1973 R75/5 - electrical issues?

[schrader7032]

On these bikes, the coils have two sets of windings, primary and secondary. The primary is only a few ohms, say 1.0 to 1.5. The secondary ohms is quite high, generally around 5K ohms. Rule of thumb that the ohms from the left spark plug cap to the right spark plug cap is 20K ohms. That's about 5K for each of the left wire, each coil, and the right wire. Some of the earlier /5s only had 1K ohms in the wires.

So make sure you measured the right part. I think the primary wiring is from the terminals on each coil. The secondary resistance would be from either of the terminals to the tower of the coil where the plug wire is.

[Kidasters]

Thanks Kurt.

But - removed from the bike, both coils should measure the same. One of my coils is steady 1.7 ohms. The other bounces all over and won't settle. In my experience, that's the sign of a coil that's shorted. My guess is that one coil is bad. We'll see what happens when I get the new coils.

[schrader7032]

It's unclear what you measured.

[Kidasters]

So - the way I was taught to check a coil - you measure across the + and - terminal with your voltmeter set to continuity (resistance, Ohms). For a good coil - you should read in the neighborhood of 1.5 ohms. On a bad coil, you won't get a steady reading and it will jump all over. Of the two I have - the "good" one was steady 1.7 ohms. The "bad" one was jumping all over - sometimes 1.5 ohms, sometimes 6 ohms, and one time 23 ohms.

I'm hopeful that new coils come and my electrical issue is gone. We'll see.

[Kidasters]

OK - update.

Well, I was wrong. Ordered new coils and installed. Once I hooked up the battery, the fuse off of terminal 15 on the board blew immediately. I am at wits end.

I think I'm going to take the switch out as well as the terminal plate and see if that's where the problem is. I do not look forward to this, but I don't know where else to look.

I am unhappy.

[schrader7032]

Replace the fuse with a large size DC volt bulb...maybe 15 watts...not real sure what would work. That way the light lights instead of the fuse blowing. Then start taking things out of the circuit until the light goes out.

[wa1nca]

Kurt
Good idea
Saves a lot of wasted fuses

[Kidasters]

Thanks Kurt. It was blowing when there was a 20 watt fuse in there. I've gutted the bucket and I'm starting over. Ordered a new terminal board. I have a switch repair kit. The good news is - both the plastic cover on the "keyhole" as well as the chrome trim were looking tired. So - will replace all of it - board, switch, everything. Also noticed where a previous owner did some wonky wiring in the bucket, so all of that came out too.

So - that's the next phase. Rebuild the ignition switch, then install a new board, and re-wire the bucket. Hopefully that sorts it.

[Kidasters]

That was a no-fun job.

Tabs BMW? Really?

Removed the old board. Getting those tabs apart was a pain. Found some sketchy wiring in the bucket, so I'm glad I did this. Getting the tabs off of the keyhole surround was also a pain. Good thing they sell a new spring for the sliding cover, becasue my fell apart when I was removing everything. Oh - and the sliding cover broke in half.

Surround was harder to get back on than off. I had to get another set of hands to hold down on it while I tapped the tabs back bent. When changing the key cylinder, I recommend getting the special tool that holds it all together until you get the circuit board back on - made that part way easier. Also - realized once I had the board back in that I left a critical part out, so I am now an expert at disassembly/re-assembly - having done it multiple times now.

Still have to re-run the wires from the harness and switches to the circuit board. But not today. And - hopefully this sorts my problem. We'll see. I will report back later.

[schrader7032]

Be careful on bending those tabs. They work-harden the more you do it and eventually they will break off. Remember the fun we had taking a coat hanger and bending it back and forth until it finally breaks!! To avoid them breaking off, one has to use intense heat from say a small propane torch to anneal the tabs so they become more ductile. The problem is that you use a lot of heat which can affect the paint in the local area. I've never had to do this, but I think one can use something like a wet cloth on the outside to pull away the heat. I believe the Barrington repair manual for the /2s has a discussion this...this is the same setup as on the /5s.