Re: Additional non-electronic speedo questions...

[Eric Maxon <emaxon@megsinet.net>]

Audial wrote:
> 
> There have been two posts in the last day that make me wish I knew more about
> the internals of the mechanical Audi speedometers.  Has anyone dismantled one
> and seen how they work?  

Yes...

>Are they magnetic?  

Yes...

>Anyone rebuilt/recalibrated one
> successfully?  

Yes...  VDO left you a BIG helper-- by replacing the needle (pointer,
whatever you want to call it,) such that it's alligned with the hash
mark at the edge of the face plate, (counterclockwise of the stop peg by
about 15deg,) you've just calibrated it!  You may have to pull and
replace the needle a couple of times to get it to rest perfectly at the
hash mark, but once you've done so, you just gently lift it over the
stop peg and you're pretty much done.  Taking the speedo apart without
totally destroying it or making the face plate ugly requires a delicate
balance between extreme brute force and delicacy.  once you've got it
apart, repairs are generally straight forward.  (I usually disassemble
them to repair a dead odometer... either the drive gear or driven gear
of the main odometer begins to spin on its chrome shaft-- all I have to
do is but a dab of polymethymethacrylate super-glue on the gear and
slide it back on.)  To test your hash mark calibration, drive the speedo
with either a square point nail or an appropriately sized square ended
tap chucked into a power drill.  By running the speedo/odo at a constant
speed for a mile, and timing how long this takes, you can figure the
"acutal" speed, and compare this with the reading on the speedo.

>Are they similar in operation to, for instance, the speedo on
> my '81 Scirocco, which bounced and hunted all over the place and intermittent
> whirring noises late in its life?  

Yes, but usually the jumping is due to either a binding cable (binding
in the sheath due to rust,) or a binding O2 sensor reiminder light
odometer, (acutally a separate mechanical odometer in a black box that
sits either in the wiper motor trough or in the engine compartment near
the firewall-- depending upon model/year.)

>I seem to remember reading somewhere that
> early Porsche 928s had the similar problems with their speedometers...
>

Hmmm.... wouldn't mind some of THOSE problems, RWD or not.  Too bad
Porsche forgot the all wheel drive in their "S4".


Eric, the presently manic "Manicured Mechanic"
 
> Times like this I wish I still had my '82 Coupe GT...
> 
> TIA,
> Alex