1972 R75/5 charging problem

[AndyFitzGibbon]

Hello, first post. Good to be here.

I recently started resurrecting a friend's 1972 R75/5. It's been parked for nearly ten years, due to a charging problem (along with various other factors not related to the bike). I attached a photo of when it first came out from under the porch...

I have it running now, and would appreciate any help diagnosing the charging issue.

Here's what I have so far:
*Battery is brand new sealed (electric starts and runs the bike with no trouble)
*All electrical systems besides charging work as they should
*Charge light seems to behave normally (on at idle, goes out when engine speed is increased)
*With engine running, battery voltage is slightly less than with engine off. When engine is revved, voltage increases by a very slight amount- around a tenth of a volt

Step one will be to go through the bike and clean every electrical connection. Beyond that, if there is any particular course of action that this set of symptoms suggests, I'd like to hear about it.

Thanks,
Andy

[jagarra]

this process may help diagnose the problem.

viewtopic.php?f=1&t=9791&p=118140&hilit=Ced#p118140

[Gibson]

I would look at the regulator first. If it is the old mechanical type (not sure what is on that machine), clean contacts and adjust from there. If still no good, test the rectifier diodes.

[hudson]

That is the first place I would look. I have a friend that just had a similar problem and it ended up being the regulator. It wasn't passing enough voltage through it to charge - should be around 14.2 volts or so.

[AndyFitzGibbon]

Thanks, guys, that gives me a lot to go on. Haven't had a chance to do anything with it these last few days. Hoping to get into it on Sunday.

The regulator is a plastic-bodied Wehrle electronic type, which I think is probably a replacement. I think this bike would have come with a mechanical one originally (though I may be wrong there).

Thanks again,
Andy

[AndyFitzGibbon]

Well, using the link above, I determined that the bike's alternator is fine. Repairing a few corroded crimp connections on the alternator output wiring did create some charging action, but I was still only getting 12.9 volts to the battery.

Yesterday, I traced the problem to the starter relay. All of the alternator output to the battery passes through two terminals on the starter relay, which are connected internally- I'm not sure why BMW did this, other than that maybe it was just a convenient place to connect wires- but I have 14.1 volts going into the relay from the diode board, and 12.9 volts coming out. The relay itself is pretty crusty, so I suspect the internal connection of the two terminals may be corroded. I'll try to pull it apart this afternoon.

Anyway, just wanted to update, in case anyone else runs into this problem. It doesn't seem to be a common one. I may run a separate charge wire from the diode board right to the battery terminal on the starter, to bypass the wire junction at the relay entirely.

Andy

[Kurt in S.A.]

Hmmm...I'm not the greatest with this, so someone will correct me. But I wasn't aware that output from the alternator to the battery passes through the starter relay. Output from the diode board (B+) goes to a connection on the starter solenoid and then on to the battery. The starter relay basically works as follows: when you hit the starter button, a connection inside the relay closes using a nominal 12v. With that closed, then current from the battery is routed to the starter through heavier wiring, able to handle the higher current.

Kurt in S.A.

[AndyFitzGibbon]

Hi Kurt,

This schematic shows how my bike is wired: http://w6rec.com/duane/bmw/electrics/Sl ... ematic.jpg I tried to post it here, but it wouldn't post the whole image, for some reason.

The red wire from terminal 30 on the diode board carries alternator output to a terminal on the starter relay ("start prevention relay" on this schematic). It then passes through the relay to another terminal on the relay, which is a direct connection, as I understand the schematic. From there, one wire goes to the battery, and another goes to the ignition switch.

My problem is that the internal connection between the two "pass through" terminals on the relay seems to be weak. My alternator is sending 14.1 volts to the terminal on the relay, but only 12.9 volts are coming out the other terminal, so that's what the battery ends up seeing.

The connection from the diode board to the starter solenoid that you mention is the one I'm considering adding. I've seen a couple references to folks doing that.

Am I entirely misunderstanding how the system works? I never rule out that possibility...

Andy

[Kurt in S.A.]

OK, that's different...I'm more familiar with the later starter relay (as opposed to the starter prevention relay). You should consider getting the book from Motorrad Elektrik called Classic Boxer Charging. It's a step-by-step guide to troubleshooting the charging system.

Kurt in S.A.

[AndyFitzGibbon]

Yeah, the start prevention relay is exclusive to the /5, I think. Kind of a weird circuit, made weirder by BMW's decision to run all of the battery charge power through a bunch of unnecessary wire length and couple little spade terminals on the relay. A direct wire from the rectifier to the positive starter terminal that you mentioned is a much better option.

I'm hoping to disassemble the relay later today and maybe find out what the problem is.

Andy