So i took the motometer speedo/odo out of a 87 325i to put in my vdo cluster in my 87 325is since the gears were good in that one and not mine. I swapped the speedo and the odometer works but my obc is not reading speed or mpg. I plugged the 2 single wires that go into the back of the speedo back into the same place so what could be the problem??