The Lucas Control Box: Voltage Regulator on the MG TD & TF

The small aluminium box on the bulkhead is the brain of the T-Type's charging system. Understand how the Lucas control box regulates the dynamo, and most "mystery" charging faults — flat batteries, blown bulbs, batteries that boil dry — resolve into a handful of checks and one careful adjustment. This guide covers the two-bobbin Lucas RB106/2 fitted to the MG TD and TF: how it works, how to fault-find it, and how to calibrate it, with particular attention to the overcharging fault and how to correct it.

Which cars this applies to

The T-Type range did not use one regulator throughout, so identify yours by the model number stamped on the box before trusting any figures below.

All TD/TF cars are 12-volt, positive earth, using a Lucas C39PV-2 (later C40) dynamo rated at roughly 19 amps. The original mechanical control box is indifferent to polarity and works on either earth; a modern solid-state replacement is not — it must be ordered for the correct polarity.

The charging system in brief

Three components share the work: the dynamo makes the electricity, the battery stores it, and the control box sits between them deciding how much the dynamo is allowed to put out and when it may connect to the battery.

A dynamo left to itself has no idea when to stop. Spin it faster and it makes more voltage, without limit, until something burns. The control box tames it using "compensated voltage control" (CVC): it watches system voltage and continuously trims the dynamo's field current to hold the output steady, letting the dynamo pour in charge when the battery is low and easing right off once the battery is full.

The RB106/2 is a two-bobbin box containing just two devices: a voltage regulator and a cut-out. Unlike the later three-bobbin boxes (RB310, RB340) it has no separate current regulator — output is limited by the dynamo's own characteristics together with the voltage regulator. Any "current regulator" adjustment you read about elsewhere does not exist on this box.

Inside the box: the two units

The voltage regulator

An electromagnet (the bobbin) with a set of contacts held closed by a spring. Two windings share the core: a heavy "series" winding carrying dynamo output, and a fine "shunt" winding connected across the system that senses voltage. As system voltage rises, the shunt winding's magnetic pull grows until it overcomes the spring and pulls the armature away, opening the contacts. Opening the contacts inserts a resistor into the dynamo's field circuit, which weakens the field and drops the output; voltage falls, the spring re-closes the contacts, and the cycle repeats — many times per second. The contacts effectively "buzz", and the average field current they produce is exactly what's needed to hold the set voltage.

A bi-metal element in the spring assembly leans on the setting so the box deliberately allows a slightly higher voltage when cold and a lower one when hot — temperature compensation, matched to what a lead-acid battery actually wants.

The cut-out

Simply an automatic switch (a relay) that connects the dynamo to the battery only when the dynamo is producing more voltage than the battery, and disconnects them the instant it isn't. Without it, a stationary or slow-turning dynamo would let the battery discharge backwards through the windings and could motor the dynamo or burn its armature. The cut-out closes ("cuts in") as the dynamo comes up to speed and opens again ("drops off") as it slows.

The single most misunderstood point: open-circuit vs on-car voltage

The Lucas regulator setting is an open-circuit figure — measured with the battery isolated from the dynamo. On a 12-volt system that figure is about 16 volts, which alarms people raised on 14-volt alternators. That is correct and normal. With the battery reconnected, the battery clamps the voltage down: on the car you will see something in the 14s dropping toward the 13s as the battery comes up to charge.

So: never "correct" a regulator to 14.4 volts as if it were an alternator. Set it on open circuit to the Lucas band below; judge health on the car by whether the charge current tapers as the battery fills, not by an absolute voltage.

Reading it correctly (and safely)

• Use a good moving-coil voltmeter or a decent digital meter, 0–20 V, readable to about 0.1 V.
• This is a positive-earth car: the chassis is +. Connect meter leads accordingly (the dynamo D terminal sits negative of earth). If using a digital meter you will simply see a minus sign; note it and read the magnitude.
• Take regulator readings quickly. The box self-heats and the reading drifts down perhaps 0.2–0.3 V within a minute, which will mislead you.
• Always begin fault-finding at the dynamo, not the box. A box cannot regulate what the dynamo isn't making.

Symptoms and what they usually mean

Fault-finding, step by step

1. Mechanicals first. Fan belt tension and condition; dynamo mountings tight; all control-box and dynamo connections clean and tight; and above all a good earth — both the battery-to-chassis strap and the control box's own earth (terminal E). A poor earth alone can cause wild mis-regulation.
2. Test the dynamo. Disconnect the D and F leads at the dynamo and link the two dynamo terminals together with a short jumper. Connect the meter between that link and earth. Start the engine and raise it only to a fast idle. Voltage should rise smartly with speed. In this linked condition the dynamo is unregulated, so keep the revs low and the test brief — it will make dangerously high voltage very quickly. No rise means a dynamo fault (brushes, armature or field) or a dynamo that needs re-polarising.
3. Check the D/F cables from dynamo to box for continuity and shorts.
4. Check the regulator setting (open circuit). Isolate the battery from the dynamo — the neat way is to slip a strip of dry card between the cut-out contacts; alternatively disconnect and join the box's A and A1 leads. Connect the meter between the D terminal (or the box frame) and earth. Run the dynamo at about 3,000 rev/min. The voltage should hold steady inside the band in the data panel below. Steady but out of band → adjust (see calibration). Won't rise, or only rises to about half the figure → dirty/high-resistance regulator contacts (bridge them momentarily with a screwdriver: if voltage now rises normally, the contacts are the fault). Rises without limit → broken shunt winding or a lost earth.
5. Check the cut-out. Remove the card. Move the meter to the A terminal and earth. Start the engine and raise speed: as the cut-out closes the reading should jump to just above battery voltage. Note the voltage at the instant it closes — it should be within the cut-in band.
6. Final on-car check. With everything reconnected and an ammeter in circuit, the charge should build with speed and then taper as the battery recovers. That taper — not a fixed voltage — is the sign of a healthy, correctly set box.

