Speedo Doesn't Work

[seanbrazendale]

:
Hi All

I know this is off topic but please bear with me.

I recently bought a 1999 Fiat Ulysse which is virtually identical to the Citroen Synergie/Evasion. It has the 2 litre PSA 8v engine.

The speedo doesn't work. On switch on, it lights up and needle moves from bottom of dial to zero position. The odometer diplays mileage done and miles to next service as it should. I have removed instument panel to confirm that it is an electronic speedo and does not have a drive cable as the earlier ones did. I can see nothing amiss electrically and speedo needle seems free to move. I know from my XM that there is a speed transducer somewhere on the gearbox/transmission.

My question is: Does anyone know what form the output from this transducer is i.e. a variable voltage which I could simulate or should I try to connect a drill to the driven gear ?

I have Noz's pictures of the transducer.

The Philips radio doesn't work either - it doesn't recognise code but that's another story.

I have tried the Fiat Forum but there is little there on the Ulysse and this Forum is much more active and knowledgeable.

Regards

Sean

[noz]

Sean,

As far as I can tell the sensor is a magnet and coil type and so emits a sinusoidal pulse in relation to speed. The receiver in the dash interprets the pulses and turns it into a speed value. The speed is integrated over time to give you distance and the distance is fed to the odometer and service interval trip. All this takes place digitally in the electronics behind the speedo. If your odometer and trip work fine then I'm afraid the sensor and receiver are working fine as is the internal maths to convert to distance. The digital circuit converts the speed value to an analogue (voltage) signal and sends it to the pointer on the speedo. The speedo will be a standard moving coil galvanometer. If it moves from the stop to the zero position when you switch the ignition on then it proves that the circuit is whole. The problem then narrows down to the analogue output from the digital processor.

Unfortunately my electronics knowledge stops there. However, as Mr Gates has taught the world to do, try switching the power off (disconnect the battery). You never know it might just reset the digital part of the circuit.

Hope this helps.

Cheers

noz

[seanbrazendale]

Hi Noz

Many thanks for such a quick reply.

I didn't explain one symtom too clearly. The mileometer/miles to service displays its digits OK but doesn't increment. Miles to next service for example remains fixed at 12500. It would seem best bet is first check obvious that transducer is connected and if it is simulate an input with a drill (space permitting).

Best wishes

Sean