The development of T300 series units is easily traced back to the TN15 transmission developed by Self-Changing Gears (this became Alvis Transmissions which was purchased by David Brown Defence Equipment in 1998) for the Alvis Vehicles Scorpion family of tracked vehicles, which is still in production.

T300 transmissions are conceptually identical to TN15, but are of a new design using uprated components throughout, with the latest T320 type designed for vehicles weighing around 20 tonnes and engines of 350 bhp. T300 is currently installed in the Alvis Vehicles Stormer range of vehicles and in the Daewoo Heavy Industries Korean Infantry Fighting Vehicle (IFV). T320 units are fitted in a heavier version of the Korean IFV and units have also been incorporated in retrofitted versions of the AMX-13 and M113 ranges. The AMX-13 and M113 applications only reached the prototype stage.

T300 series units are automatic crossdrive transmissions, which can be mounted independently or dose-coupled to the engine. Drive is transmitted from the engine through a resilient input coupling to a centrifugal clutch. Epicyclic (planetary) geartrains are used in the change speed section and the transmission is fully bidirectional. Vehicle steering is by use of an integral Merritt-Brown triple-differential system and the two transmission outputs to the vehicle final drives are mounted at right angles to the transmission input.

The T320 automatic control system comprises an electrohydraulic valve block which receives and interprets signals from the microprocessor control panel. The control panel in turn interfaces with the system peripherals which either provide condition data or actuate a subsystem to supplement the automatic operation. Signals are received by the control panel from an electromagnetic speed sensor, a microswitch on the throttle linkage indicating coast or drive conditions and the driver's gear selector. Signals are transmitted from the panel to the electrohydraulic valve block and a solenoid on the engine fuel pump which acts as a throttle dip mechanism.

The driver's gear selector has seven positions for engine start and tow starting modes, emergency low gear and two operating modes with gear hold overrides.Oil pressure for hydraulic control and lubrication is supplied by an input-driven gear-type pump. Pressure for gear engagement during tow starting is provided by a piston-type plunger pump driven from the left-hand steering annulus.

The directional gears comprise a cluster of spiral bevel pinions in constant mesh with an input bevel gear. The direction of rotation can be changed by engaging a dog clutch with either of the pinions.

The engine drives a resilient input coupling, which minimises engine torsionals, into a centrifugal clutch. This provides an automatic method of creating a break between the engine and transmission.

The gearbox section consists of a three-speed compound epicyclic gearset connected to a low/high range splitter unit giving six speeds. Low range is obtained by use of an epicyclic train and high range by a multiplate clutch.

The proven Merritt-Brown triple-differential steering system ensures power is maintained through turns and provides a full axis turn capability. It comprises a central differential, a pair of output epicyclic geartrains and acts as a split path transmission. Drive is taken from the differential carrier into the gearbox section of the transmission and from there to the annuli of the output epicyclic geartrains. Drive is also taken through the differential to each of the sunwheels of the output geartrains. A hydraulically operated disc brake is connected to each sunwheel. Vehicle sprocket drive is taken from the planet carrier of each output epicyclic geartrain.

T300 in production. Installed in Alvis Stormer and Korean IFV. T320 in production. Installed in heavier versions of Korean IFV and retrofitted into AMX-13 and M113 variants for trials.