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Ignition fire !

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OldCrow
Posts: 13
Joined: Sun May 02, 2021 6:52 pm

Ignition fire !

Post by OldCrow »

Let my sorrow be a lesson. A few weeks ago I backed my 1981 R100RT out of the garage to do a carburetor synchronization. I started the bike up normally and fiddled around with something when I heard a hissing noise that I couldn't identify. Next thing I saw was "steam", actually white/gray smoke, poring out of the right combination switch where the wiring enters. My mind froze for a few seconds. Next the wire harness from the combo to the headlight bucket started to "steam" 😯. Now I got my ass in gear. I switched off the ignition SW but the bike continued to run. And the starter started to engage. Some wires in the headlight bucket had melted the insulation and shorted together. I sprinted to my car in the front yard to get my fire extinguisher, sadly it was a dry chemical but certainly better than nothing. The engine has stopped as I sprayed and knocked out the fire but the engine starter kept running and the engine kill switch had melted and popped out. Next task was to disconnect the battery to kill the starter. Ran into the garage to get a screw driver. Flipped open the seat. Luckily I didn't have my 4 gal auxiliary fuel tank on the tail rack as the support bracket needs to be removed to open the seat. Disconnected the negative terminal and finally silence.

Lessons learned from the BMW fire.
1. Always carry a fire extinguisher of CO2 type. Precious seconds were wasted running to the car to get the dry chemical fire extinguisher. Also dry chemical leaves a mess to clean up and is corrosive. I've since gotten small CO2 extinguishers to carry on each bike, BMW, GL1200, KLR650. And have mounted a 10lb bottle in the garage. I had/have another dry chemical in the garage but couldn't remember where it was located, hence the sprint to the car.

2. The Airhead does not have a main fuse. My GL1200 has a 30amp right off the battery and the KLR650 has a 20amp. I estimate that the ignition wiring(starter relay->ignition switch->run/stop switch->coil) had 20amps to 50amp current flow. I'd never really looked at the BMW schematics before. I was shocked to see the only fuses in the system are for turn signals, marker, plate, horn, brake but not the headlight or ignition. I will be installing a main 30A fuse.

3. A battery cutoff switch should be on each vehicle. I don't know why I haven't thought of this before. I've raced sports cars and motorcycles in the past and they ALWAYS have easy to reach battery cutoff switches. So much less damage would have occurred if I'd been able to simply reach down and cut off battery power.

This will be a belt and suspenders approach with a main fuse and battery cutoff. But I've learned my lesson.

You'll see I posted a wanted ad for the right combo switch. I'm getting a couple aftermarket switches just to check run, but would like to use the OE.

Edward aka OldCrow

weh8127
Posts: 199
Joined: Tue Jan 16, 2018 11:28 am
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Re: Ignition fire !

Post by weh8127 »

Wouldn't a cutoff switch for the battery leave you open to the same problem you experienced with failed switches in the first place, i.e. the inability to shut things down because the switch had already failed? An S connector as described by Jeff Dean for his /2's allows easy installation for a fuse on the hot side and instant disconnect without tools

https://www.bmwdean.com/6_Volt_Battery.htm

Bill
Bill Husted
Barre, MA USA
1963 R60/2 w/ 1955 Steib S500
1973 R75/5

OldCrow
Posts: 13
Joined: Sun May 02, 2021 6:52 pm

Re: Ignition fire !

Post by OldCrow »

A proper battery disconnect would not melt, they are typically rated in hundreds of amps. The OE switch is just meant to interrupt the current to the coil +15 terminal, ignition control unit and starter relay. The largest wires used in the circuit are 14awg rated at about 15amps. But I think since the wires to and from the kill switch are inside the same wrap the excess heat from both carrying the current would cause failure at even lower amps.

To use a battery disconnect on a R100RT it would be needed to interrupt the D+ on the diode board which supplies voltage to the regulator once the alternator is spinning. And possibly the line from the charge indicator light since the light provides current to the field when the bike is started. Would be an interesting experiment.

I see Dean is using SAE power connector for his battery. While this will provide an easy way to plug in a trickle charger, if the bike is running disconnecting the battery will not shut the bike down since those old bikes have permanent magnet generators. Also they don't have electrical starters so the current flow in that SAE should be pretty small recharge flow. I use the SAE connectors on my heated gear and find they are too easy to pull loose so I'd never use one on my main battery power. I'm really tempted to convert my GL1200A and KLR650 over to the BMW style accessory connector. They take a determined pull and they can be plugged in without looking single handed.

OC

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skyler.robbins
Posts: 227
Joined: Sat Sep 22, 2018 4:14 pm

Re: Ignition fire !

Post by skyler.robbins »

Oh man!.. Sorry to hear about this.. but I like your idea of a quicker shut off and the correct fire extinguisher near by.

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srankin
Posts: 1054
Joined: Fri Jul 14, 2017 8:45 pm
Location: Spencerport, NY USA
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Re: Ignition fire !

Post by srankin »

Any idea what caused the problem? It will be interesting to know after the post mortem. Something was shorted badly to cause that much heat so fast. St.
Owner of a 84, R80RT and 78, R100RS

Armand
Posts: 81
Joined: Mon Jan 11, 2021 6:53 pm
Location: Binghamton ny

Re: Ignition fire !

Post by Armand »

Shovelhead harleys occasionally have this same problem, referred to as starter run on. Something to do w parts in the starter fusing together. Related to using an under charged/weak battery. Lots of the Harley guys routinely put a large shut off on the positive battery wire.

OldCrow
Posts: 13
Joined: Sun May 02, 2021 6:52 pm

Re: Ignition fire !

Post by OldCrow »

The coils seem to be the likely suspect. Only the wires on the direct path from the starter relay 87 to left coil +15 show any damage. The red wire from the start relay is larger than the remaining wires since it carries more load to other circuits. The two wires jumped off the Green/Blue to the Ignition Control Module and back to the starter Relay show no damage and they are only 16awg wires. The ignition switch survived with no damage but the Green to the ignition switch 15 was damaged.

I took the coils off and measured the ohms with my Fluke 117 multimeter. They both showed within specs as found in the OE Repair Manual. I then placed each coil on a hot plate and raised their temps to about 130degF range and actually the ohms increased as expected. So from that simple test the OE Bosch coils appear good. But I had no way to simulate the bike vibration or check when collapsing the fields. I don't currently have an oscilloscope for further checks.

I've installed 2 Bosch 12-13-1-243-452 coils. They are slightly larger, about 1mm dia and 1/2in longer, but fit the brackets and sliding them back 1/2inch just required moving my Aux tank junction. The coils actually have a higher primary side resistance so the current should be lower. I've rewired Headlight bucket to Coils with 12awg Green. Cleaned up the combination switch and an able to salvage the starter pushbutton and some proper color coded wire, about 3 inches at the switch and the terminal board. The wire between was destroyed. I'll patch in new wire crimped and soldered. I'm picking up a generic handlebar mounted kill switch.

I opened up the main harness from the headlight to the coil area to investigate for collateral damage to other wires. The over heating of the Green/Blue bubbled and melted it's insulation but there appears to be only some insulation stuck to adjacent wires. There is more damage in the headlight bucket where HOT wires looped back and crossed. So there are several in there that need repair.

I'm not getting as much time on the BMW as I would like. My real drop dead date in June 21 2022, when I have an annual summer solstice IBA ride.
OC

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