Ignition Issues

[CafePB]

My apologies for the lenghty 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.

Thanks
Peter

[melville]

The switch in your headlight appears to be the common thing in every situation. People who know /5 key switches should chime in now, but it sounds like a dodgy contact with the key, which I understand forms part of the circuit on those.

[spo123]

Easy!
Perhaps, you should SLIGHTLY TWEAK, the Spring Contact Lever (whatever it is called:)......to create a more forceful pressure against the aforementioned electrical (cleaned?) contact.
In my 120,000 miles, that I have personally put onto my /5, I have had to perform this tweak on 2 occasions.
The Result, Should be (IF I am correct:)).....that your /5, IF having the stock contact plate, will now have more RELIABLE voltage, whenever you, install your "Nail Key".

[Rob Frankham]

I would move away from generator light charge system and alternator issues, they are unlikely to be a problem even if they exist. The bike will normally start, even with the alternator completely inoperative as long as the battery is fully charged. From your description, the problem appears to be a faulty ignition switch.

To prove this one way or the other, either

1) use a meter or voltage tester to check the voltage on the out side of the ignition switch (green wire) when the key is inserted and the light is not on (i.e. in the fault mode) If the test doesn't show battery voltage, the problem is with the switch. or

2) use a length of spare wire to run a temporary power coection from battery positive to the green terminal (i.e. jump round the ignition switch). If the bike suddenly (and reliably) springs into life then the switch is defective.

Rob