On Mon, 27 Dec 2010 11:30:40 +0100, Dino
wrote:

The reason is because of the uncertainty of hourmeter
connection. The VDO is connected to the motor with two wires
that carry the RPM pulse. It is my believe that RPM pulses
are internally analyzed to originate the "motor running"
information to the original hourmeter.

Although this is possible, it is most unlikely. Just use the igntion
feed, it will be wrong by a few seconds every few hours at most.

--
Nigel M