Re: Speedo error

[paul.heneghan@bbc.co.uk (Paul Heneghan)]

From: Phil and Judy Rose <pjrose@servtech.com>
>Thanks for the useful info. I don't know yet if my speedo ('91 200q) is
>off, but it doesn't agree very well with the trip-computer speed, which
>always reads about 4 mph lower than the speedo itself. Is the computer's
>speed reading independent (electrically) from the speedometer's ultimate
>reading?

I don't have a trip computer in my cars, but I am pretty sure that they 
would be completely separate circuits.

>If so, then it's possible that changing the resistor you mentioned
>might bring the two speeds into better agreement?

It's worth a try if you don't mind removing the speedo.

> I suppose though that the
>difference could merely be from needle misalignment.

>From what I remember, the mechanical adjustment (needle misalignment) should 
be used to get the low end right (e.g. 20mph).  The high end (e.g. 70mph) is 
then adjusted using the "Galvanometer Current Setting Output Resistor".  In 
electronic terms this amounts to a offset and gain adjustments.

>BTW, did you lower the resistance in order to decrease your speedo reading
>or to _increase_ it?

>From what I remember, my speedo read too high, so I increased the resistor 
(thereby lowering the current) to get it to read correctly.  I actually 
removed the resistor, attached a resistance box and dialled up the 
resistance required to get the 70mph to read correctly.  Then I read the 
correct resistance value of the resistance box.

Paul
paul.heneghan@bbc.co.uk
1984 Audi 80 quattro
1989 Audi 100 Avant