Almost every electrical job on a T-type starts with the same question: what is this wire and where does it go? The answer is simpler than the tangle behind the dashboard suggests — there are only about eight colours, three of which are permanently live — but it comes with one important caveat. These cars predate the standardised British colour code, and Abingdon’s own diagrams numbered the wires rather than naming their colours. What follows is the reconstructed code the clubs work to, alongside the terminal markings you will actually find on the car, and a sequence for tracing a fault through it.
First, the caveat that matters
There is no factory Lucas colour code for the TC, TD and TF in the sense that later British cars have one. The codified British Standard colour system arrived in 1969. MG’s own wiring diagrams for these cars identified each wire by a number, with a cross-reference table in the Instruction Manual; colours were not printed on MG diagrams until 1962, years after the last TF left Abingdon.
What exists instead is the Ripaults colour convention the industry used informally from the mid-1930s, reconstructed for these models by club researchers — most thoroughly by Barrie Jones, whose colour notes for the TD and TF are published by the MG Car Club’s T Register community. It is reliable enough to work with, and the researchers themselves flag the loose ends: exactly when purple came in, and how the TD sits as a transitional car between the older yellow-based scheme and what the TF settled on.
So use the table below to form an expectation, and prove every wire with a meter.
The loom colours (TD and TF)
| Colour | Code | Circuit | Notes |
|---|---|---|---|
| Brown | N | Battery and charging feeds — battery to ammeter, battery to control box / fuse box A1 | Live at all times, whatever the ignition switch is doing. Never fused on these cars. |
| Yellow | Y | Dynamo D and F leads to the control box | Yellow on the TF (and on the MGA that followed). The TD is transitional: some later cars carry brown for these functions after a Ripaults colour change. |
| White | W | Ignition-switched, unfused — coil, fuel pump (control box / fuse box A3) | Dead with the ignition off. White with a black tracer is the distributor-to-coil lead. |
| Green | G | Ignition-switched and fused (A3–A4) — stop lamps, wiper motor, heater, flashers if fitted | If everything green is dead at once, suspect the A3–A4 fuse before the switches. |
| Blue | U | Headlamps, after the lighting switch | Tracers distinguish main from dip on cars with a foot or dip switch. |
| Red | R | Side and tail lamps, after the lighting switch | Also panel illumination on most cars. |
| Purple | P | Permanently live but fused (A1–A2) — horn, courtesy lamps | A later addition. Some TDs have it; the TF and MGA looms do not use it. Do not expect purple on a TC. |
| Black | B | Earth returns | Never fused, never switched. On a positive-earth car these run to the chassis. |
Tracers are read primary colour first, then the stripe: a white wire with a black tracer is written white/black, and on these cars that is the lead from the distributor to the coil. Light green appears once in the TF loom; slate, pink and orange belong to later cars and should not be there at all unless someone has added a circuit.
The practical shortcut behind the whole table: brown is live whatever you do, white comes alive with the ignition, green is ignition-live but fused, blue and red are downstream of the lighting switch, and black goes to earth. That alone tells you where to put the meter probe.
Where the TC differs
The TC sits outside the table above rather than beside it. Its factory diagram is a numbered one, cross-referenced in the Instruction Manual, and no complete colour-by-colour TC loom chart of the kind published for the TD and TF appears to exist. TC restorers work from the numbered diagram plus the manual’s table.
Two further TC-specific points are worth knowing. The early TC factory wiring diagram contains known errors, which the clubs have catalogued and corrected — the Chicagoland MG Club publishes a tech tip listing them, and the T-ABC forum has long threads working through the same ground. And the yellow-to-brown change in the charging leads happened after the TC and during TD production, so a TC does not follow the later convention even where the colours look familiar.
If you have a TC, treat any colour table — including this one — as background, and let the numbered diagram for your car be the authority.
