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post #61 of 63 (permalink) Old 11-12-2012, 05:18 PM
Main Motorcycle: 1978 Bonneville T140V
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Your good grounding of your SPG to the frame doesn't guarantee the return from engine to coil (if the engine/gearbox/plates are all bolted to a painted frame, then the engine etc is electrically isolated from the frame)
Good point koan58,

My '78 has a ground wire atop the head steady that disappers into the wiring harness.

When the zener diode was re-located due to a new side cover, the ground lead remained at the fastening lug.

Wiring diagram doesn't show the wire at the engine, so assumed it ran back to the zener?

Just ran a daisy chain wire back to the SPG.

Last edited by Morris the Cat; 11-12-2012 at 05:36 PM.
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post #62 of 63 (permalink) Old 11-13-2012, 09:20 PM
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Originally Posted by koan58 View Post
I stand to be corrected, but there seems to be one circuit which is neglected, which is the HT spark, which goes from coil to plug to engine, then back to coil. Your good grounding of your SPG to the frame doesn't guarantee the return from engine to coil (if the engine/gearbox/plates are all bolted to a painted frame, then the engine etc is electrically isolated from the frame). In practice I'd expect contact to break through the paint at some bolting point, but this is relying on luck rather than judgement.
Dave -
Thanks for the input. Your point is well stated, but overlooks some important facts. And therein lies the answer, and more importantly a learning opportunity.

You are very correct. The spark (HT) to the spark plugs is just like every other circuit on the bike. It needs a complete path back to the starting point. That is to say, it needs a complete path back to the ignition coil. Few people realize that.

However, the electrical path back to the coil is NOT through the frame and back to the metallic body of the coil, as one might think at first glance. If it were true, then the rubber mounted coils on the OIF bikes wouldn't work at all. Believe it or not, the electrical path back to the coil is through the DC wiring harness.

That's all determined by how the HT coil is wired internally. You see the secondary return lead is not tied to the coil's body, but rather to one of the low voltage primary terminals. So the coil's "grounding" is actually done by the harness.

Therefore the proposed lead from the cylinder head back to the SPC does all the "grounding" the ignition system needs. So forget about rusty, greasy, or painted engine mounting plates. They are not doing anything electrical.

Originally Posted by koan58 View Post
To be sure, I would preferably put the SPG on an engine plate/head steady etc, or run a lead to such a place.
I don't see the point in scraping the paint off the frame for your SPG, as the frame is no longer doing any electrical function, your red leads are now doing what the frame was originally expected to do.
For an education, please take a look at the mounting location for the rectifier on any 1963 to 1970 Triumph twin. The rectifier is the SPC on a stock Lucas system for those years. We can see that because all the red "ground" wires meet there. Now look closely at the rectifier mounting point.... the battery box. And the battery box is rubber mounted. The battery box is actually electrically insulated from the frame !! So the whole point of SPC is NOT to tie "ground" wires to the frame. That thinking went out in the 1940's. The object of SPC is to get all the "ground" wires gathered together in one place, whether it touches the frame or not. At that point you no longer have "ground" wires; the more appropriate term is "return" wires.

Originally Posted by koan58 View Post
IMHO you don't need so many separate leads either, one unbroken lead running the length of the bike, with spurs where necessary, would do a neater, sturdier job.
That's a matter of personal interpretation. Sure thing, one big wire will do it. But let me tell you why you might be better off to do it the way I advise. Using one big wire is "placing all your chickens in one basket". It's sort of like having your family all work for the same company. If there's a layoff, then you're all out of work. On the other hand, if you use multiple wires and one wire breaks, then you may loose your turn signals or your lights, but hopefully you can still ride home.

Another valid reason is this... In order to tap into that main wire you'll have to solder. (You wouldn't dare use connectors because connectors become unplugged.) Soldering hardens the copper making it brittle. So when you tap into this main "return" wire 4 to 8 times, that's 4 to 8 places for the vibration of a Brit bike to break the wire.

As always, you are very welcome to execute your wiring as you see best. Every owner should. But I sincerely feel like multiple wires reduces the risk of walking home. My motto on Brit bikes has always been: "Ride bikes. Pick up chicks. Get laid." Personally, I have always felt that walking home was NOT conducive to the 3rd point.

All the best.

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Last edited by GABMA; 11-13-2012 at 09:53 PM.
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post #63 of 63 (permalink) Old 11-16-2012, 07:03 PM
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Thanks for your opinion, observations and knowledge.
I appreciate education, when it derives from a knowledgable master.

But the point I was making was that if the engine/gearbox is isolated from the frame by paint, the ignition system requires an independent, dedicated return from the engine to the coil, through the SPG cabling system.

Morris's system appears to have that, tho the post I was commenting on was an earlier one by kev, which did not appear to have.

I thank you for reminding me how coils work, however if the common primary/secondary is not in some way connected to the engine, it has no way for the spark to return to the coil. If the grounding is in the harness, correctly as you say, then the harnesss must have a connection to the engine.

Rubber mounting of coils/battery has nothing to do with it,you are just confusing issues or confused yourself, and I would not take Joe Lucas's examples in the 1960's as necessarily the way to do things.

You cannot have read my post thoroughly, as I stated that the frame no longer did the work, with the red wires doing it instead, so it being pointless to ground to frame the SPG, unless hoping to luck for the connection to the engine for HT purposes, with my subsequent suggestion to run an SPG wire to the engine.

GABMA can you read AND understand?

Your comment re soldering is valid, if you run solder freely into a connector, it will permeate along the wire and make it prone to fracture. My suggestion was to use a dab of solder. Perhaps I should inlude a whole workshop manual for your benefit each time I post, but what I intended was a little soldered union at the very end of the wire to guarantee electrical continuity with the terminal forever.

What's yer problem?

You can use one unbroken lead to loop to all the vital places, there's only a small number of them. Hardly need separate leads for idiot lights etc. Why not doublebank and use frame connections as well? or is it a matter of pride and prejudice over practical reality?

The bane of frame earthing is corrosion/breaking of the connectors to the frame, SPG and a few frame earths too has to be the way to go, belt and braces!

Gabma do you have to talk down?

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