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Simple Maintenance Procedures For Car Dynamos And Alternators

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Maintenance Procedures For Car Dynamos And Alternators

Maintenance Procedures For Car Dynamos And Alternators
Maintenance Procedures For Car Dynamos And Alternators

If the ignition warning light stays on during normal driving, either the fan belt is broken or needs adjustment or the dynamo needs attention. The dynamo brushes, communicator, and armature can be examined after removing the unit from the engine and dismantling it.

The dynamo depends for maximum output on firm contact between two spring-loaded carbon brushes and its communicator.

When new, brushes are about ¾in (19mm) long, but over a period the rotating commutator wears them down. When they have been reduced to about 3/8in (9.5mm) long, they should be renewed at such length.

An armature that has lost solder is almost certainly burned out, it is important to call the service of an auto-electrician. Although armatures can be renewed, it is often quicker and cheaper to fit a reconditioned dynamo bought on a service exchange basis.


Maintenance Procedures For Car Dynamos And Alternators
Maintenance Procedures For Car Dynamos And Alternators

Most dynamos are held together by two long through bolts. Here are steps in dismantling and removing the dynamo:

  1. Loosen the clamping bolts on the engine and swing the dynamo body towards the cylinder block. Disconnect the fan belt.
  2. Disconnect the two cables from the end of the dynamo.
  3. Take out the fixing bolts and clamp bolt and take the dynamo to a workbench.
  4. Unscrew the two long bolts. If they are very tight, up-end the dynamo and grip the sides of each bolt-head in a vice, and twist the dynamo to loosen them. Remove the bolts.
  5. The main dynamo components. The commutator end bracket, casing, and armature with the pulley bracket attack can now be pulled apart. Reassembly is a reversal of the above procedure. Note that the end brackets each have a small protrusion at the edge that engages with a notch at the end of the casing to provide correct alignment. The cables cannot be refitted wrongly as the terminals are of different sizes.

Other important maintenance operations on the Dynamo are the:

  • Fitting of new brushes
  • Checking and cleaning the commutator
  • Removing the dynamo pulley


Here are steps to be used in fitting new brushes to the dynamo;

  1. Both carbon brushes are fitted in a holder on the inside of the commutator end bracket. Remove each terminal screw holding the brush wire, swing each spring clear of the holder and take out the brushes.
  2. Use a small piece of cloth moistened in petrol to clean inside of each holder. Remove traces of petrol with a dry cloth. Fit each new brush into its holder with its wire screwed tightly to the end bracket. The brush should slide in the holder without sticking. If it does stick, remove any high spots from the carbon with a fine file. Fit the wires to the terminals.
  3. The ends of the brushes should be concave to match the commutator. If they are not, wrap some fine glass paper round the commutator, abrasive side outwards. With the brushes in their holders, fit the end bracket up to the commutator and twist it back and forth to shape the ends of the brushes.
  4. Check and clean the commutator
  5. Pull each spring away from the holder and lift each brush until it no longer protrudes from the bottom. Prop it in place by resting the spring against the side of the brush.
  6. Refit the end bracket and reassemble the dynamo. Use a small screwdriver through the ventilation holes in the end bracket to lift each spring and reposition it so that it rests on the end of the brush.


  1. The commutator that the brushes bear on can be examined after removing the dynamo casing from around the armature. It is not necessary to disconnect the armature bracket at the pulley end.
  2. If the commutator is out of shape, it will cause excessive brush wear and will reduce dynamo output. Check the diameter with calipers and check that the same gap fits all around the commutator. If it is badly worn, consult an auto-electrician. It may be necessary to obtain a service-exchange dynamo.
  3. Provided the commutator is in good condition, use a strip of fine glass paper but not emery cloth, clean off the surface dirt from the copper segments.
  4. The gaps between the segments should be slightly undercut, using an old hacksaw or a sharp screwdriver blade. Wipe off all filings and dirt with a clean dry cloth when you have finished.


Though the pulleys rarely break, most replacement dynamos are sold without one, it is usually necessary to transfer the old pulley to the new dynamo before it can be fitted to the engine. Here are steps to do that;

  1. Loosen the pulley nut. Sometimes this can be very tight. If that is the case, do not hold the pulley in a vice. It will bend or break. Instead, dismantle the dynamo, pack the jaws of a vice with cloth and use it to gently hold the armature while the pulley nut is loosened.
  2. Undo the nut until it is just above the end of the shaft. Then support the pulley and tap the nut gently with a hammer to drive out the armature.
  3. If the new dynamo does not have a woodruff key locking the pulley to the shaft, use a small screwdriver to pull out the old one from its slot. Transfer the key to the new dynamo, tapping it into the slot with a nylon or copper-faced hammer. Fit the pulley, tightening the nut firmly.


The alternator is more complicated than the dynamo, and if the ignition warning light refuses to go out and the fan belt is operating correctly, its brushes may be worn out. Replacing brushes is about the only alternator job that the DIY motorist should tackle.

Alternator brushes do not carry such heavy loads as those on a dynamo and are smaller. Depending on the type, they may be mounted in a holder fixed on the outside of the alternator rear body or under a plastic cover. Both types are illustrated here.


Like the Dynamo, the alternator should be removed from the car to fit new brushes.

  1. After disconnecting the battery, loosen the mountings, push the alternator towards the engine and disconnect the fan belt.
  2. Note the position of any separate feed wires before disconnecting them from the rear of the alternator. Plug-and-socket cable connectors are usually made so that they can be fitted only one way around.
  3. Remove the mountings bolts and take the alternator to the work-bench.


Below are steps for changing the external brushes of an alternator;

  1. Undo the fixing screws and terminal nuts and withdraw the brush holder from the back of the alternator.
  2. In one type, two external spade terminals must be pushed inwards to release the old brushes. Depress the retaining tag on the terminal with a small screwdriver and push the terminal in the direction of the brush. Repeat the operation with the other terminal.
  3. Thoroughly clean the holder, removing all the carbon dust. Press the spade terminal of the new brush assembly into the holder from the brush side. Pull the spade end into position from the other side using long-nosed pliers, until the tag locks. Fit the other brush terminal in the same way.
  4. Refit the holder to the alternator.


Below are steps for changing the internal brushes;

  1. Use a socket or box spanner to remove the fixing screws. Liftoff the rear cover.
  2. Note the position of wires between the rectifier pack and brush holder. Remove the wires.
  3. Ondo the screws holding the brush holder and remove it.
  4. The brushes are fixed to metal terminals strips at their outer ends. Take out the screws holding these strips and pulls out the brushes and springs. Fit replacement brushes and strips.

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