| Diagram | Fig. 31 — IGNITION CAPACITOR AND T100C HEADLAMP · ref 22 |
| Subsystem | electrical |
| Listed in parts books | 1968, 1970, 1973–1974 |
Lighting switch, type 57SA
Lucas type 57SA lighting switch (PN 35710) fitted in the headlamp shell on a range of unit Triumphs, later superseded by 31788.
This is the Lucas type 57SA lighting switch, PN 35710, the unit that lives up in the headlamp shell on a lot of unit-construction Triumphs. Catalog appearances put it in headlamp and wiring harness figures spanning 1968 through 1974, including T100C, TR6C, TR6R, and T120R listings, so it shows up across a good spread of those models in that window.
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It's the switch that controls the lighting circuit from inside the headlamp bucket, working alongside the other switches and the harness back to the tail lamp. If you're chasing electrical gremlins or a no-lights complaint, this is one of the first things to check for worn contacts or a cracked housing.
Our records show this part was later superseded by 31788, so if you're working from a newer parts book or sourcing a replacement, that's the number you may run into instead. Bring in your old part or the number off your parts list and we'll match it up.
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