Wiring Question: Mark lll

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chasesa

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Just re-assembling the Mark lll & I'm wondering where a wire is supposed to mount.

It's 8" long, red wire with black sheath with a flat round fitting.

It emerges from the main harness just ahead of the down tubes close to the airbox.

Seems to me it must be a ground wire that should mount to the bolt that holds the front tab of the oil tank. is that right??
Thanks,
Stephen
 
chasesa said:
It's 8" long, red wire with black sheath with a flat round fitting.

It emerges from the main harness just ahead of the down tubes close to the airbox.
Click to expand...


There's no ground wire emerging from my Mk3's harness in that area.
 
If the harness is original, it is a ground wire. However, you need to ring it out to see to where the other end is attached. One quick test is to check continuity from the free end to the frame. If you find continuity, than it is NOT ment to be attached to the frame, but to some component. If you read a few ohms or more, than it's likely to be intended as a frame connection. The oil tank tab, however, is a high stress point that often fractures. Any other frame point will do, as long as it's free of paint. My common mid-bike grounding point is at one of the two holes in the web between the down tubes just above the air cleaners.

Stephen, are you sure you want to reinstall a 40-year-old harness on a rebuilt bike?
 
I'll yield to Les on this one. To the best of my knowledge, the only red wires in the original MK3 harness were ground wires. However, the first item I dumped when I prepared my Mk3 for restoration was the old harness, so I can't run out to the shop and take a look. There's always a chance that the wire is from an after-market add-on.
 
The harness is original.

With the wire only so long & there aren't many mounting point within its reach.

If I knew how to post a photo it would be easier. I shall research picture posting (then, having learned something today, I can retire).

Cheers!
Stephen
 
Where does the round end attach?

Wiring Question: Mark lll
 
That may be the ground wire that attaches to one of the cylinder head to headsteady bolts.
 
Apparently the ground wire ring is supposed to fit between the R/H headsteady plate and the rear spacer:

Wiring Question: Mark lll
 
Thanks for your guidance. That's what makes our site so good.

Evidently, I had tucked the wire through the grommet to put it out of the way. Then forgot....(second thing to go!)

Plus, I learned how to post photos.
Cheers!
Stephen
 
If that wire is indeed the ground strap for the engine, than it is adequate to return the ignition current, but is way undersized if it is to return the starter current, which can instantaneously reach 300 amps when turning over a cold engine. If you intend to use the old harness and want to use the electric start, then you can do yourself a favor now by running a second return wire, 8 gauge or larger, from the engine directly to the battery ground. Likewise from the starter relay to the starter terminal, and from the battery -12v terminal to the starter relay. These cables can be run in parallel to the harness wires, so there is no need to dig into the harness. Dave Comeau has a good discussion on starter wiring (as well as a replacement starter motor) on his Atlantic Green website. This cable change is the most cost-effective upgrade you can make on a Mk3. Damn, now you've got those "new project" juices flowing in me.
 
I agree the original starter wires need uprating, and I also agree that no secondary (harness) ground wire should be connected directly to the battery positive terminal if the electric starter is operational.
With no secondary ground connection to the battery the head steady ground wire will provide a connection from the harness to the engine-which is necessary in order for all other circuits to have a return path to the battery.
 
Sorry boys. It's not the starter wire.

And, my starter wires are 8 gauge for the upgraded starter.
SC
 
But you still should take L.A.B.'s advice and make sure that the only red "earth" (ground) wire running from the positive side of the battery is the 8 gauge wire running to a bolt on the engine. When I got my Mk3 it had two earth wires running to the positive terminal of the battery.
 
I really don't understand this focus on a single return to the battery. Both the starter and the spark plugs require a direct return from the engine to the battery; the frame is an unreliable partner. Lighting and other ancillary circuits also require returns to the battery. The most reliable way to do this in a dc circuit is through a multi-point ground system where the frame is redundant to the ground wires. Single-point grounds are only important at high frequencies, where the magnetic dipoles created by current loops couple to one another. This effect is absent at dc.
 
rick in seattle said:
I really don't understand this focus on a single return to the battery.
Click to expand...

The additional positive wire connection to the battery included in the Mk3 harness (and wiring diagram) was a serious factory electrical error.

If an additional light gauge harness ground wire is connected to the battery and for whatever reason the heavy gauge positive starter wire goes high resistance (because of corrosion or a connection vibrates loose or the owner simply forgets to reconnect it) when the starter is operated up to 300 amps of UNfused starter current will attempt to return to the battery through the light gauge harness ground wire- result = smoke and/or flames!
Electric start Triumph T160 Tridents were also wired the same, and Triumph (NVT) soon issued a service bulletin to their dealers instructing them to disconnect the harness ground wire from the battery, cut off the ring terminal and tape the wire back on every T160 they happened to encounter. I don't know if a similar service bulletin was ever issued for the Mk3 Commando, but the same thing still applies. So for the Mk3, to avoid setting fire to the wiring harness when operating the starter, all ground returns need to go to the battery via the engine and heavy starter positive cable.
 
Ron L said:
But you still should take L.A.B.'s advice and make sure that the only red "earth" (ground) wire running from the positive side of the battery is the 8 gauge wire running to a bolt on the engine. When I got my Mk3 it had two earth wires running to the positive terminal of the battery.
Click to expand...

Hi Ron,
I always take L.A.B.'s advice seriously.

But, the wire loom is stock, original, and the wire was attached to some point within its 8" last fall. My failing is not recalling where. So, it has to attach to something & the head steady bracket fits the bill.
 
Why not take that long red wire and lay it under the long head steady side plate bolts and put the ring terminal on middle allen head bolt that hold down the head steady bracket? That's a direct ground connection to the motor. The factory mounted it on a weak ground location if you ask me ?


Tim_S
 
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