Boxerworks.com

Hello all, -new to the board & new to the world of '70 & later BMW ownership (all of my experience lies in 55-69 bikes).

I will try to keep this query as short as possible, -I just picked up a 75 R90/6 that seems to be fairly solid from a mechanical standpoint and runs quite well with one issue: -The generator light stays on bright as can be at all RPMs. after jumpering the blue wires going to the voltage regulator with no results I have removed the alternator stator, rotor and diode board & had all of them tested by a local old school alternator shop. The Diode board tested bad so I replaced it and refitted the rest of the components. Now the light is still glowing. I have checked the DF and D- terminals on the stator for proper insulation/continuity to ground with results to spec and checked the slip rings on the rotor to ground with results to spec as well. Here's the kicker: the book calls for a reading range 6.21 and 7.59 ohms between the 2 slip rings on the rotor; my rotor reads 3.8 ohms. Will this cause a no-charge condition and why/why not? I would like to properly diagnose the problem without just throwing parts at the bike. Any help appreciated.

PS: Yes, I do plan on a motorrad elektrik system, and the proper literature in the future -just trying to get the bike rideable to assess the whole machine right now. And Yes, I do have the solid-mounts for the diode board. Thanks all!

Have you checked the resistance of the 2 sliprings against each other like the book says? One slipring is one side of a long winded insulated wire which ends at the other slipring. Did you isolate the sliprings from the alternator brushes with a piece of paper or something? Just trying to make sure you measured it right.

In principle though, there is nothing wrong with a 3.8Ohm rotor unless it is a 7Ohm rotor with a propagating short inside. It would create a higher magnetic field and thus, a higher current output at the stator. The diodes and wires have to be up for that additional current.

Thanks for the Reply, Stephen. I did test the slip rings isolated from the brushes. And here's an update on the status of the problem: A friend & fellow airhead loaned me a new rotor to test and it too measured 3.8 ohms between slip rings. When the new rotor was installed, the charge lamp remains lighted at any RPM besides when the starter is working.

Does the starter relay have any bearing on whether or not the alternator will charge the battery? I only ask because it looks like it has seen better days. Any broken wires that I should look for or test for shorts in the loom(s) for this no charge problem?

Thanks again!

I seem to remember a glowing GEN light problem THREAD on the newest old forum that turned out to be bad connections to the ignition switch terminals.

Your rotor is probably (99.999%) OK if it is not open or short circuit.
I've seen them ranging from 3.xx to 7.xx ohms with no measurable effect on charging output.

Is the bottom half of the diode board getting battery power?
if not check the wire from the thick lug on the bottom left of the board
It goes to the starter + , and from there to the battery positive.

With the ignition ON, does the charge light go OFF when you unplug the regulator?
It should.
With the regulator unplugged, does the charge light go ON when you ground the blue regulator plug wire?
It should.

Next, you have to try a new regulator.
There is no way to test a regulator asides from hooking it up to a known good charging system (or an appropriate bench tester).
It may not be your problem but must be eliminated as a cause before continuing.
Anyway, they're inexpensive and are a mandatory spare part to have for any 12V airhead.
You can get a VW beetle regulator at any auto parts place,
I'm sure someone will chime in with a part number for you.

If none of the above fixes it, chime back in and we'll continue...

gspd wrote:There is no way to test a regulator asides from hooking it up to a known good charging system (or an appropriate bench tester).

But you can remove or bypass the regulator to see what the alternator is really doing. If you bypass the regulator and the alternator output rises rapidly to over 15v with increasing revs, the regulator is fine.

Kurt in S.A.

I agree with you on that one Kurt
BUT - a regulator can be doing its job of limiting voltage to 13.8-14.2V as it should,
but still cause the light to glow.
I've also seen stators that charged OK. but caused the light to glow
I don't know exactly what causes this, or if it makes any sense, but I have seen it happen.

It's not exactly clear to me if Red's problem is simply a glowing light, or insufficient charging voltage.

gspd wrote:It's not exactly clear to me if Red's problem is simply a glowing light, or insufficient charging voltage.

Red, welcome to the forum.

I think it's worth stressing the point here that when evaluating or trouble shooting a charging system it's ALWAYS good to know exactly what is happening at the battery. That means with both voltmeter leads connected directly to the battery terminals.

The reason is that with these old airheads there are often wiring problems, that is bad connections, sometimes bad soldering joints, and even wires that look good all the way to the terminal, but are rotten inside the insulation there. And then there are grounding problems.

Red, I'm not exactly pointing this post at you. It did look like a good place, though, to make a somewhat general electrical troubleshooting statement. Wiring problems are hard enough to solve without chasing your tail for a day because of a bad ground or a bad connection--especially if the problem is intermittent. Knowing what is happening at the battery itself makes the job so much easier and straightforward.




Ken

Ken means to use a "shop" voltmeter to see what the battery terminal voltage is while the engine is running, Red. You should see around 13.5-14 volts at around 3000 rpm. If you don't, the alternator is not doing anything in spite of the new diode board.
Do you have a schematic of your wiring?
The lamp is IN the circuit that provides current to the rotor. If the lead that comes off the lamp and goes to the voltage regulator is grounded, it will stay lit when the engine is running and there won't be any rotor excitation and NO charging.
The schematic I have (poor) shows the blue wire from this lamp going to a terminal on the starter relay and then, thru ANOTHER terminal, going on over to the voltage regulator. The implication from the diagram is the 2 terinals are connected internally in the starter relay. THIS could be your problem. these terminals are only used as a connection point. Find them and eliminate the starter relay from the circuit!!

Ken in Oklahoma wrote:

gspd wrote:It's not exactly clear to me if Red's problem is simply a glowing light, or insufficient charging voltage.

Red, welcome to the forum.

I think it's worth stressing the point here that when evaluating or trouble shooting a charging system it's ALWAYS good to know exactly what is happening at the battery. That means with both voltmeter leads connected directly to the battery terminals.

Ken

Red, I am going to nit pick a bit here, but it can be important. When Ken says "battery terminals" it could mean two things that are sometimes quite different. There are battery terminals and battery posts. The terminals are crimped on the wires and the posts are fixed on the battery. They "should" read exactly the same, but quite often the connection between them is faulty and they have resistance. This can be measured easily, but a almost perfect fast test is to feel them after the switch has been on for some time. They should not be warm/hot. If hot, then significant current is being turned into heat. The quick fix is to just grab the wire terminal and rotate it a bit. Yes, it should be tightened down and not rotate, but usually they will. This movement is often enough to reestablish a good connection and all is well.......... for a bit. Still, fix it properly.

You can also put the voltmeter probes on the terminal and post to see if there is any voltage drop across it. It should be zero.

We commonly found this to be a problem with a battery.

The BMW wiring harness crimps were not very good. Many were failing in the first couple of years. The simple fast test is to flex the wire at the battery terminls. Is it stiff just next to the terminal compared to a couple of inches away? That is corrosion under the insulation and death to a healthy circuit.

There is more, but I am tired of typing. I really don't think this is related to your problem, but still should be checked by everybody yearly. More on my site on crimping terminals and many other places.