At the moment I know there are 2 different retarder types, hydraulic and electronic. From what I can tell, Voith Dennis Tridents and ZF (ecolife) Scania Omnicitys must work in a similar way. I've also noticed some on ZF (5hp) and Alison gearboxes on Dennis Darts appear to be allmost completley inaudible. There are also some louder retarders on Voith DIWA gearboxes.
Can anyone tell which is hydraulic and electronic?
Thanks,
LH.
All ZF and Voith retarders are mounted inside the gearbox and are hydraulic. In ZF-equipped buses there is often a dash-mounted gearbox temperature indicator and when the retarder is used this rises, sometimes a great deal for example when used when going downhill for a prolonged period.
Voith retarders are very powerful and can in the right hands slow a bus to walking pace without using the brakes.
Electric retarders were seen many years ago (this is what a Telma is) and were little more than a large magnet around the prop-shaft and were generally only used on coaches. Nowadays coaches use either gearbox-mounted examples in the case of fully-automatics, or exhaust/engine brakes in those which are manuals or have automated manual transmissions. The latter are easily identifiable as the engine or exhaust brake cuts in momentarily when changing up a gear to slow the engine down quickly and allow a quicker shift.