Ignition / Starting Issue

[CafePB]

My apologies for the lengthy post.

I have an issue with the ignition and starting of my 1972 R75/5
The charge indicator light does not come on when I push down the ignition key. With that the bike does not start. When I trigger the contact in the ignition switch several times (5-10 times), all of a sudden the light comes on and with that the bike starts.

I have already removed the ignition switch and checked for any mechanical issues. I wiggled all of the wires and cleaned the contact ‘points’ in the switch that close when the ignition key is pushed in, which did not help the issue.

Using the write up from Snowbum https://bmwmotorcycletech.info/trbleshootALT.htm “GEN lamp (alternator lamp) PROBLEMS”, I did the tests shown under ”GEN Lamp Problems - , Troubleshooting a dead (not lighted or does so irregularly) GEN lamp”.
I came up with the following results:
- With the ignition key pushed in the light does not come on. With the key still in, I connected a jumper from the Df of the brush holder to the engine case (ground) – still no light. However, if I leave the jumper in place, pull the ignition key out, and then push it in again, the light comes on. If I remove the jumper (with the key still in), the light stays on. If I then remove the key, the light goes out.
- As a crosscheck I removed the jumper and pushed the key in - the light does not illuminate. Adding the jumper back, the light still does not illuminate. Removing the key and pushing it back in, with the jumper in place, then the light comes on again.
- If I remove the key for just a few seconds and then push it back in the light comes on. If I wait just a little longer before I push the key in, the light does not come on anymore.

- Snowbum says that if the light comes on then the problem lies with the rotor, brushes or brush wires and the sliprings need to be shorted as a further test. When I do that, with the key pushed in, the light does not come on. If I maintain the short, remove the key and then push in the key again, the light comes on. If I remove the short with the key pushed in, the light stays on. If I remove the key the light goes out.
- As a crosscheck I removed the short over the sliprings and pushed the key in - the light does not illuminate. Adding the short the light does not illuminate. Removing the key and pushing it back in, with the short in place, the light comes on again.
- If I remove the key for a few seconds and then push it back in, the light comes on again. If I wait just a little longer before I push the key in, the light does not come on anymore.

Snowbum says that the ‘light on’ shows that the rotor is open and bad.
- I have a Bosch 0 120340 001 Stator (105 mm diameter) in my bike. My workshop manual says that the rotor that goes with that should measure 6.8 Ohms over the slip ring. I can only measure 4.2 Ohms.
I also read in articles on the web that there are rotors that measure 4.2 Ohms, so I am not sure what can be used, or is acceptable for the /5.

- To go a step further I tested the voltage regulator on the bike – again as Snowbum suggests.
With the ignition key in, grounding the blue wire on the VR connector the light comes on.
- With the ground removed from the blue wire, grounding the black/blue wire the light does not come on.
Snowbum says that this shows that the regulator is good.

Can anyone give me a suggestion what else I could, or need to check, or help me what could be the culprit of the malfunction. I am not much of an electrician and I am certainly at the end of my wits. Before I start randomly replacing parts, it would be nice to hear some educated advice.

Thanks

[chrishea]

I’ll be upfront, I dislike electrical troubleshooting on the internet. But I’ll give it a shot.

By “not starting”, I assume you mean the starter is not cranking.

The starting circuit and the charging circuit are totally independent. The charging light has nothing to do with starting or ignition circuits. The red “gen” light only helps you to know the key is “on”. Is the bike in neutral? Does the green neutral light come on when the key is on? Even if the alternator rotor is open, the neutral light should illuminate with the bike in neutral.

Since you have a multimeter, when you push the key “on”, see if there is battery voltage (~12.7 V) at the green wire at one of the ignition coils (they are connected in series with a short black wire between the two, and then a black wire to the points.) If there’s no voltage at the green wire at one of the coils, I do feel it’s the mechanical connection at the ignition switch. To verify, in the headlight bucket check the connection labeled “15” which should have a green wire to it. That should be powered when the key is down/on. If it’s not, the problem is the switch.

BMWs use the Bosch wiring numbers and German standard colors. In short: Red wires are tied directly to the positive terminal on the battery. Brown wires are the “grounds” directly to the negative terminal. Green and green wires with different colored stripes are “hot”’with the ignition on. Gray usually has to do with lighting. Blue are “sensor” circuits (voltage regulator, neutral switch).

Is the key you’re using a newer aftermarket key? Some have reported problems with those not making proper contact in the switch.

Do you have a /5 wiring diagram?

I feel your readings for the rotor are fine. Zero ohms would be a short and infinite ohms are an open, but as stated before, the red “gen” light has nothing to do with the starting circuit. With a fully charged battery, with an open rotor, the bike should start and run… actually for quite a while without lights. Something else is wrong.

