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sblaylock
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VBMWMO #8384
Joined: 07/22/2010
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I need some help to understand the starting circuit on an early 71 R75/5. My understanding of the start relay, and correct me if I'm wrong, is D+ needs -12 volts to disable (allow the bike to start) the start prevention circuit on the starter relay. Nothing connected to D+ or +12 volts connected to D+ will not allow the bike to start. If that is correct, then how is -12 volts accomplished on the headlight bucket circuit?

There are two blue wires connected to D+; one to the diode board and the other to the headlight bucket. If I pull the blue wire to the headlight bucket there is no voltage at D+ and I can get the starter to turn with the start button. When I connect the blue wire from the headlight I get +8.8 volts and the start button won't work.

I followed the blue wire to the charge warning light where it splits to 15 on the ignition switch board; the neutral indicator, oil press indicator, then off to 31 on the turn signal flasher. From there to the hand brake switch... This all looks as it should based on the schematic.

New battery fully charged. New starter relay. This is a no fuse, four post connector block setup.

Any ideas as to what could be the problem?

Thanks,
Scott.

sblaylock
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VBMWMO #8384
Joined: 07/22/2010
Posts: 109
Solved!

Who knew a 12 volt, 2 watt bulb could stop an R75/5 from starting.

I was asked by someone "is the GEN light on brightly with the engine stopped". It was very dim, so I switched the bulb out for another one I had and it worked. I removed the bulb and it still worked. I put the dim bulb back in and no start, so something about that bulb was causing the problem.

I hope posting this solution helps other people trying to solve /5 start relay issues.

Scott.

schrader7032
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San Antonio, TX
Joined: 10/27/2006
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Glad you found a solution. I

Glad you found a solution. I dimly remember (no pun intended) that the early /5 had some definite unusual features in the starter circuit. Snowbum has quite a discussion in his starter section on his website:

http://bmwmotorcycletech.info/startingprobs.htm

One section nearer the bottom says that if the alternator is OPEN, it could prevent the starter relay from working. I'm wondering if the dim bulb represented an open rotor to the starter relay? Generally, the bulb lights up when there is uneven voltage on either side of the bulb...the battery is down on voltage or the alternator is not putting out 13-14v. With a difference in voltage, current flows and the bulb lights. In your case, current was flowing, but given that it was not up to par, could that electrical situation simulated an open rotor? I'm sure that is far above my pay grade!!

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Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2

sblaylock
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Different Ohm Readings

Hi Kurt,

I put the multi-meter to the two bulbs and got significantly different Ohm readings.

Setting my incredibly inexpensive Radio Shack multi-meter on the 200K Ohm setting I found the following:

12.0 Ohms - Good Bulb (Starter works)
5.2 Ohms - Bad Bulb (Starter doesn't work)

Someone smarter than me will understand why the higher resistance bulb disables the start prevention and allows the bike to start. I would think it would be the other way as an open circuit also allowed the bike to start.

Scott.

mark_weiss
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Arizona
Joined: 11/17/2013
Posts: 172
The following is the way that

The following is the way that I have always believed that the system worked: The GEN bulb bridges between the battery side and alternator side of the charging system. When the two sides are markedly different, the bulb is bright and the alternator charges. When the two sides are similar and the light is dim, the battery is charged and closer to the alternator output voltage. When the two sides are markedly different, the GEN bulb has a clear + and - differential, and illuminates. When the sides are close, the bulb sees little voltage difference and is normally dim or off.

If a low resistance bulb is used, a 'lot' of power flows through the bulb, unopposed, and both sides of the bulb become similar in power levels. The bulb is dim and the starter protection circuit 'thinks' that the system must be charging, so the motor is running, and the starter is blocked. With a high (enough) resistance bulb, the two sides of the circuit remain substantially different and the starter protection circuit interprets this as 'motor not running' and enables the starter.

I have always thought that this explains why a freshly started engine would allow the starter to try to engage again but an engine that has been running for a bit (and has charged the battery) suppresses the starter.

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Mark
qualitycycleservice.com

sblaylock
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Thanks for the explanation.

Hi Mark,

Thanks for the explanation. That does make sense and is something I'll be checking going forward when trouble shooting starting issues on these bikes.

Thanks again,
Scott.

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