Control box, fuse box and terminal markings
The single biggest fork in T-type wiring is the control box, and it splits the range at TD chassis number 8142 in June 1951.
| Cars | Control box | Dynamo | Fusing |
|---|---|---|---|
| TC, and TD to chassis 8141 | Lucas RF95, nine terminals: A, A1, A2, A3, A4 (×2), D, F, E | Lucas C39 | Inside the control box |
| TD from chassis 8142, all TF and TF1500 | Lucas RB106, five terminals: A, A1, D, F, E | Lucas C40 | Separate Lucas two-fuse box, terminals A1–A2 and A3–A4, 35-amp glass fuses |
On the nine-terminal RF95, the letters mean something consistent and worth committing to memory, because the same logic carries into the later fuse box. A is the feed from the battery by way of the ammeter, permanently live. A1 is the unfused live out to the ignition and lighting switch. A2 is a fused permanent live — the horn. A3 is the return from the ignition switch, feeding the coil. The two A4 terminals are the fused output from A3, feeding the stop-lamp switch, the wiper motor, and the fog lamp or flashers where fitted. D and F go to the dynamo’s armature and field. E is earth.
The five-terminal RB106 keeps A, A1, D, F and E doing the same jobs and hands the A2, A3 and A4 fused-circuit work to the separate fuse box, where the same terminal numbering reappears — A1–A2 for the permanently live fused circuits, A3–A4 for the ignition-switched fused circuits.
The dynamo has just two terminals, D and F, on both generations, and they connect to the identically lettered terminals on the control box. That correspondence is the easiest thing in T-type electrics to get right and one of the more common things to find wrong.
One caution on labelling: I have not found a source that confirms lettered terminal markings stamped on the ammeter or on the ignition and lighting switch themselves for these years. Treat the letters shown against those components on any diagram as positions on the drawing rather than as markings you will find on the part.
System basics to settle before you start
- Positive earth, 12 volts, one battery on the TC, TD and TF. The earlier TA and TB used two 6-volt batteries instead, so parts advice written for “T-types” generally does not transfer across that line.
- Battery on TD and TF sits in a wooden tray held by a stay clamp and J-bolts — specialists list that hardware as a set, which is the quickest way to get a scruffy installation back to standard.
- Dynamo detail: early TD dynamos use a combined pulley and fan casting, later TD and TF a separate pulley with a bolt-on fan. On both TD and TF the dynamo drives the rev-counter from its rear.
- Wiper motor: the Lucas CWX, Lucas part number 732480, used from TA through TD; TD motors are often found stamped CW1.
- Bulb wattages are deliberately not listed here. I could not verify a figure-by-figure table for the TC, TD and TF from a source I would stand behind, so take them from the specification pages of the Workshop Manual or from a specialist bulb chart for your model.
Tracing a fault through the loom
- 01
Start at the battery and the ammeter, not the component
On the T-type the main feed runs battery → ammeter → control box A terminal, and out again from A1 to the ignition and lighting switch. Almost every circuit on the car depends on those three joints. Check for battery voltage at each of them in turn before you touch anything further down the loom.
- 02
Decide from the symptom whether you are chasing supply or earth
As a working rule rather than a documented law: a sudden, total loss of everything usually means a break in the supply side — the ammeter, the A terminal, a main brown lead. Dim, erratic or interacting faults, where pressing the horn moves the ammeter or the wiper slows the sidelamps, usually mean a poor earth shared by several circuits.
- 03
Prove the control-box E terminal
E on the control box is an earth junction, not an earth in itself: it needs its own good wire to the chassis, and on many cars the fuel pump, wiper and horn are earthed through it as well. A corroded wire at E produces a whole dashboard of unrelated symptoms and is regularly mistaken for a failed regulator.
- 04
Identify the wire by colour, then confirm it by continuity
Use the colour table to form an expectation — brown means permanently live, white means ignition-switched, green means fused and switched — then prove it with a meter. Fifty years of repairs, replacement looms and improvised joints mean the colour tells you what the wire was probably for, never what it definitely does now.
- 05
Check the fuses properly
From TD chassis 8142 and on all TFs there is a separate two-fuse Lucas box with terminals marked A1–A2 and A3–A4, taking 35-amp glass fuses. Glass fuses fail invisibly at the cap: test across each one with a meter rather than looking at the wire inside. Earlier cars have no separate box — the fusing lives inside the nine-terminal RF95 control box.
- 06
Work the joints, not the middles
Wire rarely fails along its length. Snap connectors, spade tags, the bulkhead grommet, the run down to the wiper motor and anywhere the loom is strapped near the exhaust side of the engine are where you look. Clean each joint to bright metal and remake it rather than testing around it.
- 07
Confirm against the diagram for your chassis number
Get a diagram for your car specifically. The nine-terminal RF95 layout and the five-terminal RB106 layout are genuinely different cars electrically, and the early TC factory diagram is known to contain errors that have been catalogued and corrected by clubs.
