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garciaae · Alfasud Joined: 01 May 2003 Posts: 25

I've unmounted the speedometer because it started to mark speed at 40 kph.
What kind of signal recives to measure speeds?
Maybe intensity?
I've also put some new white dials in it! Very Happy

Thanks in advance from Spain

Maze123 · Alfasud Joined: 26 Mar 2003 Posts: 73 Location: Denmark

Have you dissambled it completely? All I know is, that the signal comes from the gear-box, and the needle is being moved by some kind of magnetism. The needle is mounted on a round metal house, and inside that house there is a round magnet which is turning. Do you understand me? Smile

I think it's quite amazing that it works...
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Alfa 33 1.7 16v '88 QV
http://www.bilgalleri.dk/html/gal_visbil.asp?ID=2498

Matt Stolton · Posted: Thu Aug 12, 2004 8:45 am Post subject: Just sorted mine

The theory is simple.

The pickup (hall effect for the Physics bunch) in the gearbox produces a weak pulse everytime a particular tooth on a wheel in the box goes round. Because this wheel speed is directly proportionly to tyre/vehicle speed, the sender produces pulses that are directly related to the speed of the vehicle.

Inside the speedo, there is a motor, and a system of gears, as well as a circuit board. The circuit board takes the pulses, amplifies them to a useful level, and then sends a corresponding voltage/current to the motor, which moves the needle, and gears which drive the milometer.

From having repaired mine yesterday, it could be any manor of things. My original circuit board had a dry joint around the base of a large transister. However, in reparing it, I managed to overheat a close by zener diode. Shocked

However, after replacing them both, and a lot more care with the soldering Iron (the zener was surface mount, but managed to get a normal , leaded one to fit, with a little insulation on the legs) it all works fine.

From looking at a couple of them in peices, I think most of the faults with the speedos are related to the circuit board, and not the motor. On a range of different mileage clocks, most of the motors were good with little wear, but there were visibly dead components on the driver circuit board.

To maintain your milage readings, try just changing the circuit board from another speedo. This means you keep the motor, gears and readings on your current speedo, so no winding on needed, or illegal clocking going on Twisted Evil

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Regards

Matt

Ex Alfa 33 'GTA' (P4 with Knobs On)
Now cruising in a 166 3.2 Ti!!