Under the Bonnet – Lucas Generator Charging Systems

The D.C. charging system must provide a generator output sufficient to mcci the normal vehicle electrical load plus keep the battery charged. The generator is designed such that it must always be under some form of control, both to protect it against overload, and to protect the battery against overcharge. A control box (commonly called a voltage regulator) is designed to operate with a specific type of generator on a given application.

For many years the “Compensated Voltage Control” (2-bobbin) system was employed. With the increase in vehicles’ electrical requirements, this was insufficient, and the “Current Voltage Control” (3-bobbin) system was developed. The main advantage of this system is that it allows maximum safe generator output for a longer period of time when the battery is in a discharged condition. We’ll follow up on testing procedures for control boxes in a future issue.

Generators and control boxes may be replaced as individual units provided that the correct unit is fitted, and that the test procedure is carried out on both units after replacing either one. Lucas cautions that only a limited amount of work can be performed on the control boxes. If, for example, a unit does not respond to adjustment, it should be replaced.

The following instructions arc based very closely on instructions provided by Lucas. If there is any conflict between these instructions and those in the factory workshop manual for your car for the same test, the workshop manual instructions should be followed.

Battery Test

A battery fault can have an adverse affect on the charging system. For example, a sulfated battery will produce a low charge rate, whereas a battery with a shorted cell will produce a high charge rate. A hydrometer should be used to check that the battery is at least 70% charged, and in good condition.

Drive Belt

Adjust generator drive belt tension to factory recommended tightness (generally 1/2 to 3/4″ slack on the longest run of the belt). If too loose, the generator may not charge. If too tight, the generator bearings may be damaged. Replace an oily, worn, or cracked belt.

Checking the Generator

Test 1. Armature Circuit

Disconnect both wires from the generator and connect a voltmeter between the generator “D” (large) terminal and a good ground. Start the engine and slowly increase the speed to approximately 1500 RPM. The voltmeter should show 1.5 to 3.0 volts.

Test 2. Field Circuit

With the voltmeter connected as in test 1 above, again with both generator wires disconnected from the generator, connect an ammeter between the the two generator terminals. Slowly increase the engine speed until the voltmeter registers nominal battery voltage (12 volts). The ammeter should read approximately 2 amps. If the ammeter shows a higher current, the field resistance is low.

Generator Leads

If the above tests show the correct readings, but the charging system is not functioning correctly, the fault could be in the wiring between the generator and the control box. To check these, re-connect the wires to the generator, and disconnect them from the “D” and “F” terminals of the control box. With the voltmeter connected between the “D” wire and ground, run the engine at 1500 to 2000 RPM. The voltmeter reading should be the same as at “D” on the generator in test 1 above (1.5 to 3.0 volts). No reading indicates a faulty “D” wire, while a high reading indicates a short between the “D” and “F” wires.

If the reading is correct, leave the voltmeter in position, and short the “D” wire to the “F” wire. The voltage should rise with increasing engine speed. If the reading increases only slightly, there is an open circuit in the “F” wire. A zero reading indicates the “F” wire is shorted to ground.

Ignition Warning Light

The ignition warning light serves two functions: to indicate that the ignition is switched on, and, when it goes out, to indicate that the generator is producing voltage sufficient to close the cut-out switch in the control box, to enable the generator to charge the battery. However, the fact that the light goes out doesn’t always mean the charging system is functioning correctly. A warning light glowing dimly or flickering may indicate: internal high resistance in ignition switch, dirty control box cut-out contacts, or a badly slipping fan belt.

Supply Line Voltage Drop

Remove the wire from the “D” (large) terminal of the generator and connect an ammeter between the end of that wire and the “D” terminal of the generator. Connect the voltmeter between the same end of the “D” wire and the battery supply terminal on the starter solenoid. Run the engine at charging speed until the ammeter reads 10 amps, when the voltmeter reading should not exceed 0.75 volts. A higher reading indicates high resistance in the wire from the starter solenoid to the “A” terminal of the control box. This is usually caused by poor connections.

'Under the Bonnet – Lucas Generator Charging Systems' has 1 comment

  1. June 4, 2018 @ 5:56 am Dan

    Where does field wire on 69 midget connect from generator


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