Fitting a Solid-State Regulator Inside the Lucas Control Box

Why fit one

The original electromechanical box does a clever job with 1950s technology — vibrating contacts trimming the dynamo's field many times a second — but it wears: contacts pit and burn, settings drift with temperature, and a tired unit overcharges or undercharges. A solid-state regulator replaces all of that with electronics that:

• hold a steady charging voltage (around 14.2 V on a 12-volt system) from just above cut-in speed right up to maximum revs, whatever the engine speed;
• give better low-speed charging, with no battery discharge back into the dynamo at idle;
• include the cut-out function electronically, so there is nothing to set;
• add voltage-spike and reverse-connection protection; and
• never need cleaning, adjusting or calibrating again.

Crucially for an originality-minded owner, the module is tiny and runs cool, so it sits inside the original control-box case. From outside, nothing has changed.

Two kinds of solid-state regulator

Solid-state regulators come in two forms, and it's worth knowing which you're buying, because they fit differently:

• A module that fits inside your original case. You remove or bypass the mechanical internals of your own control box and mount a small electronic module in their place. This keeps your genuine original box on the car, but means working inside it.
• A complete drop-in replacement unit. A whole new box — electronic inside, but moulded to look like the original Lucas item from outside, with the same shape and the same terminal layout (A1·A·F·D·E), often with the correct screw terminals. You simply swap the entire unit and keep your genuine original on the shelf, untouched. This is the easier route and fully reversible — the widely-used ADR106-type replacements work this way.

Either way the electrical result is the same; the difference is only in how you fit it, and whether your actual box stays on the car or is set aside.

Choosing the right unit — the part to get right

This is where care pays off. Specify all four of these:

1. Earth polarity: POSITIVE (for the Aceca). Not interchangeable — the commonest mistake.
2. Case: Lucas RB106. Not every unit fits every case. The small, well-known motorcycle-type regulators are sized for the smaller boxes; for the RB106 you need a unit designed to fit that case (the larger-body regulators intended for the RB106/RF95/RB310/RB340 family). If in doubt, tell the supplier your box model and let them confirm the fit.
3. Voltage: 12 V.
4. Dynamo output: the Aceca uses a Lucas C39 (roughly 19 amps) or, on later cars, a C40 (higher output). Give the maker your dynamo model and let them set the correct current-limit variant rather than guessing.

These are made by specialist dynamo-regulator manufacturers and sold by classic-car electrical suppliers. Give them the four details above and they will supply the correct unit and its wiring diagram.

Before you start

• Isolate the battery. The Aceca is positive earth, so it is the + terminal you disconnect.
• Photograph the control box with the cover off, clearly showing which wire lands on each terminal (A1, A, F, D, E). This is your insurance and your record.
• Read your unit's supplied instructions and wiring diagram end to end before touching anything. The steps below are the general shape of the job; your unit's diagram is the authority on exact connections.
• Have the workshop manual to hand for the standard wiring.

Fitting — the general method

Exact details vary by product, so treat this as the shape of the job and defer to your unit's diagram at every wire.

If you have bought a complete drop-in replacement unit (the ADR106 type), fitting is simple, and you can skip the "disconnect the mechanical internals" step below: isolate the battery, photograph which wire sits on each terminal of your old box, unbolt the old unit, bolt the new one in its place, and reconnect each wire to the matching terminal on the new one (A1→A1, A→A, F→F, D→D, E→E). Fit a fuse if recommended, then go straight to the first-run check. Keep your original box bagged and labelled.

If you are fitting a module inside your own original case, follow these steps:

1. With the battery isolated and the cover off, note and photograph every terminal connection.
2. Disconnect the mechanical internals. Depending on the kit, you either unbolt the old regulator-and-cut-out assembly from the base, or leave the old coils in place purely for appearance and re-route the working connections. Follow your unit's instructions.
3. Mount the solid-state module inside the case — it is small and runs cool, so there is room. Secure it so it cannot rattle against live parts.
4. Connect the module's wires to the terminals exactly per the supplied diagram — typically the dynamo D and F, the battery/load A (and A1), and earth E. Do not assume the mapping from the old box; use the diagram.
5. Fit a fuse if the maker recommends one (most do) — cheap protection for the new electronics.
6. Double-check every connection against the diagram, then refit the cover.

First run — what to expect

1. Reconnect the battery (+ terminal).
2. Start the engine and hold a fast idle. The charging voltage at the battery should rise and then stabilise at around 14.2 V and hold there, steady, whatever the revs (above cut-in). That rock-steadiness is the whole point — no more climbing to 16–17 V.
3. On the ammeter, expect a healthy charge at first that quickly settles to a slight trickle (about ½–1 amp) once the battery is topped up. A high charge that persists points to a low or faulty battery, not the regulator.
4. There is nothing to calibrate — no adjusting screws, no open-circuit setting. That is the benefit.

If it does not charge: re-check that you fitted the correct-polarity unit, verify every connection against the diagram, and confirm the dynamo itself is sound. The solid-state unit manages the field, but a dynamo that has been fully disconnected may still need a brief polarise on first use per your unit's notes.

What you gain, and what you give up

You gain reliability, steady voltage, better low-speed charging, built-in protection and freedom from maintenance. You give up nothing visible — the case looks standard — and the change is fully reversible. The only real objection is originality of the internals, and since it is concealed and reversible, most owners who have fought a tired mechanical box consider that a price well worth paying.

This is general guidance, not a substitute for the fitting instructions supplied with your regulator or the official Workshop Manual. Confirm the correct unit for your car's case, polarity, voltage and dynamo before buying, and entrust the work to an auto-electrician if you are not confident.