RWD - Ringlee: How to calibrate your speedometer!

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Ringlee: How to calibrate your speedometer! 700

One of the benefits of the new millennium is that it has become reasonably cheap to check your speedometer using a GPS. If you don't own one, I'm sure that your neighbor does!

When I checked my speedo against the GPS (Garmin Streetpilot) it revealed that the speedo read 10% too high. I.e., 110 km/h on the speedo was only 100 km/h in reality.

In order to verify this, I checked the speed against the odometer. At a certain speed, 1 km takes a certain number of seconds to pass. Example: 1 km @ 60 km/h takes 60 seconds. This test also showed a 10% difference between indicated speed and true speed (assuming that the odo is more accurate).

Having this much of an error, I felt very motivated to investigate the possibility of a calibration. Since the speedos of the 700 series are electronic, another approach than replacing an internal driver wheel had to be found.

By doing a lot of reasearch on the Internet I found that in most cars it actually could be done, merely by a change of a resistor. The speedo itself is build around the ubiquitous ITT UAF2115 chip (A datasheet could be found here: http://heneghan.members.beeb.net/audi/uaf2115_1ds.pdf ). And the resistor to be changed is the one that is connected to pin 4.

Enough theory! How to calibrate?

1. Remove the instrument cluster as shown in the FAQ, i.e. remove the plastic covers and unscrew the two screws.

2. When the cluster is removed, open it by removing a number of philips screws on the rear side. Notice that the silver colored screws are longer than the golden ones so make sure that you where they belong upon reassembly.

3. Remove the speedo by unscrewing four screws on the plastic circuit board side. Two of the screws are also holding two connectors. Do not mix them!

4. Carefully remove the meter needle by turning it counter clockwise while pulling. After that remove the "number plate" which is glued onto the meter. Don't worry, you will probably not need more glue to reattach it, it is quite sticky!

5. Unscrew the three small screws which now should be visible. After that, the meter core should come lose.

6. Unscrew the two screws on each side of the the step motor (silvery thing, 3 cm diameter) and the circuit board should come lose.

7. Locate the resistor shown in the picture. Heat up your soldering iron, this is the little buggar that is to be replaced!

8. Replace the resistor. In my meter, the value of the original resistor was 51 ohm. In order to decrease the speedo reading by 10%, I increased the value of this one with 10%, i.e. 56 ohm. Of course, you should increase the value with the same amount as your error.
I double checked this by hooking it up to a signal generator, but that is not necessary unless the error is REALLY big.
It is of course possible to use a potentiometer instead of a fixed resistor, but I prefer resistors since the do not change that much over time.

9. Reassembly is almost the reverse of the assembly. It could be tricky to realign the meter needle, but I did it by rotating it counter clockwise until it rested at 20 km/h (as it did from the beginning).

10. Buy a bottle of beer.

11. Test drive your car and enjoy an extremely accurate speedo reading!

12. Drink the beer!

And my result? The GPS and speedo now read the same within 1 km/h at all speeds up to 130 km/h (didn't test at higher speeds)!


New Ringlee: How to calibrate your speedometer! [700]
posted by  someone claiming to be Volvouser  on Thu Sep 16 12:02 CST 2004 >

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