ZF 8HP

ZF's eight-speed automatic for longitudinal engines, in production since 2008 and past fifteen million units — the gearbox behind most rear- and all-wheel-drive cars sold in the last fifteen years, across more than twenty brands. BMW took it first, in the 7 Series; Jaguar, Land Rover, Audi, Alfa Romeo, Bentley, Rolls-Royce and Maserati followed, and Stellantis builds it under licence in the United States as the TorqueFlite 8 fitted to Jeep, Dodge, Chrysler and Ram.

Its trick is arithmetic rather than bulk: four planetary gearsets, two brakes and three clutches produce eight ratios with fewer rotating parts than the six-speed it replaced, because the power path runs through the sets in combination rather than in sequence. Only two shift elements are open in any gear, which is where its efficiency comes from.

The number after 8HP is the torque rating in tens of newton-metres, and nothing else. An 8HP45 takes 450 Nm, an 8HP70 takes 700, an 8HP90 takes 900 — and the range runs wider than those three, from the 8HP30 at the bottom through the 8HP50, 8HP51, 8HP55 and 8HP65 to the 8HP80 and 8HP95. All of them are this gearbox, which is why they all arrive here. It is commonly explained as petrol against diesel — because the heavier ratings did tend to land behind diesels and big V8s — but that is a correlation somebody has dressed up as a rule, and it misleads the moment you meet a petrol car with the heavy unit. Read the number as a load rating and the whole family becomes legible. A Roman suffix (8HP75/II, 8HP76/I) marks the generation, of which there have been four: 2010, 2014, 2018 and 2022, the later ones carrying 48-volt mild-hybrid and plug-in-hybrid motors inside the bellhousing.

Heat is what kills it, and the mechatronic unit is where the heat lands. The valve body and its control electronics sit inside the gearbox in the fluid, and the solenoids fail in the specific circumstance of a hot day in stationary traffic — no airflow, no road speed, the converter working hard and the fluid getting hotter than it ever does on a motorway. A gearbox that misbehaves in a jam and is faultless on a long run is describing this, and it is a different complaint from one that shifts badly everywhere.

"Filled for life" is the other half of the same argument, and it is the more expensive half. These were delivered without a service interval, which owners read as never, and the fluid is what carries heat away from the electronics above. What the trade finds at first overhaul is burnt oil, and burnt oil means the job is no longer the one part that failed:

• The torque converter's lock-up clutch, which shows vibration from around 100,000 miles and is the source of the debris that then circulates everywhere else.
• The friction clutches, replaced wholesale once the oil has been cooked.
• The valve body separator plate, a metal plate with rubber-covered tracks that degrade.
• The pressure-regulating solenoids, which are unusually sensitive to oil velocity and contamination — a specification that is fine with clean fluid and unforgiving without it.
• Splines on the E clutch hub, and the P3 planetary set, which are the mechanical end of the sequence rather than its beginning.

The honest summary for an owner: this is an extremely good gearbox with one bad habit, and the habit is inherited from the marketing rather than the engineering. Fluid and a working cooler are almost the whole of its maintenance. Ignore both and it will still do 150,000 miles — and then present a bill that assumes the whole unit is on the bench.