A generator from a positive earth car can be used on a negative earth car (vise-versa). It will be necessary, however, to polarise the generator. To do this, mount the generator on the car and connect all wires including those from the cut-out relay. When all connections have been made, and before the motor is started, simply press the cut-out relay points closed for about one second, then release. Be careful, though, some cut-out relays tend to lockup, so it may be necessary to force its release or disconnect the battery. Don’t leave the cut-out operated with the engine stationary as this could cause the generator windings to burn out.
N.B. This method of polarising is not applicable to alternators, which use diodes for rectification and are therefore polarity discrete.
Generator and Starter Motor Commutators
Commutators should be cleaned with fine glasspaper only, do NOT use emery paper. If the commutator needs to be trued, spin or lathe at high speed and cut with a sharp tool. Remove only a small amount as is necessary. After truing a generator commutator, the insulation between the segments should be undercut to a depth of about 1mm. (a broken hacksaw blade, ground to the necessary thickness, is ideal for this purpose.) The commutator of a starter motor should generally NOT be undercut. The insulation should be left level with the commutator faces.