When Mercedes-Benz E-series transmissions "lose their gears",
they're in a condition called "limp mode". Put quite simply, the
transmission's computer has detected (correctly or incorrectly) a
fault with the transmission and will only allow forward movement in
hydraulic (as opposed to lock-up) 2nd gear. Quite often, restarting
the bus using correct procedure will fix this problem.
ZF Ecomat transmissions commonly suffer a problem where the
transmission does not lock-up at all, or sometimes there will be no
lock-ups in particular gears, especially in 2nd. In this condition,
the transmission will rev the engine much higher than normal before
upshifting and during steady transients, the engine will appear to
"free-rev" (i.e. sound like it's being revved in neutral). There is
also the characteristic feel of "less gearchanges" in this
condition. As far as I know, the only way to rectify this problem
is with an overhaul of the transmission.
Regards,
Dave
