What Are MG1 And MG2 On Your Prius?

Posted on: 15 October 2021

If you drive an older Prius, you may occasionally run into several unusual problems related to the Toyota's unique design for your vehicle. Even if you're familiar with common automotive issues, some of these repairs may leave you scratching your head. Some potentially expensive problems center on the HV transaxle, MG1, and MG2. But what are these components, and why do they fail?

As it turns out, all three are essential to the hybrid operation of the Prius engine and primarily responsible for its high efficiency.

The HV Transaxle Explained

Rear-wheel drive vehicles use an independent transmission and differential to transfer power from the engine to the rear wheels. Front-wheel drive vehicles, such as the Prius, instead rely on a transaxle. A transaxle combines the roles of the transmission and differential into a single compact unit. This design saves space, which is critical with so many components at the front of the car.

The Prius' hybrid vehicle (HV) transaxle is even more sophisticated, however. Unlike a traditional internal combustion engine (ICE) vehicle, the Prius must use its electric motors for power. The HV transaxle, therefore, must balance power between the internal combustion engine and the electric motors.

Note that the transmission in the Prius is also unique. Unlike most cars, it uses a variable transmission which Toyota refers to as a "power split device" (PSD). The PSD can switch between pure electric, pure gasoline, or hybrid vehicle modes as needed. Additionally, its continuously variable nature means using a single, constantly engaged gear rather than shifting.

Understanding MG1 and MG2 Problems

Think of your car as having three energy sources: the gasoline engine and two electric motors known as MG1 and MG2. In general, the car uses MG2 in electric-only mode since it's a larger and more powerful device. On the other hand, the smaller MG1 will supplement the gasoline engine to provide increased fuel efficiency. Either motor can work in reverse to charge the batteries.

Common problems with the MG1 and MG2 motors involve wear on the internal windings, reducing their resistance below the manufacturer's specifications. Repairing or replacing either electric motor requires technicians to remove the transaxle from the car. Since this job can involve significant labor, most shops will attempt to confirm their diagnosis of a bad electric motor before beginning.

Note that while failures of either electric motor are a severe problem, they won't necessarily disable the vehicle, nor do they usually require replacing the entire transaxle. A shop with experience in engine repairs, such as Prius engine repairs, can generally identify the faulty motor and replace it without installing a new transaxle or performing a complete rebuild.