Bad earth or bad regulator?
This is the distinction that wastes the most time, because a poor earth can imitate almost any charging or lighting fault. Two things help.
The first is the pattern of the symptom. A regulator fault is generally consistent and confined to charging: the ignition lamp behaves the same way every time, or the battery is persistently under- or over-charged. An earth fault is generally erratic and crosses circuits: the ammeter twitches when you press the horn, the wiper slows and the sidelamps dim together, the indicators tick through the brake lights. If two unrelated things affect each other, look for a shared earth, not a component.
The second is the control box E terminal, and the A/A1 pair beside it. Owners have traced “the ammeter only moves when I sound the horn” to nothing more than brown and brown/white swapped between A and A1 on the control box — a wiring error, not a fault. Before condemning the regulator, confirm that A and A1 are the right way round and that the E wire is clean and tight at both ends.
Only when the earths and the A/A1 feeds are proven is it worth putting a meter on the regulator itself; the calibration article covers that adjustment in detail.
Where to get a diagram for your car
Do not work from a generic diagram. The list below is what I use, and every technical claim on this page comes from one of these.
- The Workshop and Instruction Manuals for your model — the authority for your own car. The Connecticut MG Club’s digital library hosts scans covering the TC, TD and TF.
- The MG T Society publishes colour-rendered wiring diagrams for the TC and for the TD/TF.
- Barrie Jones’ colour-code notes on the MG Cars Info site are the source of the TD/TF colour reconstruction used above, including the yellow-brown and purple caveats.
- The TC Motoring Guild’s TClinic series includes a detailed article on the nine-terminal RF95 control box — the source for the terminal functions given here.
- Chicagoland MG Club tech tips catalogue the errors in the early TC factory diagram.
- Lucas’s own “Generator and Control Box Tests”booklet, republished by the MG Car Club T Register, is the period reference for testing the charging side.
- Specialist parts diagrams from Brown & Gammons / SC Parts and MGOC Spares give the chassis-number breakpoints for the control box, fuse box, dynamo and battery; Autosparks and Moss list reproduction harnesses and fuses.
Common wiring questions
- Is there an official Lucas colour code for the MG TC, TD and TF?
- Not in the way there is for later British cars. The codified British Standard colour code arrived in 1969, long after these cars, and MG's own factory wiring diagrams identified wires by number with a cross-reference table rather than printing colour letters — they did not start printing colours until 1962. The colour tables now in circulation for the TD and TF are careful reconstructions by club researchers from surviving looms and the Ripaults conventions of the period, and they are useful, but they are not a factory standard.
- Which control box does my car have?
- The TC and the TD up to chassis number 8141 have the nine-terminal Lucas RF95, with terminals A, A1, A2, A3, A4 (two of them), D, F and E, and the fusing built in. From TD chassis 8142 in June 1951, and on all TF and TF1500 cars, the car has the five-terminal Lucas RB106 — A, A1, D, F, E — with the fuses moved out to a separate box. This is the single most significant electrical change in the TD's life and it changes almost every wiring answer.
- Which dynamo goes with which control box?
- The TC and early TD use the Lucas C39 with the RF95 control box; the later TD and all TFs use the Lucas C40 with the RB106. On TD and TF the dynamo also drives the mechanical rev-counter from its rear end, so a dynamo swap is not simply a matter of matching the pulley.
- What fuses does a TD or TF take?
- The Lucas two-fuse box fitted from TD 8142 onwards takes 35-amp glass fuses, 32 mm long, and specialists sell them specifically for TC/TD/TF. A 50-amp fuse of the same physical size exists and is sometimes offered as an alternative; fit the rating your diagram calls for rather than the largest that will physically go in.
- Are T-types positive earth?
- Yes — the TC, TD and TF are all 12-volt positive earth, with a single 12-volt battery. The earlier TA and TB used two 6-volt batteries instead. Positive earth matters for coil polarity, for repolarising a dynamo after any test or rebuild, and for anything modern you add to the car.
- Can I trust the colours in my car as a guide?
- Treat them as a hypothesis. These cars are seventy to ninety years old and most have had at least one replacement harness, several owners' repairs and a few wires added for indicators, an electric fan or a radio. Use the colour to decide what to test first, then prove every wire with a meter before cutting, joining or connecting anything to it.
Read next
Last reviewed 2026-07-17