ZF 6HP26

ZF's six-speed automatic for longitudinal engines, developed with BMW and built in numbers approaching seven million. It is the gearbox behind an enormous share of rear-drive cars from the 2000s — BMW, Jaguar, Land Rover, Aston Martin, Bentley and Rolls-Royce all bought it — and the 6HP19, 6HP26 and 6HP32 are the same design at three torque ratings rather than three gearboxes.

There are two generations of it, and the second is the half most people miss. The first arrived in 2000 as the 6HP19, 6HP26 and 6HP32. In 2007 the whole range was replaced: the 6HP21 took over from the 6HP19 and the 6HP28 from the 6HP26, on the same Lepelletier gearset and the same ratios. A 6HP34 was drawn up and never built, because the eight-speed was already coming. So a car from 2008 with a ZF six-speed almost certainly has a second-generation unit, and the number on it will not be one of the three this page opens with. Production ended at Saarbrücken in March 2014 after 7,050,232 units.

It was the first transmission to use a Lepelletier gearset, which is why it delivers six ratios with far fewer parts than the five-speeds it replaced. That matters to an owner only in one respect: there is less inside it to wear, and what does wear is concentrated in a few places that are well understood.

The mechatronic sleeve is the failure that defines this unit, and it is not the gearbox wearing out. The bridge between the internal valve body and the external wiring passes through the case on a seal that hardens with heat and age. When it leaks, fluid tracks along the loom, gear selection turns erratic and a reader is often told the transmission is finished. It is a seal, and replacing it is a fraction of what a unit costs.

"Sealed for life" is the second thing to know, and it is the more expensive misunderstanding. These were delivered with no service interval and no dipstick, which many owners and some franchised dealers read as no fluid change ever. The fluid does degrade — shift quality falls away somewhere past 100,000 km — and the torque converter's lock-up clutch is what suffers first, shedding friction material that then circulates through the valve body and the solenoids. A gearbox that shudders lightly at low speed under gentle throttle is describing its lock-up clutch, and that symptom appearing before a service is the argument for having one.

The rest of the list follows from those two: solenoid wear, valve-bore wear in the valve body, and pump-end wear at high mileage. None is sudden, and all of them announce themselves in how the car shifts long before anything stops working.