The control box — the Lucas voltage regulator on the bulkhead — is the one charging component most owners can replace themselves in an afternoon. There is no soldering and no special tool: just a couple of mounting screws and five wires. Get the polarity right, transfer the wiring methodically and re-polarise the dynamo afterwards, and the job is genuinely straightforward. This guide walks through a complete RB106/2 swap on the MG TD and TF, from choosing the replacement to confirming it charges.
When to replace rather than adjust
A Lucas control box will often give decades of service, and many "faults" are really dirty contacts, a poor earth or a mis-set regulator that cleaning and calibration will cure. Reach for a replacement when the box is genuinely beyond that: burnt or welded contacts, a cracked top, a broken winding, or persistent overcharging that survives clean contacts and a good earth — the failure traced in our TF1500 workshop health-check. If you simply want modern reliability with no adjustment, an electronic unit in a Lucas-style case is an excellent upgrade, covered in Fitting a Solid-State Regulator. For how the box actually works, see the Lucas Control Box overview.
Choose the right unit
Before ordering, note three things:
- Model. The TD (from approx. chassis 8142) and all TF/TF1500 use the five-terminal Lucas RB106/2. Earlier TDs use the nine-terminal RF95; the TC an earlier box again. Match the box to your car.
- Polarity. MG T-Types are positive earth. An original mechanical box works on either polarity, but a modern electronic replacement is polarity-specific — order the positive-earth version.
- Voltage. TD and TF are 12-volt. Confirm 12V, not 6V.
Tools and preparation
You need very little: a screwdriver, a small spanner for the mounting screws, a camera or phone, masking tape and a pen for labelling, and — for a bare mechanical box — a fine file. Work with the car cold and the ignition off.
Step 1 — Disconnect the battery
Isolate the battery before touching the wiring. On this positive-earth car the earth strap is on the positive terminal; slacken and remove it, and tuck it clear. If the car has a battery cut-off switch, use it — but only with the engine already stopped.
Step 2 — Photograph and label the wiring
This is the step that guarantees success. Photograph the box from two or three angles so every wire and terminal is clear, then label each wire with a strip of tape marked with its terminal:
| Terminal | What connects here |
|---|---|
| A | Main battery feed (via the ammeter) |
| A1 | Feed to the ignition switch, lights and fuse |
| D | Large dynamo output lead (dynamo terminal D) |
| F | Dynamo field lead (dynamo terminal F) |
| E | Earth |
If two wires share a terminal — common on A or A1 — note that on the label. Getting D and F the right way round matters: swap them and the box cannot regulate.
Step 3 — Remove the old box
Undo each terminal screw and free the wires, keeping them splayed apart so none touch. Then undo the two mounting screws and lift the box off. Keep the old unit — it is a useful wiring reference, and a repairable core.
Step 4 — Prepare the new box
On a new mechanical box, take a moment to remove each terminal screw and run a fine file over the underside of the screw head: the edges are often left sharp and can bite through a wire as you tighten it. Refit the screws. Do not tin or solder the wire ends — the connection relies on the screw clamping the bare copper strands, and a tinned end prevents a proper grip. An electronic drop-in box needs none of this — just mount and wire it.
Step 5 — Mount and wire the new box
Screw the new box to the bulkhead, then transfer the wires one terminal at a time — A to A, A1 to A1, D to D, F to F, E to E — tightening each before moving to the next. Doing them singly is what prevents a mix-up. A neat trick on the screw terminals: back the screw right out, lay the wire into the slot so no strands stray, then tighten down. Make sure terminal E lands on clean, bright metal for a good earth. Check every wire against your photograph before going further.
Step 6 — Re-polarise the dynamo
Any time the dynamo or box has been disconnected, briefly re-establish the dynamo's residual magnetism in the right direction for the car's polarity — "flashing", or polarising, it. With the battery reconnected but the ignition off and the engine not running, take a short insulated lead and momentarily touch it between the A (or A1) terminal and the F terminal. A small spark is normal. That pulse of battery current through the field sets the dynamo to charge the correct way. Skipping this is a classic cause of "fitted a new box, still no charge".
Step 7 — Reconnect and test
Refit the battery earth (positive) strap, then:
- Ignition on, engine off. The ignition warning lamp should glow.
- Start up and hold a fast idle (about 1,500–2,000 rev/min). The warning lamp should go out and the ammeter should swing to the charge (+) side — strongly at first if the battery is down.
- Let it settle. As the battery recovers, the charge should taper back toward the centre. That taper, not a fixed number, is the sign of a healthy, correctly regulating box.
- Confirm with a meter across the battery at a fast idle: you should see the mid-13s up to about 15 volts on the car, steady. If it climbs past about 15.5V or sits at 16–17V, stop and work through the overcharging section of the control box overview.
Step 8 — Set it, if needed
Modern replacement boxes are often supplied pre-set, and an electronic unit never needs adjustment. If yours is an unadjusted mechanical box, or the on-car readings are out, set the open-circuit regulator voltage and the cut-out by following the calibration guide. Remember the Lucas setting is an open-circuit figure — around 16 volts, higher than the on-car reading — so never "correct" it to 14.4 as if it were an alternator.
Safety
- Positive earth throughout — mind polarity when you reconnect and when you flash the dynamo.
- Disconnect the battery before working on the wiring.
- Keep the re-polarising flash momentary.
- Never run the engine with a battery cut-off switch open, or with the dynamo's D and F leads linked.
*Written for the T-Series Archive. Procedure based on Lucas service practice for the RB106-series compensated voltage control boxes; confirm figures for your specific unit against the Lucas data.*