits the castle is identical as the current goes in one direction as the other (this is why the water analogy is not perfect). The sand castle would be the object being powered, and circuits designed to receive AC are engineered to function with current from both directions. In the real world, batteries provide direct current and your wall socket provides alternating current. If you’re still thirsty for details, you should do some wiki research to find out how the different types are generated — fascinating.

no relation to the performing artists

All right, so now we know that there’s such things as AC/DC, and that certain devices require by their design either direct or alternating current for power. On your Highlander (or Prius, or basically any electric vehicle I know of right now), the battery stores DC and most “little stuff” on the car runs off of this current source. However, the “big stuff” is the electric motor and A/C compressor, and they run off of AC. So the question becomes how does the car create two different forms of power. Likewise, how does the car charge up the battery?

The answer is our $13,000 friend mentioned above! The purpose of the inverter/converter is just that: to invert power from direct current and change its direction, thus providing AC as needed; and, to convert power generated by regenerative braking into DC for storage within the battery. I should mention that it is more correct to say that it converts existing DC into different voltages as needed by different systems, but since we haven’t covered voltage yet, it might just muddle the mixture. When the hybrid brakes, it uses the electric motor to contribute to the slowing down. In so doing, it actually generates electricity through a concept called induction. Trust me on this for a second. Current created by induction is alternating. Thus, every time you brake, alternating current is being created then inverted into direct current by the inverter/converter. The direct current is then stored away into the battery cells. On the reverse side of things, when you accelerate the hybrid, direct current is drained from the battery into the inverter/converter. From there, it is inverted into alternating current and provided to the electric motor to contribute to the forward motion of the vehicle (synergistic hybrid). By now, you should understand that the inverter/converter is obviously integral to the proper functioning of the hybrid.

OK, so at this point, the hybrid components are described well enough that if you’re curious about what actually happens in the malfunction described above, I can talk a bit more about it. Of course, before we get to that answer, I will need to describe another fundamental behavior of electricity called voltage. Voltage and current are not interchangeable terms or concepts, but they are directly related by means of their “container.” Let’s go back to the water analogy to finish the thought. If you turn your garden hose onto maximum flow, water will leave the hose and fall an arbitrary distance from where you’re holding the hose. In this case, the hose is a flexible cylinder container. If you were to put your thumb over the hose opening, you can spray the water faster and farther. Now, you didn’t increase the flow of water, but you restricted it in a way that increased the pressure. Electricity works the same way, but with different words. Voltage is essentially the same thing as “water pressure.” Your thumb creating a water flow restriction is what we call “circuit resistance” in electrical engineering. Current, voltage, and resistance are all inextricably linked together, and their relationship can always be measured. To be clear, the inverter/converter on a hybrid vehicle is about a million times more complicated than a hose with a thumb, but the principles of how a static amount of water flow can be adjusted by restriction to create different water pressures is crucial. In fact, it’s a universal law.

Right, so now that we understand voltage, we can talk about how resistance matters. Quite simply, both water and electricity flow ALWAYS chooses the path of least resistance. If you had a bucket of water with two holes in the bottom, the biggest hole will flow the most water. Common sense, right? Well, if you conceptualize electrical circuits as basically complicated buckets, whenever you have a “big hole” in the circuit, the current will flow out of that hole and affect the rest of the circuit — mainly, it affects the electrical pressure of that circuit. A garden hose with a huge slash in the middle of the hose means you have less pressure at the outlet nozzle of the hose. Same exact concept. Electrical components are engineered to work with a certain amount of current at a certain amount of pressure — that is, voltage. So when people talk about a short circuit, they’re basically saying a “big hole” in the circuit. A hole has no resistance to the flow of water, thus it all rushes out; likewise, a short circuit is zero resistance to the flow of current, so it all leaks out. In the problem described above, there is a short somewhere within the Motor/generator (electric engine), the inverter/converter, or anything that connects the stuff together. In order to logically find out where the leak is, you are conceptually “checking the bucket for holes.” What if you couldn’t actually look at the bucket and say “yep, there’s your hole”? You would need to know that the bucket has finite measurements for the length of the sides and the width of the base, for example. If you find that the base doesn’t measure the full amount, then you could conclude that there’s a missing portion of the base that would allow for a leak when filled with water. That’s all you ever can do when testing electrical systems like these is to compare resistance (container) measurements to what are published specifications. If something has incorrect measurements, you can logically conclude the problem is there.

To clarify one last thing, if there actually is a hole in an electrical circuit where two wires stop touching, current flow will STOP. This can be thought of in the same way as turning off the garden hose. There is water present from the house up to the hose faucet, but when the faucet is off, nothing flows because there is no path for it to flow. In fact, the faucet would be equivalent of an electrical switch. If voltage is present into the switch — if water pressure is available from the house — then when you turn on the switch, current will flow to the end of the circuit just like water flows to the end of the hose. I guess you can think of auto mechanics as a type of plumber in a sense, eh? We are better dressers in my opinion.

Whew. I did not really set out to give a lecture, but I hope this helps.

just a simple diagram to show what it takes to turn on your blinkers and other gauge lights