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Fitting a Solid-State Regulator Inside the Lucas Control Box

IntermediateHow to replace the worn-out mechanical internals of a Lucas RB106/2 with a modern solid-state regulator hidden inside the original case — choosing the right unit for a positive-earth car, fitting it, and what to expect afterwards.

If you have read the companion guide on calibrating the Lucas control box and concluded that your mechanical box is simply worn out — it drifts, spikes, or won't hold a setting no matter how carefully you clean and adjust it — this is the upgrade that ends the problem for good. A modern solid-state regulator hides inside the original case, holds a rock-steady voltage from low revs upward, needs no adjustment ever again, and leaves the car looking entirely original from the outside. It is also fully reversible: keep the old internals in a labelled bag and the car can be returned to standard at any time.

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, regardless of 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 (a matchbox) and runs cool, so it sits inside the original control-box case. From outside, nothing has changed.

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 TD/TF). Not interchangeable — this is the commonest mistake.
  2. Case: Lucas RB106/2. 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 (all TD/TF).
  4. Dynamo output: the TD/TF uses a Lucas C39PV-2 dynamo rated at roughly 19 amps. The regulator's current limit should suit that — 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 — case, polarity, voltage, dynamo — and they will supply the correct unit and its wiring diagram.

Before you start

  • Isolate the battery. The TD/TF 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 your original 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.

  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 for a long time 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.

Related reading

*This is general guidance, not a substitute for the fitting instructions supplied with your regulator or the official MG 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.*

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