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Topic-icon '73 R75/5 Battery connected, dash lights on

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2 weeks 16 hours ago #5936 by DMOSER
Good morning,

I have been working on this bike as a basket case for nearly a year. I have finished the electrical connections and have come to this point where the three lights on the dash illuminate when the battery is connected. If I put the key in the headlight, it'll fry my points wire (after condenser). I have only done this once, but until something changes, I am guessing this will happen every time.

What I have done to T/S my problem?
- Pulled off the headlight and examined everything for proper connections and corrosion.
- Cleaned the starter relay of corrosion, added di-electric grease and verified the wires were connected in the right order (I used Reams' illustration to verify)
- When I disconnect the single red wire off the starter relay, the dash lights go away
- just purchased a new starter relay just in case (22 Mar)
- replaced the condenser, points wire, points, coils

What I have not done...but not sure if it matters (seriously, correct me if and where I am wrong)
- Doubled checked my diode board
- tested/ohmed out my diode board

I am not sure where to go from here. I have shared to the FB Airhead forum pages and have been able to look into other areas that folks like Tom Cutter have suggested, but not to much avail. Thank you all for your help!

V/R,

David
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2 weeks 12 hours ago - 2 weeks 10 hours ago #5938 by Wobbly
Replied by Wobbly on topic '73 R75/5 Battery connected, dash lights on
Welcome !

I'm not overly familiar with the /5 key switch to help you to spot common issues common to that physical piece. So I simply have to skip that part.

The harness consists of numerous systems (lighting, charging, turn indicators, etc), only a small part of which is the ignition circuit. The way I start all electrical work is to confine my search to the faulty system, assuming that rear brake lamp (for instance) doesn't impact the points wiring in a manner direct enough to exhibit this behavior. That's not BMW expertise, that's simply pure logic and a passing analysis of risk factors.

Then we see from the schematic that the ignition switch applies power to one side of the ignition coils. The coils consume that power. Then, the points wire provides the return path back to the battery. IF 1) the coils were not consuming power, or 2) the coil connections were made incorrectly, then power from the ignition switch would have a straight path to ground (passing thru the points), and thus make a smoke offering of said points wire.

Wiring-wise, what you should have is the ignition power (from the ignition switch) coming to the Pos terminal of Coil #1. Then the Neg terminal of Coil #1 connected to Pos terminal of Coil #2. Finally the Neg terminal of Coil #2 connected to the wire to the points (where it meets ground). (If you have German coils they mark their polarity with numbers. Thus terminal #1 is Negative, or the lead that goes to the Points.)



So my advice is to check that wiring first. It's very rare for a coil to go bad. In your case both coils would need to go bad at the same time, which the "odds makers" in Las Vegas tell us simply can't happen.

Hope this helps.

Don't hide 'em, Ride 'em !!
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1 week 5 days ago #5950 by 8053
I would go so far as to suggest that there is a wrong connection on the main switch. This assumes that the original coffin nail switch is in place.

James Strickland, IL. Airmarshal

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6 days 10 hours ago #6007 by DMOSER
Replied by DMOSER on topic '73 R75/5 Battery connected, dash lights on
Thanks Wobbly,

What I have found is that I had my wires mixed up and now all is good! I did change out my starter relay because it was original to the bike and I couldn’t test it.

I am currently trying to start the bike and it turns over but not getting any fire.

More to come!

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