The Lucas Control Box: Voltage Regulation on the AC Aceca
Whatever engine sits under the bonnet — AC's own two-litre or the Bristol two-litre — the Aceca's charging system is pure Lucas, and works the same in principle across the range. This article explains what the control box does, how to spot when it's misbehaving, and how to cure the commonest fault: overcharging.
Confirm against your car
The AC Ace and Aceca used Lucas electrics throughout — typically a Lucas C39 (later C40) dynamo and a Lucas RB106 control box, 12-volt and positive earth. Individual cars vary, especially the later Ford-engined Ruddspeed cars, so confirm the dynamo and control-box model stamped on your own car, and check its earth polarity, before trusting specific figures. Consult the AC Owners Club and, for Bristol-engined cars, the Bristol Owners Club.
The charging system in brief
Three parts share the work: the dynamo makes the electricity, the battery stores it, and the control box sits between them, deciding how hard the dynamo charges and when it may connect to the battery. A dynamo left to itself never knows when to stop — spin it faster and it makes ever more voltage until something burns. The control box tames it with "compensated voltage control" (CVC): it watches system voltage and trims the dynamo's field current many times a second, pouring in charge when the battery is low and easing off once it's full.
The two-bobbin RB106
The RB106 holds two devices, each a coil (bobbin) with a vibrating contact:
- the voltage regulator (left bobbin) — caps the dynamo output; this is the one that cures overcharging;
- the cut-out (right bobbin) — connects and disconnects the dynamo from the battery at the right moments.
It has five screw terminals, marked A1 · A · F · D · E (E is earth). There is no current regulator on this box — that is the later three-bobbin RB310/RB340.
Which cars this applies to
The Ace/Aceca family used Lucas electrics regardless of engine. On the AC-engined (2.0 OHC) and Bristol-engined (2.0) cars that means a Lucas C39 (later C40) dynamo and an RB106 control box, 12-volt, positive earth — the figures in this and the companion calibration article apply. The later Ford-engined (2.6, Ruddspeed) cars are still Lucas but later and more varied; confirm your car's specific dynamo and box. Whatever the engine, identify the box stamped on your car before using any settings.
Symptoms and what they mean
| Symptom | Usual meaning |
|---|---|
| Overcharging — battery gassing/boiling and needing frequent topping up, bulbs blowing, more than ~16–17 V at the battery when running | The regulator isn't limiting the dynamo. Almost always a dirty earth, dirty regulator contacts, or a drifted setting — rarely the dynamo itself. |
| No charge — flat battery, ignition light stays on above idle, ammeter dead | Dynamo not excited (often needs re-polarising after any disconnection), cut-out not closing, or a dynamo/wiring fault. |
| Erratic or fluctuating charge | Dirty contacts or poor connections. |
Curing overcharging — the order to work in
Work cheapest-first, because the cheap causes are the commonest:
- Clean the earth. The E terminal and its wire, and the box's mounting down to bare metal. A poor earth makes the box "see" a falsely low voltage and overcharge — the single commonest cause.
- Clean the regulator contacts lightly; they must close cleanly.
- Check the setting — see the companion guide, "Calibrating the Lucas Voltage Regulator on the AC Aceca".
- If it still won't hold, the box is worn out — fit a solid-state regulator (see that companion guide).
Reading it safely
This is a positive-earth car, so the earthed meter lead goes to the + / chassis side. Remember a dynamo cannot overcharge on its own — the box controls it — so for an overcharge fault, chase the control box, not the dynamo.
Read next
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.
About the author
An independent classic-car enthusiast in North Yorkshire with decades of hands-on experience on British classics — including the AC Aceca — writing the single-model reference he always wished existed. Read the full bio.