Re: Checking speedo

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

From: Sachelle Babbar <sbabbar@iris.nyit.edu>
>How do I check if the speedo head is fried on my car? The trans hall
>sender is good. I don't know if it is the wiring, speedo head, or another
>component. By the way, it IS electronic.

All the sender does is ground out the input pin on the speedo - on my Avant 
it does this 6644 times per mile.  If you turn on the ignition, then you 
could test out the speedo by tapping a grounded wire against terminal 31b 
(pin 9 on the IC - UAF 1025 or equivalent).  After 6644 taps, the odometer 
will have incremented by one mile (or after 50 or 60 taps, the last digit 
will have nudged forward about 1/10th of 1/10th of a mile.

I usually drive this pin with a signal from an oscillator (+12dBm, 600ohms, 
315Hz) which makes the speedo read 170.68mph!

It's worth checking for loose connections or a blown fuse before starting 
any of this.

Contact me if you need more info.  I have a circuit diagram of the VDO 
circuit and a data sheet on the chip.

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