Interior light wiring: early and late cars

Sent round by Graeme Oxley from Rob Elliott, who sorted his own car's lights with it. The write-up first appeared on the Georgia Triumph Association's website; its author drew on the factory workshop manual and the Stag Owners Club's technical pages. Summarised here.

The interior lights have caused confusion for years, not helped by Triumph never publishing an official wiring diagram for later cars. Mk1s have a lamp on each B post; Mk2s have one lamp in the middle of the T-bar with two bulbs inside. Less well known is that there are at least two wiring systems that work in completely different ways, and the change didn't line up exactly with Mk1 and Mk2.

The switch is wired with individual wires, not a plug, so if they get muddled you can end up with all sorts of odd combinations.

Which do you have?

• Door switches: early cars have twin-terminal door switches; late cars have a single terminal.
• Back of the interior light switch: early cars have two purple wires at the front; late cars have two black wires joined together.

Early type: the door switch switches the feed

Mk1s with B-post lamps, and early Mk2s with the T-bar lamp and twin-terminal door switches.

• The feed comes in on purple wires to the door switches (38 and 43). Opening a door closes its switch.
• From there purple/slate or purple/red wires feed the puddle lamps (39 and 44) and terminals 1 and 4 of the interior light switch.
• Switch off: terminals 1 and 4 connect to 2 and 5, which feed the B-post lamps (41, 45) and console lamps (42, 46). So opening a door lights everything on that side.
• Switch on: a separate feed on terminals 3 and 6 connects to 2 and 5, lighting the B-post and console lamps but not the puddle lamps.

Late type: the door switch switches the earth

Mk2s with the T-bar lamp and single-terminal door switches.

This diagram, from the Stag Owners Club's technical reprints, has one mistake on every late car the author checked: it shows a purple feed to the two roof bulbs. In practice Triumph used a single black wire as the feed to both roof bulbs, which is unusual, as black is almost always an earth. Don't assume a black wire in the T-bar or B-post loom is an earth.

• The black feed runs up the right-hand B post to one side of both roof bulbs. Purple/white and purple/orange wires come back down from the other side of the bulbs to terminals 3 and 4 of the switch.
• Purple wires feed the two footwell lamps, whose other side also joins the purple/white and purple/orange wires to terminals 3 and 4.
• Terminals 1 and 2 are joined and go to earth on a (genuinely earth) black wire.
• Switch on: 1 and 2 connect to 3 and 4, earthing them, so all four bulbs light. The puddle lamps don't.
• Switch off: 3 connects to 7 and 4 to 8. Opening a door earths its door switch, which earths 7 or 8, so the roof bulb and footwell lamp on that side light. The door (puddle) lamps have their own purple feeds and are earthed by the same door switch.

Know better, or found your car wired differently? Tell us in Tech Q&A.