Overcharging: causes and how to cure it

Overcharging is the fault that quietly destroys batteries and pops bulbs, so it deserves its own treatment. The tell-tales are a battery that needs frequent topping up or smells acrid, small bulbs that blow repeatedly, and an ammeter that keeps showing a healthy charge long after the battery should be full.

Work through the causes in this order, because the cheap ones are also the commonest:

1. Dirty or pitted regulator contacts. Oxidised contacts add resistance, so the shunt winding "sees" less voltage than is really there and never eases the field back enough — the dynamo keeps charging hard. Clean the regulator contacts with fine carborundum / silicon-carbide paper or a fine points file, then wipe with a rag moistened in methylated spirit. Never use emery cloth — the grit embeds and conducts. (The cut-out contacts, if they need it, are cleaned with fine glass paper, not carborundum.)
2. Poor control-box earth. A bad earth at terminal E shifts the reference the shunt winding works against and makes the box regulate high. Clean and tighten it to bright metal.
3. Tired temperature compensation. If the bi-metal element has weakened, the box holds too high a voltage, most noticeably when cold. If cleaning and earthing don't bring it into band across temperatures, the unit is due for replacement.
4. The setting itself is too high. Once contacts and earth are sound, measure the open-circuit voltage as in step 4 above and, if it's above the band, lower it (see calibration). As a rule of thumb, on this box the voltage-adjusting screw is turned anti-clockwise to lower the setting.
5. A sick battery. A heavily sulphated battery with high internal resistance can hold system voltage up and mimic overcharging. Rule the battery in or out before condemning the box.

Do not chase overcharging by simply winding the setting down blindly — a box with dirty contacts or a bad earth will read wrong, and you will end up mis-set. Fix contacts and earth first, then set the voltage.

Calibration

You need an accurate 0–20 V meter, a means of holding the dynamo at about 3,000 rev/min, a strip of dry card, a small screwdriver, feeler gauges, and fine abrasive for the contacts. Work with the box warm-ish but take each reading quickly.

Setting the regulator voltage

1. Clean the contacts and confirm a good earth first.
2. Isolate the battery (dry card between the cut-out contacts, or join the A and A1 leads).
3. Meter between D (or box frame) and earth. Run the dynamo at about 3,000 rev/min.
4. Read the steady voltage and compare with the temperature-corrected band in the data panel.
5. Adjust the voltage-adjusting screw: clockwise raises the setting, anti-clockwise lowers it. Make a small move, then re-check by dropping the speed to zero and running back up — don't read while continuously adjusting.
6. Repeat until the open-circuit voltage sits in band. Remove the card and reconnect.

Setting the cut-out cut-in

1. Meter between D and earth. Raise engine speed slowly and note the voltage at the instant the contacts close.
2. It should fall within the cut-in band (12.7–13.3 V). Turn the cut-out adjusting screw in to raise the cut-in voltage, out to lower it, and re-test from zero speed.

Air gaps (mechanical settings) — leave alone unless disturbed

The armature air gaps are factory-set and should not be touched unless you've replaced a contact set or someone has meddled. If you must: for the RB106/2, slacken the fixed-contact screw, unlock the armature securing screws, set the armature-to-core gap with the correct feeler gauge while pressing the armature squarely down, retighten, then screw the fixed contact down until it just touches the moving contact and lock it — and finally reset the voltage as above. Because gap figures vary slightly by exact box variant, confirm them against the Lucas service data for your unit rather than guessing.

Practical and modern notes

• Re-polarising. Any time the dynamo or box has been disconnected, briefly flash the dynamo field to restore residual magnetism in the correct sense for the car's polarity — momentarily touch a lead from the A/A1 terminal to the field (F) terminal. A dynamo polarised the wrong way, or the wrong way round after an earth change, is a classic cause of "no charge".
• Solid-state replacements. Electronic regulators in a Lucas-look case remove all adjustment and hold voltage tightly, but they are polarity-specific — order the correct earth.
• Alternator / "Dynamator" conversions. A self-regulating unit that mimics the C39/C40 dynamo removes the need for the control box altogether (the box is then blanked or used as a dummy). Worth knowing, though many owners rightly prefer to keep the original system working.

Safety

• Positive earth — mind meter and connection polarity throughout.
• Never run the dynamo for more than a moment with D and F linked (unregulated overvoltage).
• Disconnect the battery before working inside the box.
• Take settings quickly to avoid heat-drift error.

Where this article grows next

Follow-ups this piece can grow into: a labelled control-box wiring and terminal diagram; dynamo removal, testing and brush replacement; converting to negative earth safely; and fitting a concealed electronic regulator while keeping the original box. Related reading on the ignition and charging side lives in the Ignition & Charging article and in the Tuning Guide's ignition section.

*Calibration figures based on Lucas published service data for the RB106-series compensated voltage control boxes; explanatory text written for the T-Series Archive.*