Speedometer inaccuracy, 1989 Audi 100

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

Here is some info I discovered on aligning the speedometer and setting the 
odometer on a 1989 Audi 100.  It will probably be of no use to the vast 
majority of readers on this list, but it took me a long time to find out, so 
here it is anyway.

I was given a spare speedometer  with 90k on the odometer when I bought my 
Avant - it had been replaced a couple of times a couple of years previously 
because it went intermittent and the fault was eventually traced to the 
sensor.  It is an electric device which uses a moving coil meter movement 
for the speedometer and a stepper motor to drive the odometer.

The replacement speedometer showed a very low mileage (4338) and the true 
mileage of the car was 115k, so I connected the spare speedometer up to a 
12V PSU and an audio oscillator set to +12dBm, 600ohms, 315Hz.  The dividing 
ratio of the speedometer is 6644 (printed on the bit of paper stuck to the 
back, so 315Hz corresponds to 170.68mph).

147 hours later, the odometer reads 115,000miles.

I also noticed that the speedometer wasn't very accurate.  30, 60 and 90mph 
should correspond to 55.4, 110.7 and 166.1Hz.  My speedo was close at 30mph, 
about 5mph out at 60mph and more than 10mph out at 120mph (not that I plan 
on doing much of this speed).

The chip that drives the speedometer and the odometer is an ITT UAF2115.  I 
couldn't find any reference to this chip anywhere, so I e-mailed ITT.  They 
sent me a PDF file with the data sheet for a UAF1025 which apparently is 
identical.  This shows that there is a series resistor connected to the 
speedo which is used to adjust the speedo sensitivity.  The resistor on the 
circuit board was 51ohms.  I found that 54ohms gave a much better response 
along with a realignment of the mechanical zero position.  The speedo is now 
within 1mph between 30 mph and 60 mph and within 2mph over the rest of the 
useful range.

Of course, the speedo is only accurate when the tyres have the correct 
diameter, but the anal retentive amongst us could use a variable resistor to 
compensate for tyre wear (just kidding).

Note: These adjustments have no effect on the odometer.  That just divides 
the sensor pulses to give 1mile for every 6644 pulses.

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