Alternators

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Lucas’s ACR series of alternators supported the electrical systems of many British cars throughout the 1970s and 1980s; Rob Marshall insists that a DIY overhaul is neither expensive, nor difficult on your classic

Despite being more complex and expensive, the alternator is much more effective at charging a car battery than the steam age dynamo. While they share similar working principles, instead of direct current (DC) being extracted from the spinning commutator, alternating current (AC) is produced within an alternator’s stationary stator.

Lucas’s ACR alternators appeared in the late 1960s and are identified, mainly, by their maximum current outputs. Therefore, substituting a standard-fit ACR15, and its 27 amps ceiling, with a ACR16 (34 amps), an ACR17 (38 amps), ACR18 (43 amps), or ACR20 (66 amps), is a simple way of increasing the car’s recharging capacity, without fitting a more modern unit, providing that any electrical cables are upgraded, where appropriate. However, expect a few subtle differences, between ACR units of different outputs and ages.

Note: While the features majors the popular Lucas unit, the fundamentals apply to various other designs.

The Lucas ACR’S innards and common faults

The spinning rotor is supported on a pair of sealed bearings, rapid failure of which results from overtightening of the fan belt. The rotor acts as an electromagnet, the three sets of wire coils (windings) within which are powered via a pair of carbon brushes that bear against a copper-faced slip ring.

As the voltage regulator alters the power, the magnetic field’s strength is varied and, therefore, the alternator’s output can be controlled accurately. A faulty regulator causes the windings to be energised permanently and the alternator should be overhauled, if it consumes more than ten milliamps, with the engine switched off.

The AC, generated in the stator assembly, is converted into DC by the rectifier pack’s diodes. The car’s multi-plug socket affixes directly to the rectifier, the pictured version is the more common three terminal type.

The two largest connections are for the output and input (battery sensing) circuits that are wired directly to the positive battery terminal. The smaller connection serves as the earth for the ignition warning lamp, the circuit of which must be intact for the alternator to work.

While the generator’s delicate electronics deteriorate due to old age and natural wear-and-tear, disconnecting either the alternator cable, or a battery terminal, is likely to damage both the rectifier and the regulator. As testing both is fairly involved, simply replace them now – the parts are inexpensive.

Should you find that the battery voltage is not between 13.5 and 14.5 volts, with the engine running above idle speed, suspect either faulty brushes, or slip ring.

As long as you engage a reputable parts supplier, such as Wood Auto Supplies (01484 428261), and avoid the supposed online auction ‘bargain’, unless from a reputable seller, DIY rebuilding of a typical ACR unit should be straightforward, as the following steps demonstrate:

1. As the front nut can be tight, winding a fan belt over the pulley and tensioning it in a vice (or using mole grips) will help you to loosen it. Do not lose the keyway, after removing the pulley

2. Working on the alternator’s rear, remove the bolts that hold the plastic rear cover in place. As the internal wiring can differ, make a note of all connections, before disconnecting anything

3. The brush box is fixed with two bolts and the voltage regulator tends to be mounted to it but it can be removed separately. This silver 14TR regulator is also a sealed unit but, as it is inexpensive, renew it anyway

4. It does not cost considerably more to replace the entire brush box assembly, instead of just the brushes and it’s good practice to do so when carrying out a full overhaul.
However, the slip ring’s copper surfaces can wear very deep scores, or even wear through completely, so check. Replace naturally

5. Before the alternator’s rear cover can be removed, unsolder the rectifier pack’s three connections and remove the retaining nut, before lifting away the old part and discarding it

6. Remove the long through bolts that hold the two halves of the alternator together, mark them, so that they can be refitted in the same position, before dismounting the rear half from the stator

7. Pull off the stator assembly from the front casing. As this part will be reused, use a multimeter to check the continuity of its windings, first, by connecting one pair of leads at a time

8. Remove the slip ring, to access the rear bearing. Unsolder the leads, noting which one supplies the centre/outer contacts. If the surface is scored deeply always renew the slip ring and do not lose its retaining clip

9. Lift away the rear bearing carefully, taking care not to damage the rotor’s rather delicate wires that run within the slots in its shaft. A new bearing can be pressed carefully into place using basic tools at home

10 .Turn the alternator round and free the front plate from the rotor’s nose. Remove any dust guard/circlip. Press out and replace the old bearing. Check that the seal within the bearing housing is in good order

11. When reuniting the two halves, ensure that any thrust washers on the rotor are also refitted. Replace the brushes and/or brush box, rectifier and regulator, by soldering and reconnecting all wiring in their original positions

12. Tighten the securing bolts evenly and spin the rotor and ensure that there are no tight spots. Refit the front pulley, fan and nut, prior to fitting the original plastic rear cover into position. Job done, it’s saved you a considerable sum