Can I count my revs?
QI have a problem with the engine hours counter for my Vetus 12hp diesel. It is wired to the battery and records when the battery is switched on, regardless of whether the engine is running or not. I understand it needs to be rewired to the switch but I’m having difficulty locating the switch cable as all the wires are bundled together. Alternatively, is there a unit I could replace it with that would also include an inexpensive rev counter? Tony Wain-Smith
ANigel Calder replies: If you can get to the back of the switch panel, you can test with a voltmeter from the various terminals to ground (the engine block will do). You want to find a terminal that has 12v on it only when the switch is turned on. You can then wire the enginehour meter to this, where it should have been in the first place!
With respect to a rev counter, a common wiring point is the back of the alternator. An alternator actually develops alternating current (AC), which has a frequency that varies with speed of rotation. This AC is then converted to DC by diodes.
Any connection to the windings side of the diodes will have this variable frequency AC, from which you can deduce alternator speed. This is then corrected for the pulley ratio between the engine and the alternator (normally, 2:1) to get engine speed. It sounds a bit complicated, but my guess is if you go to the local autoparts store you may be able to get a relatively cheap rev counter that is not too hard to wire up.