Every MG T-Type is positive earth, and every one was wired by Lucas. Both facts shape what you can safely add to the car and how you track down an electrical fault.
What positive earth means
On a positive earth car the positive battery terminal is connected to the chassis, and the negative terminal feeds the loom. In the United States this is called positive ground. The chassis is half of every circuit, so a painted or corroded earth face drops voltage just as surely as a broken wire.
Polarity matters in three places on a standard car: the coil, whose SW/+ terminal takes the switched feed and whose CB/− terminal goes to the distributor, since a coil fitted the wrong way round still sparks but weakly; the dynamo, which must be repolarised after any test or rebuild; and anything modern you add.
Which T-Types, which voltage
| Model | System | Battery |
|---|---|---|
| TA and TB | Positive earth | Two 6-volt batteries |
| TC, TD and TF | 12-volt positive earth | Single 12-volt battery |
All TD and TF cars charge through a Lucas dynamo controlled by a bulkhead control box; there was no alternator from the factory. The TC, TD and TF wiring diagrams show the full circuits.
Adding modern parts
Electronic ignition. Any electronic ignition module, tacho pickup or solid-state accessory must be a positive earth part, or be wired through the maker's own positive earth kit. A negative earth module fitted without conversion is destroyed the first time it is switched on.
Radios, USB chargers and LED lamps. These are almost all negative earth. Either isolate them fully from the chassis or fit an isolated DC-DC converter.
Control box. The original mechanical control box works on either polarity, but a modern solid-state replacement does not, so order the positive earth version. Ordering the wrong polarity is the most common mistake. See the Lucas control box guide.
Alternator. The original dynamo is easily overwhelmed by halogen lamps, an electric fan and a modern radio. A small alternator in a dynamo-shaped case is the usual answer; the modern upgrades guide covers it. Confirm with the supplier that the unit suits a positive earth car.
Converting to negative earth. Do not do it casually. The dynamo, control box, ammeter and any surviving original electrics all assume positive earth.
The Lucas reputation
Owners of old British cars sometimes call Lucas “the Prince of Darkness”. On a T-Type the joke is usually aimed at the wrong target. These cars are seventy to ninety years old, and most have had at least one replacement harness, several owners' repairs and a few extra wires for indicators, a fan or a radio. A high-resistance earth or a tired connection can mimic a failed component, such as a dynamo that seems not to charge. Clean earths and sound connections cure a large share of Lucas faults before any part needs replacing.
Treat the wire colours as a starting point and prove each wire with a meter; the Lucas loom colour codes explain the system, and Lucas dynamo not charging walks through a charging fault.
Common questions
- What is positive earth?
- On a positive earth car the positive battery terminal is connected to the chassis and the negative terminal feeds the wiring loom. American owners usually call it positive ground. Most modern cars are the other way round.
- Are MG T-Types positive earth?
- Yes. The TC, TD and TF are 12-volt positive earth with a single 12-volt battery. The earlier TA and TB are also positive earth but use two 6-volt batteries.
- Can I fit a modern radio or USB charger to a positive earth MG?
- Yes, with care. Modern radios, USB chargers and LED lamps are almost all negative earth, so either isolate them fully from the chassis or fit an isolated DC-DC converter.
- Should I convert my MG T-Type to negative earth?
- Not casually. The dynamo, control box, ammeter and any surviving original electrics all assume positive earth, so a conversion touches the whole system.