Date:         Sat, 20 May 2006 22:25:27 -0700
Reply-To:     Robert Fisher <refisher@MCHSI.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Robert Fisher <refisher@MCHSI.COM>
Subject:      Re: Speedometer Ok -Odometer and trip meter dead!?
Content-Type: text/plain; format=flowed; charset="iso-8859-1";
              reply-type=response

The problem with my piece isn't the main gear, it's the grey gear at the
opposite end of that shaft. All the forces applied to the mechanism, whether
from the main gear during normal operation or from the trip odo being reset
are applied to that gear. The shaft from the main gear doesn't connect to
anything else- the grey gear drives the upper mileage wheels and the lower
trip odo. I noticed when turning it by hand that it catches and begins to
slip each time when the grey gear is engaging the 'mile' wheel- the one just
inboard of it. At the same time that it's trying to turn that wheel it's
also trying to provide the force for the white 'tenths' wheel to turn the
'mile' wheel on the trip odo. In other words, 98% of it's revolution it's
just driving the fat gear that turns the 'tenths' wheel on the trip odo via
the little gear between them. However when it reaches the 'top' of its
travel it's turning the trip odo drive gear which in turn is driving the
'tenths' wheel which is driving the mile wheel (and if you went far enough
it would be driving all four wheels in the trip odo over at the same time);
at the same spot in its revolution it's driving the mile wheel on the main
odo, and every ten miles the mile wheel and the ten mile wheel and every
hundred miles all three of those plus the four on the trip odo... and so on.
It's just a little metal gear pressed onto a little metal rod. It isn't held
on by anything but faith and friction. I can hold it still with one thumb
and turn the main gear with the other thumb without much effort. It isn't at
all difficult to picture the press fit failing under those combined
stresses, particularly when the one (reset) applies a force opposite to the
rest of the forces acting on it. Add to that the additional force of having
wheels stuck together that are supposed to spin free during the reset and
there you go. In that sense perhaps it was 'bound to happen'.

However I don't think your analysis works in this case- we're talking about
two different gears with very different forces acting on them. At any rate
the tolerances that Mark described on his piece are apparently much greater
than those on mine, in that there is a point on mine where the trip odo
drive is still partially engaged while the reset levers are well into the
wheel cams. There's something wrong with the design of this thing- the first
thing that jumps out at me is that the teeth on the intermediate gears on
the trip odo could be shorter by as much as a couple of millimeters.

Which leads me back to the question: Are there different versions of this
thing, and are some more prone to this failure than others?

Cya,
Robert


----- Original Message -----
From: "John Bange" <jbange@GMAIL.COM>
To: <vanagon@GERRY.VANAGON.COM>
Sent: Saturday, May 20, 2006 5:41 PM
Subject: Re: Speedometer Ok -Odometer and trip meter dead!?


>
> I can readily picture that these two gears got gunked or gummed up and
> that
> while it didn't require a great amount of force to move them past the
> sticking point it was almost certainly enough to dislodge that crappy pot
> metal gear from its press on the main shaft after repeated applications.


That may be the case, but if so, the difference in the amount of "dislodging
force" applied by a gunked/stuck tenths wheel on the trip meter to the main
odo gear will not be much between a moving and a stopped vehicle. In the
case of a stuck tenths gear causing the main gear to come loose from the
shaft, you'd have two opposing forces:
 -the reset "fingers"on the cam lobes of the trip odo, to the stuck-on lower
tenths gear, through the tiny idler gear to the upper tenths gear on the
main odo shaft

this opposed by

 -the main gear's desire to NOT rotate due to being connected to the speedo
cable via a worm gear, which as we all know can only drive, cannot be
driven.

Now, the amount of pressure from your finger necessary to break the gear
loose would be the same whether that worm gear is moving or stationary, the
only difference being the distance you have to push the lever. There's just
no appreciable difference between pressing it while in motion vs pressing it
while stopped. I'm not saying pressing didn't pop the gear loose; just that
it was bound to happen either way.


--
John Bange
'90 Vanagon - "Geldsauger"