Posted: Thu Jan 08, 2009 9:27pm

I own a narrow boat and it has a Beta 1903 engine and two alternators. When I start the engine the rev counter doesnt kick in unless I give it some throttle; why is that?

Posted: Fri Jan 09, 2009 9:12am

The rev counter is driven by pulses form one coil inside the alternator so no rev counter means no/very low alternator output. Going back a few (or more) years dynamos "held onto" sufficient magnetism to allow them to start generating all on their own. Alternators in the main are not like this. They use the tiny amount of current that passes through the warning lamp to create a magnetic field. This tiny field starts to produce electricity when the engine is started but not very much. In theory this tiny current is fed back into the alternator to make a stronger magnetic field and thus a higher output. This builds up until the electricity from the alternator can extinguish the warning lamp and then charge the batteries. Unfortunately theory and practise are a little different because that initial small current has to "open" a diode so it can get through to the electromagnet and this takes about 0.6 volts. Now if the speed of the alternator in conjunction with the warning lamp current produces less than 0.6 volts the diode will not open so the magnet can not be made any stronger thus the alternator just sits there producing less then 0.6 volt with the warning lamp on. As soon as you give the engine a bit of a rev the extra speed increases the voltage to above 0.6 volts so the diode opens and the magnetic field and thus the output builds up to the normal level. If the alternator in question has no warning lamp then fitting one may help as may fitting a higher wattage warning lamp bulb but make sure it is within the specification for the bulb holder. Tony Brooks