How to… overhaul your classic car’s dynamo

By Classic & Sports Car Editorial team

| 23 Jun 2020

Classic & Sports Car – How to… overhaul your classic car’s dynamo

When coils of wire are rotated inside a magnetic field, a current is produced – which is the principle used by a dynamo.

The field is provided by magnetic ‘pole shoes’, enhanced by ‘field windings’. These are referred to as the field coil, which is static.

The second working part is the armature, supported in bearings and rotated when the engine is run. It has multiple windings of copper wire that pierce the magnetic field many times each turn.

The wire is connected to a commutator, which transmits the electrical power using carbon brushes held in ‘shoes’ that are designed to allow the carbon blocks to slide and take up wear.

The electrical current is fed via a control box that is first a voltage regulator to avoid overcharging. Its second function is to cut out battery current back to the dynamo when the ignition is switched off.

If this doesn’t happen, and your ignition lamp stays on when the car is turned off, disconnect the battery straight away. This voltage will burn out the armature because when it is applied back to the unit it will try to make it turn (as an electric motor) and the fanbelt will prevent that, hence it will overheat and suffer damage.

1: BELT TENSION

Ensure the belt is at the correct tension. Slacken the adjuster and both pivot bolts, then gently lever the dynamo out to tighten the belt.

Aim for a 10mm deflection at half-way: over-tightening stresses the armature bearings. Re-tighten the fixed bracket first, then the steady/adjuster bar, then the sliding sleeve.

2: REMOVE AND STRIP

If the unit needs overhauling, then take off the dynamo and remove its endplate, easing the brushes out with it.

Place all parts in order of removal, then carefully withdraw the armature to examine the drive-end bearing.

Clean the case and endplates in Jizer or a similar degreaser, and repaint as necessary.

3: CHECK CARBON BRUSHES

Brushes should be at least ¼in long (6mm), slide easily in the shoe and not rattle. Inspect the connector wire for damage.

The commutator should be bright copper without large marks or excess carbon deposits. Clean in white spirit. Wedge in a retracted position for rebuild using long cocktail sticks.

4: INSPECT BEARINGS

Armature support bearings should have no appreciable play or noise, and still contain sufficient grease. Read the numbers from the sides or take them with you as patterns.

Press in square using a socket of similar size to the outside diameter of each bearing, then support the inner as you install the armature.

5: INSTALL AND POLARISE

Polarise the refitted dynamo by connecting the battery terminals, then connect a wire to the positive (on negative-earth cars) or negative (positive) and ‘flash’ it for a couple of seconds on to the field terminal ‘F’ (or ‘14’) on the back of the unit.

This creates a residual field in the pole pieces. Don’t short the lead!

6: ADJUSTMENT

The two- or three-bobbin relays inside a regulator control voltage and cutout, or voltage, current and cutout. Setting needs an accurate 0-20V ranging voltmeter.

The cutout works as an on-off switch, but the current and voltage coils oscillate at up to 200Hz. Each unit has exact values for correct set-up.

7: MAINTENANCE

Keep the belt at the right tension and your battery conditioned to 13.6V if your car isn’t used often.

But, ideally, keep commutators, brushes and contact points in good health by using the car regularly.

Some dynamos have tiny oil holes for lubricating the bearings: you only need two drops though.

8: DASH WARNING LAMP

The circuit should be charging when the ignition lamp goes out. It’s controlled from the regulator, and may flicker at low revs.

Should it come on while driving, the fanbelt is too loose or has broken, or the dynamo or regulator aren’t working correctly. Never run the car with a faulty ignition lamp.

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