Electrical Meltdown 1974 R75/6

[spencer_p]

Had an exciting ride last Friday on my 1974 R75/6. Bike started smoking, and when I pulled over, flames (small flames) were issuing from beneath the tank. No worries. I’m a firefighter and quickly fixed that. But I’m not an electrical engineer so I can’t quite be sure why it happened.

From what I could tell, the green wire on the left coil was at fault. It burned hottest down by the coil. It also burned up in the headlight bucket where it meets the key. Interestingly, BMW did not design a fuse into the wiring between these components.

I’ve ordered a new wiring harness, but I’d feel a lot better if I could pinpoint the source of the problem. Maybe it’s a simple as old, cracked wires? Any ideas?

Most of my electrical components are new, and I have an EME electronic ignition, though I’m not sure that would matter in this case.

TIA!

 

[olironrider]

Seeing that no one has “stepped-up” on your “melt-down”, here’s my thoughts:

Confirm that your 1974 R75/6 has the five(5) position key switch ?
That it also has the Slash 5 style “thumb” switches for headlight, horn, turns and starter ?
And, a mechanical tachometer ?

I looked at your first image of the left hand coil; right off, the 1.5 green wire, 12VDC feed to the coil terminal 15,
has aftermarket shrink tubing ! Secondly, I see a newer style 1.5 wire, appears to be clear spade connector, on the lower
terminals of the left coil ? That wire connects terminal 1 of the left hand coil to terminal 15 of the right hand coil.

Unseen, there should be a black wire from the right hand coil terminal 1, going to the points area under the front engine cover.

What really caught my attention was: the melted wire, co-joined at terminal 15 of the left hand coil with the unfused green wire, created a direct short ! It should not have been there !

Switched 12VDC flows to: left hand coil terminal 15; left hand coil terminal 1; right hand coil terminal 15; right hand terminal 1; to capacitor and breaker points; ground.

The direct short at the left hand terminal 15; traveled forward through the main wiring bundle; being in very close proximity with the five(5) wires tight to the five(5) position switch; melting “Snake’s Nest” of wires therein !

To compound your misery, and I apologize, the 1974 only, main wiring loom has been no longer available from BMW for decades.

I believe the 1975 on loom can be fitted but needs modifications… not for the faint of heart.

D.

 

[stevenrankin]

EME, has a fellow in Germany making replacement wiring harnesses for most bikes. They are very good but because it is a one man show making them, availability can be an issue.
That aside, after you rewire things with the new harness you installed, VERY carefully take a look at the EME electronic ignition system's wiring diagram as well as the wiring diagram for your bike.
I believe like olironrider, D, is correct in asserting that something was wired incorrectly causing a short. As I don't know what EME system you used, I am afraid I can't point out a place to look in the system for a short. Some of the ignition systems on the market require a bit of "modification" to the stock wiring. I have found almost any time stock wiring is "modified", there is an induced weakness input. Modifying stock wiring without a good idea as to what does what can lead to problems. LOL, sometimes the company making the part has less than helpful instructions for installation.
LOL, now so as not to totally throw rocks at you for that possible mistake, also look to see that wires weren't rubbing on the tank or someplace else.

Sorry I can't be more help. Good luck and good hunting. St.

 

[olironrider]

… a few more thoughts and, a couple of pictures.

I also noted in your first image, a new, not OEM, brown wire, signifying ground connection at the left hand coil bracket;
co-joined with the master chassis ground bundle ?

These two(2) images are of a 1974 R 90S I’m restoring;



The area circled in RED, is the short connector from left hand coil T 1 to right hand coil T 15. The black wire, near the lower portion of the red circle, is the wire going forward to the capacitor, breaker point, chassis ground.

It would be interesting to know the color and, where the melted wire co-joined with the green wire on the left hand coil T 15, went ?

 

[spencer_p]

Hi all, thanks for the detailed and thoughtful replies. Let me try to answer questions and add more details…

- Yes, my bike has a 5-position key, old-style combination thumb switches, and a mechanical tach.

- The green wire co-joined at terminal 15 of LH coil was wired to the electronic ignition module.

- The new brown ground wire on LH coil bracket also ties in with electronic ignition.

- My short connector is going from LH coil T1 to RH coil T15.

More photos attached. Also, I have ridden this upgraded ignition and coils for several months so it seems a bit odd they’d go so far before failure, if they were indeed wired wrong. Thanks again!

 

[spencer_p]

Oh one more thing. Here is the underside of the tank. It got a little hot and smoky but it doesn’t seem to me like this was the origin of the short.

 

[AntonLargiader]

It looks like something (possibly the choke cable) shorted the upper LH coil terminal to the case. Your later set of pics look like it could have shorted the lower right terminal, but the first set shows something at the upper terminal that I can't make out clearly. That would be a dead short to ground.

 

[spencer_p]

Hey Anton, that’s a good point. Of course I didn’t see the ignition, but when I stopped the choke cable was clearly on fire, so if it wasn’t the source of the short, it certainly was very close to the action.

 

[olironrider]

… Image 6872: the key switch has main power at T30, 1.5 Red; ground is T15, 1.5 Green; T56 is power to Hi/Lo beam, 1.0 Yellow with white tracer; T58 is power to other lighting, 1.0, Gray (wire sizes in square millimeter area, ie. 1.5 mm2

There was a huge draw of current at T30, even melted down to the spade connection at the board ! T58 got the spade connector cover melted; T56 escaped the conflagration; T15 melted along-with with T30 !

T15 runs to the connection board and co-joins another 1.5 Green wire, at the unfused, power side of a fuse; guess where the unfused power goes directly… T15 on the LH coil !

Image 6875: the EME processor brown, ground wire, co-joined at the main chassis ground coil bracket, shows evidence, as a result of routing it, of having rubbed thru the insulation at a male spade coil connection; just below the spade insulation cover at LH T15 !

