Volkswagen 01M transmission
The Volkswagen 01M transmission is a hydraulic four-speed automatic transmission, developed in-house by Volkswagen, and deployed in Cabrio, Jetta, Golf, GTI, New Beetle, and transverse engine Passat cars manufactured between 1995 and 2002.
It is an electronically controlled based transmission with a lockup torque converter. It does not have provision for a dipstick. VW determined that the dipstick and fill might invite owners to introduce wrong or inferior fluid. Some areas of failure on this transmission include damage to plastic internals due to fluid over-temperature conditions, internal fluid pressure leaks from torn piston diaphragms, worn bores for solenoids in valve body, and the resulting worn clutches and bands. This transmission has a separate oil for the differential than for the transmission, so there are two fluid levels to check.
The shifting of this transmission is controlled by the transmission control computer. This computer is located under the back seat on Volkswagen Golf Mk3 /Jetta models, under the right side dashboard cover on Volkswagen New Beetle models, and in the wiper area plenum on other Volkswagen Golf Mk4/Jetta models. After about 10 years, the resistance in the wires and/or connectors grows. The additional resistance prevents the computer from reading the faint pulses from the transmission speed sensors. This causes the transmission to go to "fail safe" mode. This mode keeps the transmission in second gear and the gear indicator in the instrument panel indicates all gears are selected simultaneously.
It has since been replaced by an Aisin designed 5-speed automatic (09A) Tiptronic, and later 6-speed automatic (09G) Tiptronic transmission in New Beetle Convertibles, as well as a Direct-Shift Gearbox (DSG) based dual clutch transmission in recent models of the above cars. The DSG does not have a torque converter, and is more akin to a manual transmission.
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