speedo not working

hi , just wondering if anybody can help with this weird problem. if we turn the speedo drive in the gearbox with the ignition on and engine not running the speedo works, as soon as the engine is started the speedo stops working.we can push the car with ignition on and it works as soon as engine is running it stops.we have checked the battery, charging and fitted a complete speedo drive even knowing there is nothing wrong just to proove a point.if anybody can give me some sort of a clue that would be great. thanks for reading vinny.ps we are very capable of checking wiring and values of units and i have the equipment to check items like these. thanks again.vinny