Toyota Hybrid System — THS

Toyota's hybrid system is the most common hybrid drivetrain on the road. It has been in production since 1997 with the original Prius and is now used in the Camry, RAV4, Highlander, Corolla, Sienna, Venza, Crown, and every Lexus hybrid. If you work on hybrids, you will work on Toyota's system. Understanding it is non-negotiable.

The heart of Toyota's system is the power split device — a single planetary gear set that connects three power sources. The engine connects to the planet carrier. Motor-Generator 1 (MG1) connects to the sun gear. Motor-Generator 2 (MG2) connects to the ring gear, which also drives the final drive and wheels. This one planetary gear set replaces a conventional transmission. There is no torque converter, no clutch packs, no shift solenoids — just gears and two electric motors. MG1 acts primarily as a generator (converting engine power to electricity) and also controls the effective gear ratio by varying its speed. MG2 acts primarily as a drive motor (converting electricity to wheel torque) and also performs regenerative braking.

At low speeds and light load, the engine stays off and MG2 drives the wheels using battery power alone. When more power is needed, the engine starts (MG1 cranks it through the planetary gear set — no traditional starter motor). At cruise, the engine runs at its most efficient RPM while MG1 generates electricity that either charges the battery or powers MG2 directly. During hard acceleration, the engine runs, MG1 generates, and MG2 adds torque — all three power sources contribute. During braking, MG2 becomes a generator and captures kinetic energy as electricity stored in the battery.

Toyota's system is a true series-parallel hybrid. In series mode: the engine drives MG1 as a generator, MG1 sends electricity to MG2, and MG2 drives the wheels — the engine is not mechanically connected to the wheels. In parallel mode: the engine drives the wheels mechanically through the planetary gear set while MG2 assists. Toyota's system seamlessly blends between these modes thousands of times per minute. The driver never feels any of it.

The hybrid battery on most Toyota hybrids is a nickel-metal hydride (NiMH) pack — 201.6V on the Prius, 244.8V on the Camry Hybrid. Newer models use lithium-ion. The battery lives under the rear seat or in the trunk. The inverter (which controls MG1 and MG2) has its own dedicated coolant loop — separate from the engine coolant. Inverter coolant must be serviced per the maintenance schedule, and using the wrong coolant can damage the power electronics. The most common maintenance issue is the 12V auxiliary battery dying — because the HV system is what starts the engine, a dead 12V battery prevents the HV system from powering up, and the vehicle is completely dead. Always check the 12V battery first on a no-start hybrid.