Where'd my headlight go?

Not the first startup of the season but close to it. Anyway, giving it the pre-ride check through and no headlight - neither high beam nor low. I've got 13.12V at the connector to the H4 bulb, the high beam indicator on the shell lights up when high beam selected, and I've tried several bulbs, including a few brand new ones. No joy.

Worked fine last time out (few weeks ago) and has simply sat (and sumped....) in the meantime.

This is a new one on me.

Perhaps the selector switch on the lamp shell has a problem like a wire disconnected or might have corroded internally on contacts?

The one for the parking light or whatever it is (was)? That's disconnected and has been since I did theH4 conversion many moons ago.

My '74 850 has a three way toggle switch on the shell. I've wired it to only toggle the headlamp on/off but I think stock it is supposed to allow pilot/parking lamp on/off and headlamp on/off.
If you've bypassed this, then you need to test the wiring leading into the headlamp bulb and trace them back to all their connections, including at the ign switch. My headlamp only gets powered if the ign switch is turned fully clockwise (2nd position). So you could have a fault in the switch....original lucas ign switches are a common failure point.

BrianK said:

I've got 13.12V at the connector to the H4 bulb,

This is a new one on me.

Click to expand...

Continuity to ground from the connector?

BrianK said:

Anyway, giving it the pre-ride check through and no headlight - neither high beam nor low. I've got 13.12V at the connector to the H4 bulb, the high beam indicator on the shell lights up when high beam selected,

Click to expand...

Power must be reaching and the handlebar selector switch if the high beam indicator works as it uses the same blue/white feed as the headlamp bulb high beam filament. Neither high nor low beam working points to a bad bulb holder ground or holder to ground (harness red, not the shell connector).

Tornado said:

Perhaps the selector switch on the lamp shell has a problem like a wire disconnected or might have corroded internally on contacts?

Click to expand...

BrianK said:

The one for the parking light or whatever it is (was)? That's disconnected and has been since I did theH4 conversion many moons ago.

Click to expand...

Tornado said:

My '74 850 has a three way toggle switch on the shell. I've wired it to only toggle the headlamp on/off but I think stock it is supposed to allow pilot/parking lamp on/off and headlamp on/off.

Click to expand...

Pre-'71 had 'Parking lights' switched by the three-position headlamp switch.
1971-on parking lights were switched by the four-position ignition switch.



The '71-'74 two-position headlamp switch turned off the headlamp when the ignition switch was turned to the 'Ignition and Lights' position.

It's working! Appears to have been the fuse to the relay that operates the headlight had squirmed its way a bit loose. EIther that or some other wire I jiggled did the trick. Thanks all - BrianK

"Lucas Prince of Darkness" LOL