Under the Bonnet – Control Boxes

Control boxes (“voltage regulators”) for “our” cars arc of two basic types as described below. While there are some variations of design and configuration within each type, the test instructions are applicable for all of the appropriate type.


(control boxes with removable cover retained by a wire bail, terminals marked A1,A,F,D,E. Al=power to ignition switch, A=Accumulator=battery, F=Field coil of generator, D=Dynamo=generator, E=Earth=ground)

TEST 1. Open Circuit Voltage Setting

With all other wires connected to the control box and the generator, remove the “A” and “Al” wires from the control box and join them together. Connect the voltmeter between terminal “D” and ground, and run the engine until the voltmeter reading settles. As the engine speed is increased slightly, regulation should occur within the limits of 16 to 16.5 volts.

If the voltmeter reading is outside these limits, turn the regulator adjusting screw clockwise to increase the voltage, or counter-clockwise to decrease. If adjustment has no effect, check for a faulty control box ground connection (“E” terminal) or an open-circuit shunt winding.

TEST 2. Cut-in Voltage

Leaving the “A” and “Al” wires disconnected from the control box but connected to each other, and the voltmeter between terminal “D” and ground as in test 1, connect an ammeter between the “A-Al” wires and the “A” tenninal.

Switch on the headlights, start the engine, and gradually increase the engine speed. When the cut-out points close, the voltmeter needle will “kick back” within the range of 12.7 to 13.3 volts. If outside these limits, adjust the cut-out adjusting screw in the same manner as the regulator screw in test 1, but with the engine off. Turn the screw only a fraction of a turn at a time and test each setting by increasing the engine speed from zero and noting the voltmeter reading at the instant of point closure.

Having made this adjustment, slowly increase the engine speed. The ammeter reading should increase with speed. (The actual reading depends on the state of battery charge and the electrical load I. Gradually reduce the engine speed to a slow idle. The ammeter reading should fall through zero until it shows a discharge (or reverse current) of 3 to 5 amps. The ammeter should show zero when the cut-out points open. Replace all control box wires, ensuring correct connections.


(control boxes with rectangular covers riveted to base, terminals marked E,D.W/L,B. E=Earth=ground, D=Dynamo=generator, W/L=Warning Lamp, B=Battery)

Note: Since removal and replacement of the covers on this type of control box is difficult, and the warranty on them is void once the cover is removed, we advise replacing them if faulty. While adjustments are possible, a special tool is required. For these reasons, we are not publishing adjustment instructions for this type of control box.

TEST 1. Voltage Regulator Setting Test

Remove the wire(s) from terminal “B”. If there arc more than one wire, connect them together. Connect a voltmeter between terminal “D” and ground, and run the engine at 1500 to 2000 RPM. For practical purposes, the voltmeter reading should be between 14.25 and 15.5 volts. If outside these limits, the regulator should be considered faulty and the control box replaced.

TEST 2. Cut-in Voltage Test

With the “B” wires as in test 1, connect an ammeter between them and terminal “B” on the control box. Keep the voltmeter between terminal “D” and ground. Switch on the headlamps, start the engine, and slowly increase engine speed. The voltmeter needle should “kick back” at 12.7 to 13.3 volts. If outside these limits, consider the unit faulty.

If that test is correct, increase the engine speed whereupon the ammeter reading should exhibit a corresponding increase. Gradually reduce engine speed to slow idle. The ammeter reading should fall through zero to indicate a discharge of 3 to 10 amps.

TEST 3. Current Regulator Setting Test

Turn on the headlights 5 minutes before starting the engine. With the headlights still on, start the engine. With the ammeter still connected as in test 2 (between the “B” wires and the “B” terminal), run the engine at above charging speed and check the ammeter. If the reading is incorrect to the specifications listed below for your generator, replace the control box.

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