sorry for the very very late raply. i forgot about this thread 
well no it doesn't works on that principal, basically it gets the signal from the diesel pump, but here is where the simplicity ends, the signal direct form the pump doesn't drives the tacho, there is an accessory control module under the dash, the signal goes there and from there a signal come out which drives the tacho, took me a lot of experimenting to find it out, but the result is wroth it, 100% accurate tacho reading.... 