I believe you've lost the Return ("ground") for the lamp cluster. Imagine the sewer line in your front yard gets clogged or blocked. In that situation water from the sink will eventually "backup" into the bath tub.

The same thing happens with electricity. Without a way for the power (probably from the speedo unit dial lamps) to return, the power is finding another way to "return" to the battery through the other lamps. That makes them light up as if those circuits were faulty. But they are just fine.

If you'll look around inside the headlamp, I bet you forgot to connect a Brown wire to the body of the speedo instrument that supplies the Return function. OR, the unit was previously getting this function through (say for instance) the old drive cable, but the new one isn't doing it. So you need to run a NEW wire to fulfill this function.

Hope this helps.

Thanks I definitely did not connect a brown to it, there wasn't one on the existing speedo, but that was probably my fault form the rebuild, 13 yrs ago.
BTW are you a Manxman or just like the flag LOL.
Michael

BTW are you a Manxman or just like the flag LOL.
Michael

I simply love the island and the TT Races.

On the /5 speedo/tach, there are 2 different bulb holders. The illumination bulbs are fitted in to holders with brass bases and are grounded to the instrument housing. The indicator bulbs (idiot lights) are fitted in plastic bases and have 2 wires attached. These circuits are activated at the end effector that they are supposed to display. The only lights in the speedo/tach that share a ground are the bulbs for the back lighting. Example; The neutral light switch grounds inside the transmission. (this assumes the bike has the original type 4speed transmission, not a later 5 speed) There is a nub on the shift mechanism that contacts a tang inside when the mechanism is in the neutral position and provides a path to ground to light up the neutral indicator light. In original configuration, all the indicator lights have their own circuits, but the function is achieved in slightly different ways. Note; the charging light is essential to the function of the circuit and the methodology to the manner in which that light works is a bit more sophisticated than a simple switching action like the neutral light. In reference to your specific indicators (neutral and oil pressure) Neutral is normally open and closes when the transmission finds neutral, turning on the light. Oil pressure is normally closed to illuminate the light when not running, and goes open when the oil pressure from the pump exceeds a minimum value and the light goes out.

Thanks James,
I’ve checked all the bulb connections and they are as you stated.
For the neutral I’m suspecting a broken wire shorting to ground or the connection at the gearbox grounding all the time. For the oil pressure, I’m going to try a new switch.
Michael