ZF 4HP22 / 4HP24

ZF's longitudinal four-speed — in the Land Rover Discovery and Defender, and across BMW, Jaguar, Peugeot and Volvo in the 1980s and 1990s.

On a Land Rover it is the transmission behind the vehicles that work, and heat is what kills them: towing and low-range crawling put load through the torque converter rather than the gearbox, and the fluid pays for it first.

Its recurring failure is the torque converter's lock-up clutch and the valve body. A shudder at light throttle is the converter; a delayed engagement into drive that clears as the vehicle warms is pressure rather than friction, and points at the pump and the valve body rather than the clutches.

These predate sealed-for-life entirely, so the service interval was specified and is simply often skipped — a Discovery with unknown fluid history is worth a service before any diagnosis, because worn fluid produces every symptom above on its own.

It is not the 4HP20, PSA's transverse four-speed of similar name in the Peugeot 307 and 407; the two share a maker and a ratio count and nothing else.