An ignition warning lamp that glows on the move is the T-type’s way of saying the battery is doing all the work. The charging system is a simple, three-part affair — a Lucas C39 dynamo, an RB106/2 control box and the wiring between them — and the fault is almost always in the cheap end of that list. Work through it in order and you will usually know within an hour whether you are cleaning a connection, adjusting a regulator, or taking the dynamo off.
Owner photograph to follow
What the system actually does
The dynamo is a simple generator: as the belt turns it, current is taken from the large D terminal, while the smaller F terminal feeds its field windings. On its own a dynamo has no idea what the battery needs, so the RB106/2 control box on the bulkhead does the thinking — a regulator winding that trims the field current to hold the system voltage, and a cut-out that connects and disconnects the battery so it cannot discharge back through the dynamo at rest.
That division of labour is the key to fault-finding. A dynamo either generates or it does not, and you can prove which in five minutes. Everything to do with how much it charges, and whether that charge reaches the battery, belongs to the control box and the wiring.
The fault-finding sequence
Start at the cheap end. Most “dead dynamos” turn out to be a slipping belt, a corroded spade or a poor earth.
- 01
Read the warning lamp
Switch on the ignition: the lamp should glow, and go out as the engine picks up above a fast idle. A lamp that stays on means no charge is reaching the battery. A lamp that never lights at all is usually the bulb, its holder or the wiring to it — prove the bulb before condemning anything electrical.
- 02
Check the belt and pulley
A glazed, slack or oily fan belt will let the dynamo pulley slip, especially under load. You should be able to deflect the belt about half an inch at its longest run and no more; check the exact tension figure against your Workshop Manual before adjusting the dynamo on its cradle.
- 03
Clean and test the earths
T-types are positive earth, and the earth path is as important as the live one. Clean the battery earth strap, the engine-to-chassis strap, and the dynamo's own mounting faces back to bright metal. A high-resistance earth mimics almost every charging fault there is.
- 04
Check the dynamo terminals and leads
With the ignition off, remove the two leads from the dynamo — the larger D (output) and the smaller F (field) — and inspect for corroded spades, chafed insulation and broken strands where the loom flexes near the engine.
- 05
Run the output (flick) test
Leave both leads off. Join the dynamo's D and F terminals with a short jumper wire. Connect a voltmeter between that jumper and a good earth. Start the engine and hold it at a fast idle only — perhaps 1,000 rpm — and the reading should climb steadily well above battery voltage. Do not rev the engine hard with the terminals linked: an unregulated dynamo will produce a damaging voltage. A rising reading means the dynamo generates and the fault lies downstream, in the regulator or wiring. No rise, or a barely moving needle, points at the dynamo itself.
- 06
Inspect brushes and commutator
Take the band off the end of the dynamo. The brushes should move freely in their boxes with springs that still press firmly, and stand well clear of their wear limit. The commutator should be clean and even, not blackened, scored or clogged with brush dust. Clean it with a rag and petrol or methylated spirit — never with emery paper, which embeds grit — and re-check the brush length against the figure in the Workshop Manual.
- 07
Flash the field if the dynamo has lost its magnetism
A dynamo that has stood for years can lose its residual magnetism and produce nothing even though it is perfectly serviceable. With the engine stopped and the leads off, briefly touch a wire from the dynamo's F terminal to the live (negative, on a positive-earth car) battery terminal for a second or two, then repeat the output test. If the dynamo now generates, that was all it needed.
Reading the symptom
| Symptom | Most likely | Prove it by |
|---|---|---|
| Lamp on all the time, battery going flat | Belt, earth, dynamo output or open field circuit | The output (flick) test in step 5 |
| Lamp goes out only at high revs | Slipping belt, worn brushes, or a regulator set low | Belt tension, brush length, then regulator setting |
| Lamp flickers with engine speed | Dirty commutator, sticking brush, or chattering cut-out | Band off: watch the brushes; then watch the cut-out points |
| Bulbs blowing, battery boiling and losing water | Overcharging — regulator set high or contacts welded | Measure the charging voltage and re-set the control box |
| Dynamo generates on test but nothing at the battery | Control box, its earth, or the brown wiring to the battery | Voltage at each control-box terminal, engine running |
Specific voltage and current settings for the RB106/2 vary with model and control-box version, so take them from the Workshop Manual for your car rather than from a general figure — the calibration article below walks through the adjustment itself.
Two traps worth knowing
The first is polarity. T-types are positive earth, and a dynamo that has been flashed, tested or rebuilt with the polarity reversed will fight the system rather than charge it. If the car has ever been jump-started the wrong way round, or the battery has been fitted backwards, re-establish polarity before chasing anything else.
The second is over-eagerness with the output test. It is a genuinely useful check, but with D and F linked the dynamo is unregulated: hold it at a fast idle, watch for the voltage to rise, and stop. There is nothing to be learned from revving it, and a real risk of cooked windings.
When to take the dynamo off
If the output test shows no rise after the field has been flashed, the brushes are sound and the earths are clean, the dynamo has an internal fault — an open or shorted armature or field winding, or a commutator that needs skimming. These are straightforward, worthwhile rebuilds: the units are simple, parts are still made, and a rebuilt dynamo with a fresh commutator and bearings will outlast most of the other electrics on the car. The alternative — an alternator conversion — solves charging at idle but changes the look of the engine bay and the polarity of the car, so it is a decision to make deliberately rather than as a repair.
Read next
This guide is a general introduction and is not a substitute for the official Workshop Manual for your car.