Electrical problems....

[electric_monk]

Last thursday evening I left work facing a 100 mile run home in torrential rain, the spray coming up off the road was so heavy that visibility was down to about 80 metres at times. When I hit the motorway section the 2 lanes were practically stopped so I knocked on the high beam and set about filtering. After about a mile I could smell burning and noticed smoke coming from the left hand switch gear. I immediately switched the high beam off and pulled onto the hard shoulder. The smoking stopped and I noticed that the indicators horn were not working but that the high b.eam warning light was still coming on when they were switched on. So I elected to leave them off and limp down the road keeping a close eye on the switchgear. Some miles down the road I pulled into garage ( a proper one with a working mechanic). The fuses were blown and unfortunately neither I or the mechanic had 7.5 fuses, after blowing out the switch gear with an airline and replacing the fuses with 10 amp ones everything was working fine and the mechanic assured me that if there was still a problem the 10amo ones would blow before any damage was done. So onward I headed as the rain stopped.
20 mile later the bike suffered a complete electrical failure. Quickly heading to the battery I discovered that the connection from the battery to the starter had vibrated until it had broken. A quick dig into the topbox turned up the jumper wire for testing the charging circuit and a roll of insulating tape.....just enough to fashion a quick bodge to get me home. So onward I headed again...
2 miles further down the road the bike went on to 1 cylinder and I had to limp home the last 5 miles while the rain returned.

On Friday afternoon I got a chance to investigate and discovered that the wiring from the high beam switch was completely melted all the way from the switch down to the six pin connector block under the tank. It is a complete mess and I need to replace the switch and wire. Some heat damage is apparent on the other side of the block connector but I don't know to what degree yet. Further investigation is needed and will hopefully take place this week.

I have only recently stopped using an 80w/100w headlight bulb, so am I am wondering could that be the root cause or could it have been caused by the combination of a 25+ year old switch and the heavy rain.
Any thoughts, observations or advice would be greatly appreciated.

 

[markjackson]

I can only say what I'd do, but I do enjoy a good electrical fault…

To melt automotive wire insulation with just 12v would require a pretty low resistance (like a starter motor), if not a straight short circuit.

At 12v I've never known water to do this, and I doubt a 100w headlamp would either (as the switch takes the full lamp current on the GS wiring diagrams I've seen).

I also don't think the wire could melt while only pulling 10amps, so it looks as if the problem area wasn't be protected by the 10amp fuses (although these will need more than 10a to blow them).

So it seems more likely that a component has failed or 'rubbing'/'bending' and other general frottage has caused the insulation between wires to be rubbed/pushed away allowing the conductors to touch.

The other possibility (given the failure of the big thick wire from battery to starter motor) is that a high power circuit (starter motor) has been drawing current through the wrong route, i.e back from the bike via the smaller wires that meet at the rectifier. Looking at a wiring diagram I can't see what route it may take tho.

So… If it were mine I'd start by ignoring it for a few weeks/months/years, while playing with a random 80s moped. After I'd tired of 15mph into a head-wind, I'd look at the switch gear, 'cos most faults occur where mechanical (thumbs) and electrical (wires) things meet. If there is a way that the live feed into the switchgear could be earthed, that would melt the live wire.

Years ago, (helping to find an underground cable fault) I was taught by a wise cable-fault finder to sniff rocks around where fault-tracing stuff said the problem may be. The smell of arcing was a good clue, so sniffing the switchgear (honestly) would be the first thing I'd do, then I'd take it apart.

The proper way would probably be to leave the switch on the bike, with the 6 pin connector disconnected, and (at the switch side of the connector) test the switch and its tail with an Ohm-meter (or old horn or light) for a circuit between the live feed (yellow?) and earth (the handlebars). Any path to earth, if the connector is split, would be a bad thing

Would it be this loom?

 

[ahutcheon]

Any chance a connection became high(ish) resistance, dirty or corroded maybe, heated up to the point that the insulation near it failed, then the ensuing short or arc destroyed more insulation until the fuse blew? Just throwing a guess out there, but other parts of circuits do have a habit of blowing to protect the fuse...

 

[electric_monk]

Mark that wiring diagram is for the earlier G/S and has differences to my 88-90 100GS. By the looks of the diagram the high beam is not operated through a relay so the bigger wattage bulb may have overwhelmed the system. However as I weight up the current and incomplete evidence I reckon it has been caused by a short at the high beam switch..because the wiring from the switch down to the connector is completely destroyed and the wiring on the other side of the connector is much less damaged.
New switchgear has been ordered and further investigation should follow tomorrow
.

 

[markjackson]

Manky switchgear or switchgear tail certainly sounds like the likely culprit. Good Luck!