Try Brook Reams’s site for further information. https://brook.reams.me/bmw-airhead-moto ... -circuits/

Good Luck

[CafePB]

Chris,
thanks for the details.
I will look into this in the next couple of days and get back with you on the findings.

Regards
Peter

[srankin]

I won't add anything to the troubleshooting as I know very little about /5 bikes. I will say, EME sells a very good color wiring schematic for reasonable cost. For any BMW bike I have found the more schematics you can get ahold of from various sources such as EME, Clymer or Hayes, are a great help. BMW sometimes made small changes in wiring and not all the changes make it into print. One source may have those small change but another may not. St.

[cbclemmens]

PB:
It sounds like an intermittent contact of some type. Since pushing the key in and out makes it come and go, I would look there first. One way to check would be to bypass the switch. Run a jumper from pin 30 on the switch to pin 15 on the switch. You could also jumper from the battery (+) to the green wire (+) on the first coil as these are electrically the same points as pins 30 and 15. If the charging light comes on then the problem is the switch. If not, then start checking wires. There may be a break in one of them - not visible through the insulation.

Craig

BTW my '72 R 75/5 did not have a neutral start switch. You could engage the starter with the bike in gear

[chrishea]

There’s also a similar /5 thread…. “1973 R75/5 - electrical issues.”

viewtopic.php?t=44751

[CafePB]

Thanks everybody. I am traveling and likely will get into your suggestions over the weekend.
I will let you know what I can find out.
Regards
Peter

[CafePB]

I worked on my /5 today
I first submerged the entire ignition switch assembly, with the wires removed, in white vinegar for a couple of hours, to dissolve any corrosion that might be present.

Then I did the tests that you have suggested.
With the key in I have 12.7V on the coils
On connection ‘15’ in the headlight bucket I show 11.8V
Whenever I push the key in, I see the green neutral light and also the yellow oil pressure light – no red light.

I got a little improvement though, because now it now only takes 2 or 3 tries with the key and then the red light comes on.
If I disengage the contact ‘points’ that are closed with the key, then I clearly see a small arc. Could that be an indication of any malfunction somewhere?

Chris you are correct, with ‘not starting’ I meant that without the red charging light on, the starter does not engage and rotate - hence the bike does not start. With the charging light on, the starter engages and rotates and the bike starts.

I have an original BMW key, but do not know how old that is. You suggested that it might not make a proper contact. Would that be the contact that is established at the ‘points’ of the spring-loaded copper strip, or is there another contact that needs to be closed?
The points are firmly pushed together by the key.

One other thing. During my early troubleshooting attempts I replaced the thin flexible wire that is soldered to the flexible copper strip with the ‘point’ on it, with a thicker wire. Could that cause any issue with the charging light?
I have a wiring diagram – one from ‘Prospero’s Garage’ and now of course the copy that Craig posted above.

Any further suggestions from you are very welcome.

Thanks

[chrishea]

So with the key pushed in and power at 15 on the coil and at 15 on the terminal board is good. The power then branches off, one of the green wires goes to the fuses, and then I think green/black branches off, one wire goes to the alternator light. The other goes to the horn. When there’s no red light have you tried the horn? I’d try to jiggle that fuse anyway, they do corrode over time.

If the horn does work and there’s no red light, then follow the blue wire from the red light, per the diagram it goes to the starter relay then to the diode board.

[CafePB]

I have checked the horn and it works with the red light on, or without it being on.
Checked the fuse holder and fuse – everything is clean and shiny

I measured the voltage on the blue wire at the socket for the red light, at the D+ on the relay for the starter and the D+ of the voltage regulator:
11.4 V with the key pushed in, but the light not illuminated
2.4 V with the key pushed in and the light illuminated

As soon as I create a little spark at the ‘points’ on the flex copper strip, by lifting the points off, the light comes on – everything else untouched.
That is the same as jumping a wire rom 30 to 15. If I touch the connection – no light. If I lift the jumper slightly off, thus creating a spark and touch the connection again, the light comes on.

I even found out that if I wiggle the bulb for a short while it comes on. I checked the inside of the socket everything is shiny – sprayed it with contact spray nevertheless.
Maybe wiggling the bulb creates a spark somewhere as well.

Once the light has come on and I pull the key, then wait for 3 or 4 seconds and push in the key again, the light comes on. But if I wait 10 seconds the light does not come on. Almost like something is energized for a short period, but looses that energy a few seconds later.
Is it possible that the above mentioned spark energizes a component, which then helps the light to come on?

Does pushing the key into the ignition switch create any electrical contact other than pressing the ‘points’ together?