Lucas C42 type Dynamo for Classic vehicle applications

The C42 generator is a shunt-wound two pole two brush machine with 4.2" generator casing (yoke) diameter designed to work in conjunction with a compensated voltage control regulator unit. Output (up to 30 Amps +/- 0.5A) is controlled by the regulator control box and is dependent on the vehicle battery state of charge and the electrical loading during vehicle operation.
When the battery is in a low state of charge the generator gives a proportionately higher output to keep the battery in good order and supply additional electrical loads such as lighting.

2023 Replacement C42 Dynamo

Lucas Part Nos: 22900, 22900B, 22900F, 22900L, 22906, 22906A, 22906B, 22909, 22910, LRD104,

C42 Dynamo Rebuild Parts Bill of Materials

Lucas C42 Dynamo Checks and Tests

Check condition and tension of dynamo drive belt.

Ignition warning lamp (WL) should illuminate with ignition key on and extinguish when the engine is running. Check the dynamo and control box is wired correctly.

Residual field : Disconnect cables from dynamo , connect voltmeter between D terminal and chassis ground. Engine at approx 1500 rpm , Volt meter reading should be 1 to 3 volts

C42 Dynamo field circuit : Voltmeter connected between D terminal and chassis ground. Ammeter connected between D terminal and F terminal. Increase engine rpm until voltmeter reads nominal system voltage. Ammeter should read aprox 2 amps.

C42 Dynamo armature circuit : Disconnect cables , connect volt meter Black lead to D terminal , Red lead to Earth, run engine 1500rpm , meter reading should be 2 to 4 volts.

On dynamos with open ventilation the commutator condition may be partly seen without major disassembly. The commutator should be clean and free of grease with a polished appearance, free from pitting . The brush gear should be examined for wear ensuring carbon brushes travel freely within the brush holders and contact the commutator under brush spring tension. Brushes below 1/4 inch length (6mm) should be renewed. The armature may be checked for condition after disassembly of the dynamo by use of an armature growler, or otherwise by substitution. Field coil resistance may be checked using an ohmeter between the field terminal and the yoke (casing). Resistance of approx 6 Ohms is expected. Alternatively field coil continuity can be checked using a 12v DC supply and ammeter which should read approx 2 Amps. No reading on the Ammeter indicates field coils to be open circuit and requiring renewal. To replace field coils a pole shoe expander an wheel operated screwdriver may be required. Armature shaft bearing bushes should be replaced if they allow excessive side movement of the armature shaft. Before bushes are refitted they should be immersed in a thin engine oil for 24 hours. A shouldered mandrel will be required to press in new bushes/bearing to commutator end and drive end brackets.

Flashing the C42 Dynamo

Flashing the dynamo is required to polarise dynamo output to suit either Negative earth or Positive Earth vehicles.
With the dynamo mounted on the vehicle, remove D & F leads, touch a wire between the dynamo F terminal and battery live terminal.

C42 dynamo Car & Light Commercial applications

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