Figure 1 shows the wiring layout of a battery sensing Lucas 11AC alternator. When the ignition is switched on and the engine is not turning, the warning lamp glows, because one side of it is connected to the battery, whilst the other side is connected to earth via a set of closed contacts in the 3AW warning light unit. These contacts are held closed by a taut wire in in the unit. One end of the taut wire is connected to an earth terminal whilst the other end is connected to the AL terminal on the alternator.
When the engine starts rotating, current from the AL terminal flows through the 3AW unit to earth, causing the taut wire to expand and the 3AW contacts to open, resulting in the warning light extinguishing. The 6RA relay closes and connects the battery positive wire to the rotor field windings, creating a magnetic field within them. The rotating field windings generate an AC current in the stator windings. Diodes convert the AC output into a DC one.
As the engine speeds up the voltage output rises and unchecked could exceed 100 volts. To prevent this the 4TR regulator switches the current off when it reaches 14.2 volts, and switches on again when the voltage falls below this. By rapidly switching on and off, a constant 14.2 volts is maintained.
When the ignition is switched off, the 6RA relay disconnects the rotor field windings, which prevents the battery from running flat.
Lucas ACR15 Machine Sensing AlternatorFigure 2
The alternator fitted to my car was an ACR15 machine sensing alternator, which contains internal electronic controls and only needs two connecting wires as shown in Figure 2. The ACR15 has a maximum output of 28 amps. They are rarely available today, but ACR17 models which are rated at a maximum 38 amps output, are readily available on the internet. It would be wise to upgrade the B+ wire if you replace an ACR15 with an ACR17.