Remove the brushes together with their terminal strips and springs.
Unlike dynamo brushes, alternator brushes normally last for several years. Electrical arcing between brushes and commutator, a major cause of dynamo wear, is far less common.
Alternators carry only the field current of 2 or 3 amps. The brushes are in contact with slip rings, which have a smaller, smoother surface than a dynamo commutator. The slip rings are solid rather than segmented, as in a dynamo. The clicking over the segments is another cause of brush wear in a dynamo.
The brushes are longer than those on a dynamo. Some alternator brushes are marked with a wear limit, but generally they should be renewed when worn about halfway down.
Before starting work, check which make of alternator is fitted to your car. The maker’s name should be marked on it — the types used most are illustrated in this section.
Depending on the type of alternator and regulator, worn or damaged brushes or slip rings may be indicated by the ignition warning light not coming on, not going out, or just staying dim. Check to find the cause of the problem (See Testing an alternator and checking output).
Before doing any electrical work other than circuit testing on the alternator, disconnect the battery.
Generally you need to take the alternator out of the car to replace brushes. Remove it in the same way as a dynamo (See How to fix a dynamo).
The way in which the brushes are fitted varies according to the make or model. On some alternators the brush holder can be removed from outside. On others the end cover must be removed. Guidance for different types is detailed below.
Slip rings are seldom worn or scored enough to need serious repair or renewal. If they are slightly damaged, smooth them in the same way as a dynamo commutator (See Replacing dynamo brushes), although there are no insulation strips to cut back. Be careful, however, because the metal on the slip ring is far thinner than on a commutator.
Usually, it is only necessary to clean a slip ring in position. Use a cloth or soft brush damped with methylated spirit.
Clean the brush housing and guides at the same time in the same way.
The brushes must slide freely into their holders. Carefully rub down any high spots on them with a fine file.
Always fit the correct type of replacement brushes and springs (check with the car handbook); make sure all connections and terminals are tight.