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Topic-icon 1978 RS100 troubleshooting dead electrical

  • Wursthead
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4 days 1 hour ago #5056 by Wursthead
Hey all, new to group.
Just acquired a beemer from my buddy who wouldn't get it going. it runs fine but as he handed it over its electrically dead. I looked at all the "duh" issues battery good. He did seem to jiggle the harness under the gas tank a bit when it wouldn't start and that would solve it. But now it dosent. Im limited electrically speaking but wondering if anybody knows how to troubleshoot to find out whats up? where do you start? any help would be great. glad theres a group like this. thank you

Wursthead

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3 days 23 hours ago #5057 by Wobbly
Replied by Wobbly on topic 1978 RS100 troubleshooting dead electrical
Welcome Aboard !

Your problem is most likely NOT electrical !! Remove the fuel tank and on the LH side of the frame, just under the brake m/c is a mini-cube relay. That's the starter relay. You probably need a $7 replacement off of Amazon. But that's only a symptom.

The real problem is that the brake m/c leaked corrosive DOT4 fluid all over the starter relay... and that fluid is corroding all the electrical contacts in that highly populated area of the main harness. You need to wash off all the old brake fluid with HOT soapy water. Then you need to rebuild the brake m/c so it doesn't leak anymore.

Hope this helps !!

Don't hide 'em, Ride 'em !!
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1 day 18 hours ago #5067 by Wursthead
Replied by Wursthead on topic 1978 RS100 troubleshooting dead electrical
Hey Mr Wobbly
Checked out the situation you spoke of. replaced starter relay . didn't see any issue with M/C really or leakage. still no results. any plan B ideas past this? I appreciate your time and experience. Thanks in advance my airhead friend

Wursthead

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1 day 17 minutes ago - 23 hours 36 minutes ago #5071 by Wobbly
Replied by Wobbly on topic 1978 RS100 troubleshooting dead electrical
Several things....

• There's no need to start a new thread. Keep ALL your correspondence about this problem in one single thread, PLEASE.

• I didn't know you were actually going to order a new relay. There are numerous 5-prong relays on the market, but only one type will work. It's called a "Dual 87" and if you didn't buy that specific relay, then that's a new problem which needs correcting.

• The RED wire is "hot" all the time and straight from the battery. When the relay is in place you should have 12V on both RED wires at the relay socket. From the relay socket, that wire usually shows up on the rear of the ignition switch, or possibly on the RED section of the muti-color circuit board inside the headlamp.

If you've got a lighted test probe, connect the clip to a cyl head fin and poke around the RED wires under the tank and in the headlamp trying to make it light up.

• There are NO fuses in the stock harness until the power has already passed through the ignition switch. And the 2 fuses that are there in the headlamp do not usually protect the ignition circuits.

A Simplified Schematic

Hope this helps.

Don't hide 'em, Ride 'em !!
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Last edit: 23 hours 36 minutes ago by Wobbly.

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1 day 3 minutes ago - 1 day 1 minute ago #5072 by Wobbly
Replied by Wobbly on topic 1978 RS100 troubleshooting dead electrical
You most probably bought a relay with a schematic LIKE THIS and that absolutely won't work. That's 87 and 87A. Not the same.

You need a relay with a schematic LIKE THIS . That's an 87 and 87.

Don't hide 'em, Ride 'em !!
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Last edit: 1 day 1 minute ago by Wobbly.

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22 hours 12 minutes ago #5076 by Wursthead
Replied by Wursthead on topic 1978 RS100 troubleshooting dead electrical
Maestro Wobbly
Your help is amazing! thank you so much. the relay i received was just from the previous owner. and your right its the one that you pictured that is the wrong one!.
By any chance is there a link or photographs/video of the harness for that bike that depicts how everything should be plugged in and laid out? they were both hanging down when i received and I'm not even sure where they mounted originally. Again thank you so much! this is huge help

Wursthead

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