Single-speed reduction gear (EV drive unit)

The gearbox in a battery-electric car — a single fixed reduction gear, in 159 models across 55 makes on this site.

It is on this site because it is the transmission those cars have, and because the most useful thing to say about it is what it does NOT have. There is no clutch, no torque converter, no belt, no pulleys, no synchros, no valve body, no solenoids and no gears to change. An electric motor makes its full torque from rest and revs to fifteen thousand or more, so a single reduction — typically somewhere near 9:1 — covers the whole speed range.

So almost every symptom this catalogue teaches you to read does not exist here. There is nothing to slip, nothing to flare, no shift to be harsh, no lock-up clutch to shudder, no fluid whose viscosity holds the drive together. A car with one of these cannot have a transmission fault of the kind the rest of this catalogue describes, and a diagnosis that starts by assuming it can will go a long way wrong.

What it does have is a gearset, bearings and a differential, and those are what fail.

• A whine that rises and falls with ROAD SPEED, present under power and on the overrun, is the reduction gearset or its bearings. Some whine is normal — there is no engine noise to mask it, which is why EV owners report gear noise that a combustion car's owner would never hear.
• A clunk on taking up or releasing drive is backlash in the gearset or the driveshafts, and it is far more noticeable on an electric car because torque arrives instantly.
• A noise that changes with steering or cornering load is a wheel bearing or a driveshaft, not the drive unit — the same distinction as everywhere else here.

The oil is the part people do not know about. A reduction gearbox holds gear oil, it is not sealed for life on most cars whatever the handbook implies, and a great many EVs have never had it changed because their owners were told there is no maintenance. On a vehicle that tows or does high mileage this is real.

Two-speed EV gearboxes exist and are the exception, in the Porsche Taycan and Audi e-tron GT — where the second ratio is for top speed rather than for launching. A car with two ratios can have a shift fault; a car with one cannot.

A single-speed reduction gear is not a CVT, despite both appearing under automatic in a listing, and it is not the e-CVT of a hybrid either — that is a planetary power split with two motors and an engine attached. Three quite different things share one word in a spec sheet.