Alternator tachometer signals may be derived from the (“W”) stator windings. This signal is, thus, dependent on the amount of energy being produced by the alternator. When batteries are being charged, signals are strong enough to produce good tach stability. When multi-step regulators are used to charge batteries, the tach signal strength can vary according to the charge state. When the regulator switches from the absorption state to the float state, there may be complete loss of tachometer signal during the time that battery
voltage decays. During this period some regulators shutdown completely, and the tach signal does likewise.
When the regulator finds that the battery
is full, alternator field current
is reduced, as necessary, to avoid overcharging the batteries. This often results in a tach signal which is too weak to trigger some tachometers. Erratic tachometer readings result; particularly at low RPM. One possible solution is to turn on some electrical
loads; forcing the alternator to produce more output, and hence, more tachometer signal strength.
Virtually all diesel
engines have a port which will accept an RPM sensor*. This mechanism produces a reliable tach signal without regard to battery state of charge. The RPM sensor does not suffer from belt slippage, and continues to operate even if the alternator has failed.
* See ➥ Magnetic Proximity Tachometer - Tachometer Wiring Diagram Cruisers & Sailing Photo Gallery