Wiring issue

I was installing new turn signals on my 1974 commando and now I have a tail light/brake light issue...when turning switch(not original) to the start position, I have brake light,although very dim..when I turn switch to the headlight/taillight on position, nothing lights...flopped wires and still same issue...I read a pervious post about a grounding issue and started there..the tail light is only 2 wire and must be grounded somewhere internally si I hooked an alligator clip to the metal surrounding the outside of the socket..not the socket itself but the metal plate that touches the rubber gasket...hooked the other end to ground on the frame and voila! It works..my question is how can I make this permanent? I cant have an alligator clip hanging off my bike and it wouldn't work since I'm connecting it inside the lens cover and that has to be put back on....how would it ever be grounded internally since the majority of the body of it is plastic and doesn't touch anything metal

Your '74 tail light should be a three-wire L917, therefore should have a (red) earth/ground wire connected between the bulb-holder and harness red as the bulb does not ground through the bulb holder bracket.

L.A.B. said:

Your '74 tail light should be a three-wire L917, therefore should have a (red) earth/ground wire connected between the bulb-holder and harness red as the bulb does not ground through the bulb holder bracket.

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I guess my tail light is not original then..its the bee hive style..maybe 72 or earlier? Either way..only 2 wire..black and red..bulb holder doesn't seem accessible from front or back...

rodngonzo said:

I guess my tail light is not original then..its the bullet style..maybe 72 or 73? Either way..only 2 wire..black and red..bulb holder doesn't seem accessible from front or back...

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That is the tail lamp used before 1973 which is two-wire and should ground through the mounts so you need to add a ground connection (ring terminal?) to one of the lamp mounts.

The mounts are on the inside aren't they? Sorry to be such a noob...

rodngonzo said:

The mounts are on the inside aren't they? Sorry to be such a noob...

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Is the L679 fairing fitted?


According to the parts diagram (as I don't have a 679 tail lamp) the "Bolt Tail Lamp Fixing" (item 2x 30) should be accessible from beneath the fairing and fix the lamp assembly from the front.

I figured it out..all is well now!! Thanks for the help..much appreciated...now to figure out where the crankcase drain is so I get get the oil back into the tank..it was recently changed...what a pain in the off season having to drain and re fill.....

rodngonzo said:

now to figure out where the crankcase drain is so I get get the oil back into the tank.

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Sump plug, item 31.

So, was that style light only meant to be a brake light only or is there just a problem with the grounding that I just worked around with a wire from mounting bolt to frame? Just trying to increase my knowledge..I imagine at some point there was an extra wire ran from hot towards the back because normally there would only be 2 connections..1 to the brake light switches (hot) and one to ground, correct?

rodngonzo said:

So, was that style light only meant to be a brake light only

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No, the two wires are for a tail light and a brake light as the bulb should have a double filament, both need a ground, which, on that type of tail light assembly is through the mounting and you should see that if you remove the lens.
The fact that the wires are a red and a black and not brown/green (tail) and brown (brake) indicates that it's a replica tail lamp assembly and not original Lucas.

Makes sense.....thanks!! On another quick question...the left side panel is held on by a weird connector at the front with a wire and a little twistable connector that is almost impossible to turn due to the fact that the mikuni carb fitted is in the way... is there a work around for this weird little device?

rodngonzo said:

.the left side panel is held on by a weird connector at the front with a wire and a little twistable connector

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It's what's known as a Dzus fastener.
Not all that weird as they've been around for years but more commonly used on aircraft and in motorsport for securing panels, fairings. etc.

There are various different types of Dzus fastener.

Yeah, seems to work well..just can't easily turn the other side as it sits right against the rear of the carb....

My Commando isn't fitted with a Mikuni and it's an 850 Mk3 which has the Dzus in a different location so I can't offer you any advice but I expect some of our other forum members can.

It is a PITA. But you'll get used to doing.it with two fingers. Key is aligning the slot with the wire before you try to insert the fastener thru the side cover arm and using the other hand to keep pressure on the arm.