Tesla Model S Drive-Unit Replacements: How Big A Problem?

It was a car company's worst public-relations nightmare.

Edmunds.com, the widely-read automotive website, bought a Tesla Model S for long-term testing in February 2013. After nine months and 11,000 miles, the car developed an ominous grinding noise under acceleration and deceleration.

Under warranty, Tesla replaced the drive unit, which is essentially the car's entire drivetrain--motor, inverter, and gearbox.

Four separate drivetrains

A few months and 8,000 miles later, the car stopped dead on a freeway on-ramp at night. The Model S was flat-bedded to the Tesla service center, where the drive unit was again replaced.

Then, just last month, the third drive unit started to make a "milling sound" after completing a 5,000-mile coast-to-coast round trip. The solution: Install drive unit No. 4.

Four drive units in 30,000 miles, all in full view of thousands of car-savvy readers. Cosmic bad luck?

Apparently not.

Motor Trend magazine's long-term test Model S has also had drive-unit problems. During a routine maintenance visit, a Tesla service tech noticed a funny sound during a test drive, and the drive unit was replaced.

Tesla Model S: the magazine's Car of the Year indeed.

How serious is the problem?

As a Model S owner, I certainly sat up and took notice. My car, a standard 85-kWh version (VIN 3996), has had no drivetrain issues in 18 months and 22,000 miles.

But what happens when its four-year / 50,000-mile warranty (including the powertrain) runs out? For me, given my annual mileage, I go out of warranty in only another 18 months or so. What were the odds I'd be on the hook for a drive-unit replacement, rumored to cost $15,000?

Tesla owner forums gave scant comfort. A Tesla Motor Club poll of 87 Model S owners revealed that a startling 28 of them had had their drive units replaced, a rate of 32 percent. Five of them (6 percent) had had multiple replacements.

(We should note that due to the poll's built-in selection bias, that percentage of drive-unit replacements is probably not representative of the larger Model S population.)

On Tesla's own website forum, dozens of owners weighed in with their tales of drive unit woes.

"Every car in my area has had at least one DU replaced," noted one. "I'm on my fifth drivetrain at 12,000 miles," reported another.

One poor fellow was on his sixth--as far as we know, the record for drive-unit futility.

The good news

Looking beneath the surface, though, all is not gloom and doom.

For one thing, most drive-unit replacements are the result of funny noises--not an actual failure.

Moreover, In the Tesla Motors Club forum poll, only seven of 28 owners with new drive units reported that the original drive had actually failed.

Most simply reported milling sounds or clunks. In a few cases, it was the Tesla technicians who first detected the suspicious noise during routine service--and replaced the drive units pre-emptively.

"Milling sound" has become standard argot among Tesla repair techs. In at least one case, a factory tech apparently diagnosed the problem by listening to the car over the phone.

And in all cases, the drive units were replaced under warranty, and didn't cost the owner a cent.

Moreover, the procedure is apparently quick and easy; most owners reported getting their updated cars back within a day or two.

(The forums mention an unfortunate car-service operator who used a Model S commercially--beyond its warranty, to boot--and supposedly had to pay $15,000 for a new drive unit. I was unable to confirm this, however.)

Too generous?

From an owners' point of view, the drive-unit problem doesn't seem to be a huge deal, at least in the short term. The fix is quick and free, and Tesla has been aggressively generous to owners who have those problems.

According to Tesla CEO Elon Musk, in fact, that eagerness to replace the drive units, combined with poor diagnosis, may be part of the "problem."

"In some cases, we (thought) that something (was) wrong with the drive unit, but it's actually something wrong with another part of the car," Musk said.

"And then we'd replace the drive unit, and that wouldn't solve the problem, because the drive unit wasn't the problem."