The power unit fuel supply is controlled by a valve operated by a solenoid with current fed via a protection circuit so that should the power unit overheat or lose oil pressure then the fuel supply is cut off and so stopping the diesel engine. The protection circuit is manually overridden when starting the diesel engine.
The gearbox selection valves are operated by pneumatic cylinders, one for the drive gear and one for direction, and these are controlled by solenoid valves operated by an ex-British Railways diesel mutliple unit gear/direction controller. The controller also provides interlocking between cabs, ensuring that it can only be driven from one end at a time.
The throttle is controlled pneumatically by a solenoid valve and uses a brake valve as a pressure regulator to a bellows unit reacting against a spring. The locomotive brakes are air operated through a pressure regulating valve to four bogie mounted cylinders. When hauling a train of vacuum braked stock a combination valve operated by the vacuum brake valve applies the locomotive brakes in unison with the train.
There are two modes of operation: Shunt and Passenger. Shunt mode is for yard operation eliminating the deadman system and the removes the ability to generate vacuum for braking and so makes it impossible to pull passenger trains in this mode. Passenger mode activates the vacuum brakes and the deadman system. The deadman system requires resetting every 40 seconds, indicated by a siren. If it is not reset then 5 seconds later the power unit shuts down to tickover, the gears disengage and the brakes come on simultaneously. A Park switch disables the deadman system, applies the locomotive brakes and disengages the gears.
Vacuum for train braking is generated by a vane type exhauster driven by a hydraulic motor, which in turn is driven by a hydraulic variable displacement pump driven off the gearbox power take-off. Flow to the motor is restricted through alternative flow restrictors giving two running speeds 750 and 1400, available whatever the power unit speed. Four high vacuum receivers provide additional "suck" availability. The parking brake is cable-operated on one bogie actuated through a screw jack and powered by electric motor or hand in case of power failure.
Cab heating and demisting is by a diesel powered unit situated centrally feeding to both cabs simultaneously. Windscreen wipers and washers are pneumatically powered, as are the warning horns. The electrical system, 24 volts DC, is supplied by a power unit driven alternator and storage by two batteries. Air is supplied by a compressor driven by the power unit.
The cabs are modules mounted on neoprene pads and the interiors are lined with sound-absorbing material. The control panels are covered in leather cloth.
The bonnet side and roof panels are carried on three portal frames with the centre frame carrying the silencer, air filter and cab heater. The bonnet and roof panels do not carry any services and can be removed by two people in less than 30 minutes so making access for heavy maintenance easy.
Under the bonnet the layout is modular, with radiator, power unit/gearbox, exhauster package, brake package, electrical/pneumatic package, fuel tank and handbrake units. Each of these units can be lifted out separately after disconnection.
During 2007-8 Vale of Ffestiniog was out of service and rested for a long while on ambulance bogies, during which time parts were cannibalised to keep Castell Caernarfon running on the Welsh Highland Railway. By August 2008, with parts refurbished, returned, replaced, and a new repaint into a two tone green livery, Vale of Ffestiniog was ready for service again and on 12 March 2009 it was the first locomotive to cross Britannia Bridge in the new linked Festiniog / WHR era, with Empty coaching stock for Dinas.

























