Mercedes-Benz 722.3 / 722.4 (4G-Tronic)

Mercedes' four-speed automatic for longitudinal engines, launched with the W126 S-Class in 1979 and built until 1996. 4G-Tronic is what owners call it; Mercedes' own documents call it the third automatic transmission generation, and the workshop literature uses the type numbers — 722.3 for the W4A 040 and W4A 028, 722.4 for the smaller W4A 020.

Its defining property is that there is almost nothing electrical in it, and that is why it lasts. Control is mechanical and pneumatic throughout: a throttle pressure cable, a vacuum modulator, a governor. The only electrical parts in most cars are the kickdown solenoid and, on some, a solenoid that delays the 2-3 shift while the engine is cold to warm the catalyst faster. Read that against its own successor — the 722.6 on this site fails electrically before it fails mechanically, from speed sensors sitting in contaminated fluid — and the contrast is the whole character of this gearbox.

Everything that goes wrong with one goes wrong through the fluid. The rebuild trade's account and the enthusiast record agree on this from different evidence, and the list is short:

• The brake band is the single most-replaced part at overhaul.
• The reverse package springs fatigue with age, which then damages the piston and, on the 722.4, the B-3 damper cone ring behind it.
• The pump cup needs replacing in any car that has run low on fluid, or whose torque converter hub has overheated behind a worn converter.
• The valve body's only real problem is dirt. Channels clog and the plunger springs tire; the trade cleans and reuses these rather than re-parting them, which is unusual enough to be worth knowing before paying for a replacement.

So the maintenance that matters is fluid and cooling, and then the torque converter. Serviced fluid and a cooler that is clean inside and out will take one past 300,000 km without an overhaul, and past 300,000 miles in the enthusiast record. The one piece of genuinely preventive work is the torque converter, whose wear is what overheats the pump hub and starts the sequence above — the trade's advice is to do it at around 200,000 km rather than wait for a symptom.

Two behaviours read as faults and are not. Many calibrations move off in second gear to stop the car creeping and to smooth the take-up, dropping to first only with the selector in 2 or under sharp throttle; a driver who thinks the car feels flat from rest has found the calibration. And on V8 cars the shifter's B range exists for engine braking, not for anything else — it holds first longer on the overrun.

A detail worth knowing on an early car: everything built up to 1990 can be push-started, because a second fluid pump driven by the output shaft will pressurise the gearbox while the car rolls. Get to about 20 mph and move the selector from N to 2. That pump was deleted for the 1991 model year, so the same trick on a later car does nothing at all.

Do not confuse the four-speed with the 722.5. That is the same gearbox with an electrically controlled fifth-gear overdrive bolted on in a second housing — a transitional design with two housings and two control systems, replaced within six years by the 722.6, which does the same job with one of each.