The GM400 gearbox of the Silver Shadow: Technical information Operation Animation Maintenance

Maintenance of the GM400 gearbox

It is necessary to check the ATF level frequently, preferably every month when the car is used on a daily base. For that purpose the gearbox is provided with a dip stick in the engine bay. While checking the fluid level, the engine must run at idling speed with the parking brake firmly applied. Furthermore the engine must be warm, after a ride, or after 4 minutes running idle. It is necessary to put the gear lever to all gears before checking the fluid level. During checking the gear lever should be in 'Park' ('P'). Top-up if needed, using correct ATF. As long as the ATF is kept on the right level, the GM400 gearbox is very reliable. But if any fault should occur, it usually can be noted by a less smooth operation. Faults often will be found in one of the 'outside' parts. Parts that could easily be replaced are the vacuum hose (fitted between the vacuum modulator and the carburettor system), the vacuum modulator and the speed governor. It is always to be recommended to use original parts. For instance, some aftermarket vacuum hoses can cause a drop in the vacuum pressure system, that can cause a fault in operating the vacuum modulator. For replacing the modulator, be aware that some changes were carried through during production. Mind that the speed gear mechanism, connected either to the mechanical speedometer cable or to the electrical speed sensor (from 1977), can also be replaced without opening the gearbox. The only maintenance inside the gearbox consists of fitting a new fluid filter and fluid pan gasket. If any other malfunctioning occurs other than mentioned above, it's likely caused by an internal fault, and should be checked by a professional gearbox specialist. While purchasing parts always mention the right chassis number or VIN of the car, to be sure to receive the correct parts. If you are not sure your car is fitted with the original gearbox, you can best check the identification plate fitted on the left-hand side of the gearbox, stamped (for instance) 'R-R 69'. The ID tag on a GM400 gearbox.