For Voith, if you notice, the 1st gear needs to be revved to high
rpm because axle and engine are not connected. You'll get the
feeling that the engine is pushing hard at the last stages of 1st,
and yet the acceleration seemed to have plateued off. That's
because of power loss in the high rpm regime without lock-up.
Upon transition to 2nd, the bus transits gears and at the same
time locks-up. The difference with and without lock-up is obvious.
Most voith buses don't stay long in 2nd.
The biggest clue to look out for the lock-up is the feeling that
the acceleration does not match the engine's output. And if you
notice, if the BC lets go of the pedal in 1st, the engine rpm drops
at once. If its locked-up, the rpm should match the axle speed. For
2nd onwards you don't get that.
So if there's lock-up, when the hell did it engaged?
The answer came from taking a worn-out 792Z and a few other
MKIIIs. These Voiths prove interesting. During transition to 2nd,
unlike the normal voiths that lock-up and transit in a split
second, these ones attempt to lock-up, and then transit. So what
you hear is the high pitch whine of the engine suddenly getting
lower and lower as the engine speed slowly matches the axle speed
and then with a terrific jerk, the lock-up engages and the bus goes
into 2nd at that same moment.
For that, this is where the biggest strength of ZF lies. A Voith
down-shifting will not disengage lock-up unless its a 2nd to 1st.
Consequently you'll notice that most Voiths prefer to kick-down all
the way to 1st at relatively high speeds to achieve high
torque input to the axle. For ZF, if the bus needs to disengage
lock-up upon down-shifting to achieve that, it is capable of doing
so. So for Voith, if it goes back to 1st at high speed, what you
get is a overworked engine and practically no effect on the
axle.
Just a side note, just like a manual where you lift your foot
off the pedal and depress the clutch, for the lock-up clutch to
engage the same should occur. But for some worn-out ZFs, the engine
does not stop and what you get is a nauseating grinding sound as
the lock-up engages.
I get what you mean on the Voith part. That makes sense. See
when I have time to pong 792Z.
By the way, for ZF when disengaging lock-up and revving up from
second gear, techinically the gearbox does not drop to first gear,
except in rare cases when the speed of the bus is really low, or in
some LO's, which will result in a very strong jerk.
