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DMU Technology - The Basics

Control Systems

The vehicles are controlled by an electro-pnuematic (EP) system. Most of the cab controls work by electricity, which send signals to electrically controlled pnuemantic (EP) valves throughout the train which control the distribution of compressed air. The air operates the engine throttles, gearboxes and final drives, and auxiliaries such as the horns and wipers.

As DMUs evolved so did the control systems, meaning some of the early types were incompatible with others, so only vehicles with the same coupling code could be used in multiple.

Coupling Codes
Red Triangle The first system used for the West Riding hydraulic Derby Lightweights. These were withdrawn in 1964 and the code was reused in the '70s for Class 127s, although they were still wired as blue square.
Yellow Diamond The rest of the Derby Lightweights, the Met-Cam Lightweights and the Class 129s. These had the 'A' type control system. All the EP valves operated down the train without relays, powered by the battery in the car being driven from, which proved troublesome due to voltage drop.
White Circle Used on the Class 126 Swindons only. In truth there were two batches of these cars which were initially incompatible. The first had 'A' type controls and was based on Yellow Diamond but with the addition of relays. The second had 'L' type controls and was based on Blue Square. Modifications were done two allow the two to run together.
Blue Square The most common code. It introduced relays, so that the driving car only sent a signal to the other cars which then used their own battery power to operate the EP valves.
Orange Star Used only on Class 125 hydraulic sets. Based more on locomotive style multiple working, with pneumatic control of the engine speed.

There also some 'sub' variations on these codes over the years. Click here for some more information on coupling codes.

Electric connections between the cars were by jumper cables, normally 4 between each car. Other connections between cars were two vacuum brake pipes, on most cars an air pipe, and on some a water pipe. Along with the coupling, this meant that between 6 and 8 connections between each car!


All diesel multiple units used the Gresham & Craven twin pipe system. As on British Railways coaching stock the brakes operated on 0 - 21" of vacuum, using standard cylinders. But on DMUs the exhauster was belt driven from the engine, meaning that at the time the brakes require to be released when the vehicles were stationery, the engines were at idling and the exhausters running slow. The two pipe system incorporated large vacuum reservoirs which stored vacuum at up to 29" ready for when required. The second pipe was the 'release pipe' to distribute this 'high' vacuum. Also associated with the brakes was the AWS (automatic warning system).


As well as the exhauster other auxiliary components were required:

These were normally engine mounted and so ran constantly. They supplied air to operate the throttle motors, gears, final drives, windscreen wipers/washers and horn. The air was stored in reservoirs.
Belt driven from the cardan shafts, above a certain speed they generated electricity to charge the batteries and ran the controls, lights and heaters.
These diesel burning heaters each ran on a single electric motor and distributed warmed air through ducts to outlets in the vehicles floor. They could be controlled individually, or all at once, and could also feed through unheated air for ventilation.
Used to start the engines and operate the controls / heating / lighting when the vehicle was stationary or at slow speed. They could be formed of either lead acid or alkaline cells.

DMU Technology:
The basics

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