I have just had a look at the spare rev counter I have in the garage & wonder if the drive mechanism, for the pointer, is a stepper motor. Thus needing power to return the pointer to zero. So once the ignition is turned off the rev counter uses stored power (capacitor ?) to return the pointer to zero, but the process gets screwed up by the emf from the fan motor. If I get a rainy day I will run a test on the spare rev counter (VDO with Morgan wings)
