A T-type’s electrics look worse than they are. There are only five or six circuits on the whole car, and every one of them starts at the same three joints. What follows are two schematics drawn for this site — not scans of somebody else’s print — showing where the current actually goes on both halves of the range, with the terminals named as you will find them stamped on the car.
What these diagrams are — and are not
These are original drawings. We have not reproduced the factory prints or the clubs’ colour redraws, because those are other people’s copyright, and in any case a scan of a seventy-year-old print is rarely what you want when you are lying under a dashboard. What you want is the shape of the thing: which feeds are live all the time, which die with the ignition, which pass through a fuse, and what shares an earth with what.
Every connection shown here comes from a source we could check — Lucas control-box literature, the TC Motoring Guild’s terminal-by-terminal bulletins on the RF95, and the documented production change at TD 8142. Where we could not verify a detail, it is left off the drawing and listed openly further down rather than invented. Your own car’s diagram, for your own chassis number, remains the authority.
Which car you have: RF95 or RB106
One chassis number splits the range in two. The TC, and the TD up to chassis 8141, carry the nine-terminal Lucas RF95: regulator, cut-out and both fuses under a single cover. From TD chassis 8142 in June 1951, and on every TF and TF1500, the car has the five-terminal RB106 with the fuses moved out to a separate box. Diagrams from one side of that change will mislead you on the other.
Everything else on the car — ammeter in the main feed, unfused live side, switched ignition side, lamps downstream of the lighting switch, chassis as positive earth — is common to both. That is why the first diagram below is the whole car and the second is only the box that differs.
Diagram 1 — TD from chassis 8142, and all TF
Read it left to right: sources on the left, control gear in the middle, loads on the right, chassis earth along the bottom. Colours group the feeds rather than representing loom colours — for those, use the loom colour-code guide, which also explains why no factory colour code exists for these cars.
The circuits, annotated
Seven things in that drawing explain most of the electrical faults owners actually meet.
- 01
Battery → ammeter → terminal A
The whole car hangs off one path. The battery feed runs through the dash ammeter and into control-box terminal A, which is why the ammeter reads total charge and discharge rather than the dynamo alone. A poor joint anywhere along that path takes out almost every circuit at once, and it is the first thing to test when nothing works.
- 02
A1 is live whatever the ignition is doing
A1 is the outward feed to the ignition and lighting switches, and on the later cars to the separate fuse box as well. It is not switched and not fused. That is why the horn and the lamps can work perfectly on a car whose ignition circuit is dead — if they do, the fault is at or after the ignition switch, not in the control box.
- 03
The dynamo returns on two wires, D and F
D is the output; F is the field the regulator controls. The cut-out is a simple relay that only connects D to A once dynamo voltage exceeds battery voltage, so the battery cannot discharge back through a stationary dynamo. Early TDs use the Lucas C39; later TDs and all TFs use the C40, which also drives the mechanical rev counter from its rear end.
- 04
The starter is drawn on its own heavy cable
Starter current is far beyond what a period ammeter is built to carry, so the starter switch and motor are fed straight from the battery and sit outside the metered circuit entirely. Both TC and TD/TF use an inertia (Bendix) starter — the TC with the Lucas ST-10 pull switch. A car that cranks but shows nothing on the ammeter is behaving normally.
- 05
Ignition-switched, but not all of it fused
The coil feed is switched and taken through the box as a junction rather than regulated a second time. The SU electric fuel pump — fitted to the TC as well as the TD and TF, these cars never had a mechanical pump — and the fuel gauge sit on the same switched side. Stop lamps and wiper are switched and fused, which is why the fuse is the right first check when those three die together.
- 06
Lighting hangs off the switch, not the ignition
The lighting switch takes its feed from the always-live side, running side and tail lamps, number-plate and panel illumination on one position and the headlamps on the next, with main and dip separated by a floor-mounted foot switch. Lamps that dim when another circuit is used are almost always sharing a tired earth rather than losing supply.
- 07
E is a junction, not an earth in itself
The control-box E terminal needs its own sound wire to the chassis, and on many cars other components earth through it. A corroded wire at E produces a dashboard full of unrelated symptoms and is regularly mistaken for a failed regulator. Every load on these diagrams returns through the chassis, which is the positive side on all of these cars.
Diagram 2 — the nine-terminal RF95 (TC and TD to 8141)
On these cars there is no separate fuse box to look for: both fuses are inside the control box, and several terminals are simply junctions that happen to live under the same cover. That is the single most misread thing about early T-type electrics, so the drawing below names each terminal by what it feeds rather than by where it sits on the moulding.
One practical consequence: on an RF95 car, A3 receiving the ignition feed and passing it straight out to the coil means a dead coil with a live A1 points at the ignition switch or its wiring, not at the regulator. Owners replace a great many serviceable control boxes for want of that distinction.
