Nov 23, 2004
I really love the look and the sound of a Norton...... But this electrical system is driving me NUTS...While waiting to put the head on, I figured I would try to get the electrical system checked out.... What is with this positive ground stuff! Every time I touch something one compnent starts working and another component stops working. I have only worked on negative ground American and Japanese systems. I have never been this frustrated before. The taillight has two leads and NO GOUND.....How can this work? You must have three wires, one for the taillight, one for the stop light, and a ground...Please help! Before I dismantle it and put it on EBAY.......P.S........Yes I do have the service manual with the wiring diagram.
Sorry for venting.... :cry:
Why positive ground? Why ask why? That's just the way it is.

If there's no ground wire then the light would be grounded through the fender to the frame. That of course means you need a nice clean metal-metal contact at each mounting point all the way back. Any rust or corrosion or paint or whatever will result in sub-optimal lighting ;)

I can relate to your frustration, more than once I've threatened mine with being parted out on ebay. The bike just laughed at me though. It knew as well as I did that I wasn't serious :lol:

good luck,
Positive vs negative has no effect other than which way the current flows.
Nortons do ground through the frame and this where most problems lay.
There are two grounds, the motor and the frame. Since the motor is rubber mounted it is isolated from the frame.
The frame ground is usually behind the zener diode on the right Z plate. Check it out.
Hey Debby,
I kind of agree with your diagnosis....except one thing, the rear taillight assembly is plastic and the housing that attaches to chrome fender is there is no ground, and I can't find any kind of ground wire. Also with just the two tail light wires attached (brown & brown/green) and the taillight assembly isolated from any metal(ground) I can activate the brake light, but the tail light will not operate. That's what's confusing me....Is it safe to assume the either the brown or the brown/green wire has shorted to ground somewhere completing the circuit?
Try running a wire directly back to the battery positive from the light unit bulb holder as a test. The brake light circuit seems to be using the tail light circuit as the ground? But it may not be shorted.
The housing (or fairing as it's sometimes called) is fibreglass but the license plate bracket is metal, and it bolts directly to the fender IIRC. That's probably your ground path. I'm pretty sure there's no ground wire. I'd go look at mine but it's buried in a box somewhere.

As for the wiring, you should find a brown (brake light), brown/green (taillight), green/red (left blinker) and green/white (right blinker). This, of course, assumes you have a standard wiring harness and that some PO hasn't done a rewire job.

Triple John, I think the original tail-light/indicator/number plate bracket that goes from the light assembly to the rear mudguard was steel. However I generally run a seperate earth wire from an unpainted part of the frame to the rear light as I find it 'efficacious' to rubber mount my rear mudguards. It is important to mount any earth wires on a part of the frame which has had the paint scraped off or else the paint itself acts as a very effective insulator.
My late type 850 harness includes a red wire to the tail light.
tail light ground wire

LAB is correct. There is a red wire in the harness going to the tail light. Also should be a brown (brake light) and brown with green (running light). There should be a red wire with eyelet on one end and bullet on the other. The eyelet is attached to the right bolt holding the tailight assembly to the tailight fairing. The bullet plugs into the red lead in the harness. Electrical problems on Nortons are ususally a result of hamfisted previous owners.

Oh and they also shift on the right with an upside down pattern.
The taillight fixture should be mounted in a rubber grommet, otherwise the vibration will destroy the bulb filament in short order. So, a ground wire is required. The ground wire is red and is bundled with the two brown wires that run along side of the right-hand seat/fender rail to the taillight assembly.

At least this is the way it works on my MK III. And I think my 69 S-type was the same way. Sadly, I no longer have the S-type, so I can’t say for sure about how it's wired.

Keep looking.

OK...Now I'm getting somewhere......Dave you are correct I looked through the box of parts I was given with the bike, and the parts diagram...and you are correct, there is a metal plate! that must be the earth path! but since mine is a 70 there are no blinkers on it...I will try to get it together later this week....I will keep everyone informed...As usual THANK YOU for everyone's help and encouragement! ALSO I did get the cylinder head back from the disintegrating place on Friday....And that was the best $30.00 I ever Spent! the spark plug was removed and the threads are in perfect shape! I would highly recommend this place! :D
Well I stepped in it this time and want to apologize for the faulty info I gave on grounding through the frame on the tailight.
Everyones comments made me go and check and sure enough there is a dedicated ground running to the rear. Then grounds through the taillight assembly. Sorry, sorry sorry.

John, congradulations on fixing your head.
Hey Micheal,

Where is this ground wire located? all I have coming off the wiring harness are the brown & brown green wires? Is the red at the base if the harness? Is it safe to assume I can use any red ground wire?
Is it safe to assume I can use any red ground wire?

Should be, provided the harness hasn't been hacked about or modified?

Possibly the early models didn't have the red wire to the tail light unit but probably a good idea to fit one?
John, the two bikes I checked were a 72 and 73. Both have 5 wires running to the rear, one being a red ground. The difference being the 73 has a 3 wire tailight with internal ground, the 72 has a 2 wire tailight with an external ground as Illf8ed explained.

Someone with a 70 is going to have to explain how it is done originally.

The 72 has a jumper wire running from the right side mounting screw to the harness. If you can't find a ground in the harness, I would run one from the right rear screw to some place appropriate.

Yes, RED is always ground. And repeat, red is ground, red is ground ...

Good luck!!
I have an early 750 (1970)with ammeter and no indicators, there is no seperate earth wire in the wiring loom to the rear light and I had to add one from the battery and enclose it and that section of loom in shrink-fit before it would work. I also found it beneficial to run an extra earth wire to the headlight which doesn't always seem to earth through the rim. and an additional earth wire from the frame to the well isolated (and insulated) engine.

Ah ! Lucas electrickery,
OK this is not for the 'purist' but best to junk all those bullet connectors etc. and solder the joints. run independent earth wires instead of relying on the tenuous electrical connection between cycle parts and battery + terminal and use relays for the heavy duty stuff like lights and horn (you can then fuse these seperately).
colour coding is fairly straight forward: brown/blue - main supply from battery/genny, Blue with tracer colur - lighting, white - ignition on cct. , green with tracer colour - indicators, red - earth.... etc.