How to service your automatic gearbox

Extend the life of your autobox and keep it shifting smoothly

Automatic gearboxes have grown in popularity since their mainstream commercial introduction by General Motors in the Forties. At first, an auto ’box was available only as an expensive option on high-end luxury cars, but in the Sixties the BMC Mini could be ordered as an auto. Another decade later, most cars had an auto ’box on their options list.

Preselector, constantly variable and clutchless transmissions were employed successfully by various manufacturers. The system that became the industry standard, however, features a fluid-drive torque converter that delivers rotation from the crankshaft to the gearbox in place of

a conventional clutch. The ’box typically has three ratios and uses epicyclic gears, which operate on a similar principle to the overdrive units frequently fitted behind manual ‘boxes. Gearchanges are made by unidirectional sprag clutches that work like a ratchet with rollers in a toothed gear or – more commonly – by cone clutches operated by hydraulic pressure.

Gears can be selected manually or automatically, the latter governed by inputs including road speed, engine speed and sometimes manifold vacuum. Most also have a ‘kickdown’ facility, where flooring the accelerator pedal causes the ’box to change down to maximise acceleration. Cars from the mid-eighties onwards often have more sophisticated multiple hydraulic gears that are controlled by a microprocessor similar to the ECU of an engine’s fuel injection system.

Most classic auto ’boxes can be serviced at home. Adjustment of the kickdown and selector is also possible, too. Regular fluid changes and cleaning or replacement the gearbox’s internal filters are recommended by most manufacturers, and this routine maintenance is essential for longevity and correct function. Repairs are complex and generally best left to experts, so neglecting maintenance can lead to big bills. Follow this handy guide to keep your classic’s auto ’box healthy and shifting sweetly.

You will need Fluid change intervals Level checking procedure Inspect fluid Basic tools, fluid receptacle, gearbox filter and sump gasket, correct grade of ATF. Intervals can be as long as 40,000 miles. A few claim to be ‘filled for life’ – but how long a life the manufacturer had in mind is questionable. Most automatics need to be up to temperature – but not unusually hot – before the level is checked. Some also require the engine to be running. Check your workshop manual for the procedure. Automatic transmission fluid (ATF) should be clear and red or pink in colour. A dark or burnt appearance can mean the internal brake rings or clutches are wearing – and definitely signify that replacement is due.