| Diagram | Fig. 33 — HEADLAMP, WIRING HARNESS, SWITCHES AND TAIL LAMP · ref 52 |
| Subsystem | electrical |
| Listed in parts books | 1966–1968, 1970, 1973–1974 |
Ignition switch, type S45 (99-0558)
Lucas type S45 ignition switch, the main on/off switch for the ignition circuit, factory part 31899.
This is the Lucas ignition switch, catalog type S45, listed under part number 31899. It shows up in the wiring harness and switch panel figures alongside the headlamp and tail lamp circuits, so it lives up front near the headlamp shell where the rider can reach it to turn the electrics on and off. On these bikes it's the key point of control for the whole ignition circuit — no juice gets to the coil or the rest of the harness until this switch is on.
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Our records show this number appearing across catalog listings from 1966 through 1974, including entries for the TR6R and T120R. If your parts book lists 31899 or the older 99-0558 reference, you're looking at the same physical switch.
Heads up that this part number has been superseded by 30608 in later factory numbering. If you're ordering a replacement today, that's the number you'll likely be quoted, and it's worth checking that it matches up with your existing switch and harness connections before you install it.
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