Calibrating the Lucas Voltage Regulator on the AC Aceca

What you're adjusting

Two sprung brass screws, one per bobbin: the voltage regulator (left bobbin — cures overcharging) and the cut-out (right bobbin). There is no current regulator on this two-bobbin box. Don't confuse the sprung brass electrical adjusting screws with the plain armature-securing screws (which set the mechanical air gap — a separate, rarely-needed job).

Why it must be set on open circuit

The current a dynamo pushes out depends on how flat the battery is, so with the battery connected the reading is a moving target. Lucas therefore set these boxes on open circuit. Isolate the battery either by slipping a piece of dry card between the cut-out contacts, or by removing the A and A1 cables and joining them together temporarily. Note: on open circuit the regulated figure is around 16 volts — higher than the ~14–15 V you see at the battery when running normally. That is correct; do not "correct" it down to 14.

What you'll need

A multimeter on DC volts, 20 V range; a small screwdriver; a piece of dry card; a way to hold a steady fast idle (~3,000 dynamo rev/min); and a rough idea of ambient temperature (the target depends on it). Positive earth: the earthed lead goes to the + / chassis side.

Step 1 — Prepare

1. Engine off, battery isolated. Remove the control-box cover.
2. Isolate the battery (card between cut-out contacts, or A+A1 joined).
3. Voltmeter: one lead to the D terminal (or the regulator frame), the other to a good earth.
4. Work promptly once running — the coil self-heats and the setting drifts as it warms.

Step 2 — Set the voltage regulator

1. Start up and hold a fast idle (~3,000 dynamo rev/min); the voltage should rise and hold steady.
2. Compare with the temperature-corrected target:

Rule of thumb: subtract about 0.2 V per 10°C above 20°C, add the same below.

3. If outside the band, turn the voltage-regulator sprung screw: clockwise to raise, anticlockwise to lower, a fraction of a turn at a time.
4. Re-read from zero each time — drop to idle, raise again, read the settled figure.
5. Read promptly (the figure drifts down as the coil warms).

Step 3 — Set the cut-out

1. Remove the card (or reconnect A/A1).
2. Voltmeter on D (or frame) to earth.
3. Slowly raise the speed; the reading rises then dips at the instant the cut-out closes — that dip is the cut-in voltage.
4. It should close between 12.7 and 13.3 volts.
5. If not, turn the cut-out's own screw — in to raise, out to lower — and re-test from zero.

Step 4 — Final check on the car

Refit the cover; with the battery reconnected, at a fast idle with the headlamps on, the voltage at the battery should settle and hold in the mid-13s to about 15 V and the ammeter show a charge that tapers. It should not climb past ~15.5, sit at 16, or spike to 17 — that would be the overcharge fault.

The mechanical air gap (advanced — only if you've disturbed the armature)

If it still won't hold, the armature-to-core air gap may need resetting: slacken the fixed-contact screw and the armature securing screws, insert the correct feeler gauge between armature and core face (the figure depends on the core type — of the order of 0.38–0.53 mm — take the exact value from the Workshop Manual), press the armature squarely down, re-tighten, then screw the fixed contact down until it just touches, and re-do the voltage setting. Fiddly; leave it alone if unsure.

When to stop and fit solid-state

If, after cleaning contacts and earth and setting carefully, it still won't hold a steady figure, the mechanical unit is worn out — fit a solid-state regulator (see the companion guide). And note: the adjusting screws ARE the calibration, so the moment you turn one you've changed the setting — always re-check afterwards.

This article is a guide written for owners and is not a substitute for the official Workshop Manual. Confirm every figure against the manual and the owners' clubs for your own car.