Alternator W Terminal
The W terminal is an output taken from an unrectified phase leg of the charging alternator, giving an alternating voltage at a frequency proportional to engine speed. It is used to feed tachometers. Its advantage is that it requires no separate sensor; its greatest weakness is that the measurement is corrupted when the belt slips.
Since the signal is taken before the rectifier, it is alternating in its raw form; at the instrument input it is thresholded and converted into a pulse train. Its frequency depends on the alternator's number of poles and the pulley ratio; that is why the instrument is calibrated separately on each engine. It is converted to rpm with a fixed multiplier, and the multiplier is set against a known speed in the field.
- The signal amplitude varies with speed; at low speed some instruments cannot read it.
- When belt tension drops and slipping starts, the instrument shows below the actual speed. This is a typical fault that is taken for a calibration error but is in fact mechanical.
- If the number of poles or the pulley ratio has changed when the alternator is replaced, the multiplier must be reset.
In practice the W terminal is the easiest way to read speed through an existing instrument; it can be used as a calibration reference. Where exact measurement is required, a magnetic pickup reading from the flywheel teeth is more reliable, because no belt is involved.
Context in which this term is used: Electronic speed and throttle control on diesel engines