Hi there, i have a 2002 Focus saloon which has developed a weird electrical issue. It will drive perfectly well for week after week but all of a sudden the speedo needle will swing round to 140mph and the radio volume will increase. This drops again within seconds but will repeat over and over for a period of time spanning several journeys, then will stop. Has anyone experience this? Do you have a solution? My regular mechanic said that when he's seen it previously it has lead to a whole new dashboard cluster. Any help will be gratefully received.

Shaun.

The fact that both the speedometer shows an incorrect value and the radio volume increases suggests that there is a problem with the vehicle speed pulse signal. Both the instrument cluster and the radio need the vehicle speed pulse signal (the instrument cluster to display the speed and the radio for the speed dependent volume control).

On the focus mk1/mk1.5 problems with the vehicle speed pulse signal are usually caused by a defective vehicle speed sensor (VSS). This is quite a common problem. The VSS is installed on top of the gearbox and measures the revolutions of a magnetic sensor ring inside the gearbox. 

That was also my first thought but I would have expected a failing sensor to be losing pulses causing the speed to under-read. This fault implies the pulse rate is approximately doubling (assuming a road speed of 70mph) and I can't imagine what kind of sensor fault would produce that.

However changing the sensor is probably going to be the cheapest thing to try. They are a PITA to change though, by all accounts, as they get corroded in and may snap off whilst being extracted from the gearbox casing. Hopefully your mechanic will be wise to this and proceed with caution.

As a matter of interest we used to have a 2003-plate Mk1.5 and had problems with the speedo dropping to zero. After having had the speed sensor and instrument cluster replaced without effecting a cure the problem was eventually found to be the sensor loom chafing on the gear linkage so before shelling out any cash I would recommend a check of the wiring and the connector on the sensor.

Cheers gents. I will pass this info on to my mechanic. Much appreciated.