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1978 RS100 troubleshooting dead electrical

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chris wilson
(@wursthead)
Posts: 32
Eminent Member
Topic starter
 

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

 
Posted : 07/12/2019 15:50
Richard W
(@wobbly)
Posts: 2546
Member
 

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 !!

Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.

 
Posted : 07/12/2019 17:43
chris wilson
(@wursthead)
Posts: 32
Eminent Member
Topic starter
 

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

 
Posted : 07/14/2019 22:25
Richard W
(@wobbly)
Posts: 2546
Member
 

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.

Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.

 
Posted : 07/15/2019 17:07
Richard W
(@wobbly)
Posts: 2546
Member
 

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.

Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.

 
Posted : 07/15/2019 17:21
chris wilson
(@wursthead)
Posts: 32
Eminent Member
Topic starter
 

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

 
Posted : 07/15/2019 19:13
Richard W
(@wobbly)
Posts: 2546
Member
 

Start up Google, input "BMW R100" or "Airhead Repair", and then select "Images". Tons of Airhead photos will pop up of Airheads in all stages of dress and undress. Should be something there for you.

Hope this helps.

Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.

 
Posted : 07/16/2019 22:53
chris wilson
(@wursthead)
Posts: 32
Eminent Member
Topic starter
 

thanks again my friend

 
Posted : 07/17/2019 12:26
chris wilson
(@wursthead)
Posts: 32
Eminent Member
Topic starter
 

This is what I’m dealing with. Just trying to figure out what goes where. The schematic I have for this year bike doesn’t match up with numbers on the dual 87 relay.. maybe I’m missing something? Thoughts?

 
Posted : 07/19/2019 16:28
chris wilson
(@wursthead)
Posts: 32
Eminent Member
Topic starter
 

If I connect direct from battery straight to the starter the bike kicks over

 
Posted : 07/19/2019 16:31
Richard W
(@wobbly)
Posts: 2546
Member
 

If I connect direct from battery straight to the starter the bike kicks over.

No doubt. Since the starter is permanently wired directly to the battery this news comes as no huge surprise.

► First step was all about insuring power is flowing to the entire electrical system. That's what the starter relay issue is all about. That is to say, are you getting power into and then out of the ignition switch. (In computer troubleshooting lingo, this is the equivalent of asking: "Is the computer plugged in ?" 😛 )

► Then there is a second tier of investigation at the starter interlock, which keeps you from starting the bike in gear. The bike either has to be in neutral OR the clutch has to be disengaged at the lever. Either one of those sensors can be faulty.

Hope this helps.

Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.

 
Posted : 07/19/2019 16:54
chris wilson
(@wursthead)
Posts: 32
Eminent Member
Topic starter
 

I’m here wrenching on it as we text. I got power to to the starter and bike but there’s no turning the power off even with the key in the off position ??
Thanks again for your time. Really appreciate this

 
Posted : 07/19/2019 17:27
Richard W
(@wobbly)
Posts: 2546
Member
 

I got power to the [snip] bike but there’s no turning the power off even with the key in the OFF position ??

Your communication is not clear. Are you saying there is power applied to either or both the Gray and Blue wires coming out of the ignition switch, no matter what key position the ignition switch is in ?

If so, you may need a new ignition switch. Test first. Unplug the Blue and Gray wires from the back of the ignition switch. See if power is still applied to the Blue and Gray wires.
• If there is power, then someone has installed a jumper somewhere in the harness to route power around the ignition switch.
• If there is no power on the Blue and Gray wires once they are disconnected, then you simply need a new ignition switch. BMW does not sell these, so you need to call our friend Rick Jones at Motorrad Elektrik.

Hope this helps.

Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.

 
Posted : 07/19/2019 18:56
chris wilson
(@wursthead)
Posts: 32
Eminent Member
Topic starter
 

thanks again for your time.

I guess what I'm saying or trying to figure out being a bit of a novice with electrical is that this whole thing was really just about hooking up the right wires from the harness to the right areas on the starter relay harness. if I follow the color codes that the schematic says its confusing because colors don't match up an the numbers differ from what the dual 87 says as to the diagram. so i was poking around a bit but was hesitant to go too deep in fear of frying something . I wish i just had a picture of the wires going into the harness and somehow they were labeled whats what

 
Posted : 07/19/2019 19:31
chris wilson
(@wursthead)
Posts: 32
Eminent Member
Topic starter
 

i found a simple schematic backed with photographs of the starter relay and horn relay hookups. so according to the diagram all wires are where they are supposed to be. still dead, upon startup . I'm going to go back and check all connections in the harness next.
any other thoughts?

 
Posted : 07/20/2019 20:18
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