Mon Jan 15 09:27:54 2007
From: Jerry Kaidor 

> My 83 300D is shaky between gears - engine revs up before shifting - I
> assume that is what you mean by "flairing".  I was told that there is a
> "vacuum assist thing" that can cause funny shifting if it is not working
> properly.

*** The "Vacuum assist thing" is called the Vacuum Modulator.  Basically,
it works off manifold vacuum.  In a gas car, manifold vacuum is inversely
proportional to your amount of hurry.  When you stomp on it, the vacuum
goes down ( absolute manifold pressure comes up ) because you have opened
the door to the outside world ( the throttle ).  When you have only light
pressure on the throttle, contrariwise, manifold vacuum will be higher.

  So the vacuum modulator looks at your manifold vacuum to figure out how
much of a hurry you're in.  If you're in a big hurry ( low vacuum or high
absolute manifold pressure ), it causes the transmission to shift faster.
Clunk!  If you're not in a hurry ( high vacuum ), the transmission can
take its time and make things smoother.

  Unfortunately, a diesel car does not have manifold vacuum, because there
is no throttle.  So Mercedes simulated it with a gadget on the injection
pump called the "VCV" or "Vacuum Controller Valve".  The VCV works off
the vacuum in the vacuum bus, which is created by the vacuum pump at the
front of the engine.  It ( the VCV ) acts as the lower leg of a vacuum
voltage divider.  The upper leg is a restriction orifice in the line to
the
VCV.  The output of the restriction orifice goes to two places:  The upper
end of the VCV, and the vacuum modulator.  The lower end of the VCV
goes to ambient - call it "ground" :).

   My 300D was delivered to me with the vacuum modulator misconnected -
effectively disconnected.  The transmission was shifting so abruptly that
it *hurt my back* and made rubber.

                          - Jerry Kaidor