Current flow thru the 1.5, green, unfused power from forward, at LH T15, was dramatic and melted the factory female spade cover !

Image 6912: the coil cases have a ground connection internally for primary & secondary windings. EME must have an internal cut-out to protect itself from a shorted ground ? With the enricher Bowden cable having rubbed to bare steel under the RH coil; the beginning of the short started at the EME ground & LH T15; the bridge connection LH T1 to RH T15 was not harmed; the coils case were warm and, perhaps, the RH coil case gave in to the short ?

Now, you got big ground, big unfused battery power, big “Magic Smoke” in the area

What say you all ?

Don

 

[stevenrankin]

I was wondering what the strange looking twisted wire is in the picture, I can see as perhaps Anton has seen it looks like it was rubbing on the cable.
What is it? Could any of the other ignition wires rubbed on it and shorted? St.

 

[AntonLargiader]

To me it looks like the remnants of a choke cable (airbox-mounted). If it wedged itself between a coil terminal and the coil case (like it sort of seems in 6912), it would short that terminal to ground. If it happened at the upper left coil terminal, it's a dead short that would fry that green wire. If it only happened at the right coil's crossover terminal, it would be an overload with twice the normal current draw. I see what looks like arc marks on the right coil. Maybe some better pics of the left coil would shed a bit more light on it, but I think its pretty clear what happened in any case.

 

[stevenrankin]

Blush, I have never seen a choke cable naked, LOL. St.

 

[Lendog]

Had an exciting ride last Friday on my 1974 R75/6. Bike started smoking, and when I pulled over, flames (small flames) were issuing from beneath the tank. No worries. I’m a firefighter and quickly fixed that. But I’m not an electrical engineer so I can’t quite be sure why it happened.

From what I could tell, the green wire on the left coil was at fault. It burned hottest down by the coil. It also burned up in the headlight bucket where it meets the key. Interestingly, BMW did not design a fuse into the wiring between these components.

I’ve ordered a new wiring harness, but I’d feel a lot better if I could pinpoint the source of the problem. Maybe it’s a simple as old, cracked wires? Any ideas?

Most of my electrical components are new, and I have an EME electronic ignition, though I’m not sure that would matter in this case.

TIA!

If your negative battery was loose the entire electrical system could have attempted to ground thru other wires,
the ignition switch should be checked as well

 

[spencer_p]

Hi everyone, thanks for the continued assistance. Yes, the weird, tangled, burnt wire is the enricher cable. I uploaded a video, which might make it easier to see what happened. LH coil appears unscathed (superficially). RH coil is very charred and some rubber from the enriched cable is still melted onto it. So is that the source of the problem? Seems likely. Certainly my enriched cables are old and worn to the point that their sheathing is cracking.

 

[20774]

Just curious...are those stock coils? My /7 has (what I remember) are black bodied coils.

 

[spencer_p]

They are Enduralast coils from EME.

 

[20774]

Thanks. I see that EME says they are slightly larger and come with their own clamps.

 

[spencer_p]

Thanks. I see that EME says they are slightly larger and come with their own clamps.

Yes, so the leading theory is that, due to their larger diameter, the new coils were close enough to the choke cable to rub it to the point where it shorted on the coil housing.

 

[olironrider]

… The Verdict, please !

The enricher cable going to the RH carb, was in hard contact with the RH coil casing; the casing is thoroughly grounded to the frame, a necessity for the primary & secondary windings. As time progressed, the outer sheathing of the enricher cable rubbed through to the steel spiral-wound support of the solid wire core, which is the actuating means for the enricher.

Un-fused 12VDC via a Red, 2.5 mm2 wire from the battery, branches to an un-fused Red, 1.5 mm2 wire at T30 on the connector board in the headlight nacelle. The un-fused Red, 1.5 mm2 then connects to the key switch T30 spade.

In all positions of the key switch other than OFF, un-fused 12VDC is supplied to the T15 spade at the key switch: a Green 1.5 mm2 wire. That 1.5 mm2 wire returns to the connector board and joins with another Green, 1.5 mm2 wire at the power side of the doubled T15 spades.

Still un-fused, the Green, 1.5 mm2 wire runs within the main wire loom to T15 on the LH coil. Also at the LH coil, T15, an un-fused(?) Green wire from the Enduralast system joins, for un-fused power.

In very close proximity to that Enduralast 12VDC, Green wire at T15 of the LH coil, is the Enduralast Brown, ground wire at the master chassis ground.

911 Operator: “Station 10, Engine 10-1, Rescue 10, Medic 12, Ambulance 14… a fire call; motorcycle smoking with possible fire; operator not, repeat not, injured at this time… SR 293 and I-495N… responding units use Fire Frequency 7 for comms.”

Engine 10-1: “responding and has command !”

911: “Engine 10-1 rolling and in command, all other units use Fire 7.”

Sorry guys… couldn’t resist

It appears to me, that a smaller short circuit occurred between the Enduralast Brown wire and the LH T15 multi-spade lugs ? Too small a wire to draw much current, just a little burn on the Brown wire insulation.

Things lit-off when the worn, fifty year old, Bowden steel sheath/wire of the RH enricher, provided a significant chassis ground, via the RH coil case ! The bridge wire between LH T1 to RH T15, does not appear harmed. I didn’t see evidence of the RH T1 making contact outside normal pathway. With such a monster ground, maximum current flowed in the Red and Green, un-fused ignition circuit, within wires not able to support the load.

I support BMW’s decision to not fuse the power feed to the stock ignition system. I’d rather two blown fuses and still be able to ride to a safe location.

Inattention to motorcycle maintenance: guilty as charged

 

[omega man]

Sounds like you figured it.
OM