This fully automatic transmission has been developed under contract to the UK MoD by David Brown Defence Equipment Limited and is installed in the Challenger 1 MBT in service with the British Army and in Khalid tanks used by Jordan. More recently it has been fitted to the Challenger Training Tanks. Over 1,000 TN37 transmissions have been built. Further development of the TN37 resulted in the TN54 which is now in production and covered in the previous entry.

The TN37 has a single input and two in-line outputs at right angles to the input. Drive is transmitted from the engine through a splined coupling to a torque converter. The gear and direction of travel are engaged by correctly sequenced application of multiplate clutches, actuated by electro-solenoid valves. It has four forward and three reverse gears and the steering module is of the hydrostatic double-differential regenerative type.

Automatic control is by means of an electronic control unit operating hydraulic clutches through electro-solenoid valves. The transmission has integral filtered lubrication and hydraulic systems. Main vehicle brakes are of the integral oil-immersed multiplate type. A Power Take-Off (PTO) for the power pack cooling fans is mounted on top of the main casing.

Alternatively, the transmission is supplied with a mechanical steer unit incorporating controlled slip multiplate clutches.

Production as required. In service in the Challengerl (to be phased out of service with British Army, although some may be converted to other roles), Khalid MBTs and the Challenger Training Tank. In service with Jordan and the UK.

In early 1999, it was stated that part of the British Army's fleet would be passed onto Jordan with first deliveries taking place in the second half of 1999.