In fixing a loose connection in the instrument panel which prevented the engine from starting, my more knowledgable crew has moved onto another problem. The tachometer no longer registers the revs but swings at about once a second from one side of the dial to the other. We have checked the connections and found a steady 12 volts from the alternator lead to the tachometer on the + side whatever the engine speed. What we are wondering is where the lead from the - side goes (It is not shown on the Penta handbook wiring diagram) and what current varies to cause the tachometer to register the revs.
Can anyone give a laymans guide to this, please?