If you are shopping for a used Tesla, you might assume the biggest risks come from early Model S or Model X examples with high mileage and fading battery packs. But according to a detailed buying guide published by The TSK Club, the single worst Tesla motor ever produced never actually powered a Tesla car. It was built into two vehicles you would never associate with the company: the Toyota RAV4 EV and the Mercedes-Benz B-Class Electric Drive.

That claim sounds provocative, and it is. But the story behind it reveals how Tesla's early strategy of selling electric powertrains to established automakers created a small fleet of orphaned vehicles with a shared, problematic motor. For anyone considering a used EV from that era, understanding this piece of history is more than trivia. It could save you thousands of dollars in repairs.

The powertrain supplier phase

Before the Model S made Tesla a household name, the company's survival depended on selling its technology to larger automakers. In 2010, Tesla signed a deal with Toyota to supply the electric powertrain for the second-generation RAV4 EV, which launched in 2012. A similar arrangement with Mercedes produced the B-Class Electric Drive, appearing in 2014. Both vehicles used a Tesla-designed and built drive unit: a combination of electric motor, inverter, and gearbox.

That motor, sometimes referred to as the "large drive unit" or simply the Tesla-supplied motor, was built in limited numbers. Toyota and Mercedes handled the rest of the vehicle. Tesla's own cars from the same period used a different motor design, refined in-house. The Toyota and Mercedes applications were effectively a beta test for a powertrain that never made it into a production Tesla.

What made it the worst

The buying guide from The TSK Club does not mince words. It describes this motor as the most failure-prone Tesla ever built, citing early rotor bearing failures, coolant leaks that damaged the inverter, and overall reliability that fell far short of Tesla's own standards. Owners of the RAV4 EV and B-Class Electric Drive reported sudden loss of power, strange noises from the drive unit, and expensive repairs that could easily exceed the car's resale value.

Why did this motor struggle? One theory is that the packaging constraints imposed by Toyota and Mercedes forced Tesla to compromise on cooling and vibration isolation. Another is that the motor was simply an early, less mature design that Tesla never had to support long-term in its own lineup. Whatever the exact cause, the result was a powertrain that earned a reputation among enthusiasts and repair shops as the one to avoid.

What this means for used EV buyers

If you are shopping for a used EV from this era, the RAV4 EV and B-Class Electric Drive can be tempting. They wear the badges of established brands, and they carry the allure of Tesla technology without the Tesla price. But the TSK Club guide warns that these vehicles should be approached with extreme caution. Replacement drive units are scarce and expensive, and independent repair expertise is rare.

For the RAV4 EV, which was sold mostly in California in limited numbers, a working motor is the single most valuable component. A failure can total the car. The B-Class Electric Drive, more common in Europe and some U.S. states, shares the same vulnerability. The buying guide recommends that prospective buyers insist on a full service history and, if possible, a pre-purchase inspection by a shop familiar with Tesla powertrains.

A lesson in platform history

The story of this motor is also a lesson in how automakers cover for each other's weaknesses. Toyota and Mercedes built what they knew: bodies, interiors, crash structures, and dealer networks. Tesla built what it knew: batteries and motors. Neither partner fully controlled the other half. When the motor failed, Toyota and Mercedes dealers often had no training to diagnose it, and Tesla had no obligation to support a product it sold to a competitor.

The result is a small fleet of orphan cars, each with a ticking time bomb under the floor. If you already own one, the TSK Club guide suggests proactively checking coolant levels, listening for bearing noise, and maintaining a relationship with a Tesla-capable independent shop. If you are looking to buy, the safest move is to steer toward a Model S from the same era, even if it costs more upfront.

The broader takeaway

Tesla's early powertrain deals were a survival tactic. They kept the company alive through the hard years between the Roadster and the Model S. But they also produced a motor that, by any measure, was Tesla's worst. The fact that it ended up in a Toyota and a Mercedes, not a Tesla, is a quirk of history. For used car buyers, it is a warning that not all Tesla-powered vehicles are created equal.

The TSK Club buying guide puts it bluntly: you are better off buying a used Tesla than a used EV from Toyota or Mercedes that happens to have a Tesla motor. The company learned from its mistakes, and every drive unit built after 2015 is more reliable. But for those few thousand vehicles from that partnership era, the risk is real.

Before you hand over cash for a RAV4 EV or B-Class Electric Drive, do your homework. The motor might have the Tesla logo, but it carries the worst record the company ever produced. That is a secret worth knowing.