For what the regulator and cut-out are doing inside the box — and how to test them rather than replace them — see the ignition and charging guide and the symptom-led dynamo-not-charging walkthrough.
What we have deliberately not drawn
Rather than fill the gaps with plausible guesses, here is exactly what we could not verify to the standard the rest of the drawing meets. If you can document any of it from an original source, we will draw it in and credit you.
- Warning lamps. Whether an ignition (no-charge) warning lamp was fitted as standard on each model. No lamp is shown.
- Fuse ratings per position. That the box takes 35-amp glass fuses is well attested; which rating the factory specified for each of the two positions is not, so no rating is assigned to a position.
- Stop-lamp switch type. Mechanical pedal-operated and hydraulic switches were both used across the range; the changeover point is not something we can pin to a chassis number, so the switch is drawn generically.
- Starter switch on TD and TF. The TC’s Lucas ST-10 pull switch is documented; the exact later arrangement is drawn simply as “starter switch”.
- Dynamo part numbers on the TC. Period Lucas documents and modern club sources do not agree, so no TC dynamo model is stated on the drawings.
- Instrument internals. Fuel-gauge and tank-unit resistance values, and gauge terminal markings, are not given.
Where to get the factory and club diagrams
Use these drawings to understand the car, and one of the following to wire it. All are copyright to their publishers, which is why we link rather than reproduce.
- The factory Instruction and Workshop Manuals
- The authority for your car. The TC diagram numbers each wire and cross-references it in a table rather than naming colours. Buy a reprint from a specialist or your club rather than working from a photocopy of unknown provenance.
- MG Car Club T Register
- Publishes technical material and Lucas service literature for the T-types, including the Lucas Generator and Control Box Tests booklet.
- New England MG T Register — Totally T-Type
- Club journals have carried colour wiring diagrams and corrections for the TC over many years; a good route to a diagram checked by people who own the cars.
- TC Motoring Guild — TClinic bulletins
- The clearest plain-English account of the RF95 nine-terminal control box terminal by terminal, and the source we worked from for the RF95 drawing on this page.
- Harness makers' installation diagrams
- Autosparks, Advance Autowire and Moss publish coloured diagrams as fitting aids with their looms. They are modern redraws rather than factory prints, and they note differences between the nine-post and five-post cars — useful as a cross-check.
Before you order a loom
Establish your control box first. A harness cut for a five-post car will not suit a nine-post one, and the difference is not something to discover with the dashboard out. Take the chassis number, confirm it against the chassis and engine number guide, then order to that.
Questions
- Are these the factory MG wiring diagrams?
- No. Every drawing on this page is our own, made from verified descriptions of the circuits — Lucas control-box literature, the TC Motoring Guild's RF95 bulletins and club charging-system notes. The factory prints and the clubs' colour redraws are copyright to their publishers, so we point you to them rather than reproducing them. Our drawings show how current actually flows, which is what you need at the roadside; your car's own diagram remains the authority on wire numbers and colours.
- Which wiring diagram applies to my car?
- It depends on one chassis number. The TC and the TD up to chassis 8141 have the nine-terminal Lucas RF95 with both fuses inside the control box. From TD chassis 8142 in June 1951, and on every TF and TF1500, the car has the five-terminal RB106 with a separate two-fuse box. Electrically these are two different cars, so establish which one you have before you follow any diagram.
- Why does the starter not appear in the main circuit?
- Because it deliberately bypasses it. The starter draws hundreds of amps, far more than a period ammeter or the control box can carry, so it is fed by a heavy cable straight from the battery through the starter switch. This is standard Lucas practice of the era; it means the ammeter needle sitting still while the engine cranks is correct behaviour, not a fault.
- Does the T-type have an ignition warning lamp?
- We have not been able to verify this to our own standard for each model, and rather than guess we have left the lamp off the drawings. Many British cars of this class relied on the ammeter alone to show charging state. Check the dash layout in the Instruction Manual for your model and chassis number before adding or chasing one.
- What fuses should be in the fuse box?
- The Lucas box fitted from TD 8142 onwards takes 35-amp glass fuses in the original size, and specialists sell them specifically for TC/TD/TF; a 50-amp fuse of the same physical size also exists and has caused decades of confusion about which position takes what. We have not verified the factory rating for each position, so fit what the diagram for your chassis number calls for rather than the largest that will go in. Test each fuse with a meter — glass fuses fail invisibly at the cap.
- Are the factory diagrams reliable?
- Mostly, with a known caveat: the early TC print has been discussed for years in club circles as containing errors, and the clubs have published corrected redraws. Part numbers in secondary sources conflict too — the TC dynamo, for example, is cited variously in different documents. Where a detail matters, go back to an original document or ask the register rather than trusting a summary.