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Bigsieuk
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Hi all,

I've bought a new ignition board, a new wiring loom and I've downloaded every wiring diagram that there is for R12's. Problem is that '61' is identified as pole on the ignition board on some diagrams, and no. '51' on others. Am I right to assume that this is a typo and should be '51' instead?

Also the loom comes with two red cables to the board but the diagrams only not one red cable?

The ignition board also came with two white wires coming from it yet the diagrams only show one!

I have a tip for any of the vendors of looms - label them up please.

If you are thinking of buying the wiring looms that are out there don't expect their wiring diagrams they send out to make any sense. Might have been easier said if I'd just made my own loom.

Feeling frustrated - time for a cup of tea and a cigarette.

John

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Bruce Frey
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It is confusing!

Most of the wiring harnesses I have seen are, at best, a starting point. I have never seen one that was really correct. Some have additional wires for brake lights and some do not.

Also, the replacement boards I have seen are all set up for a battery ignition and not magnetos (I assume you have a D2B magneto).

I am attaching an English language D2B manual which has a wiring diagram that may be useful. I have some more manuals, pictures and diagrams in my archives, so if this does not answer your questions, I will look for more.

Good luck,

Bruce

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bosch_manual_d2b_1938_eng.pdf1.21 MB
Bigsieuk
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Wiring

Thanks Bruce,

It certainly seems the most logical. As you suggest I'll just accept the loom as series of wires that I use rather than a prescribed configuration. I could have probably saved myself the money and made up the wiring.

Yes, the board does seem to be set up for battery ignition.

Once again, thanks.

Regards,

John

Bruce Frey
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With the schematic, it should

With the schematic, it should hopefully be more clear.

My R12 has a battery ignition with an original board, so I can't offer any first hand insight to your situation.

I suspect the hardest part will be figuring out which terminal on the board to attach the magneto grounding wire (2), assuming there is not a "2" terminal identified.

Some work with a multi-meter may be in order. You don't want to put voltage to (2), so don't use (15) which would provide power to the coil with a battery ignition.

I am sure someone has crossed this bridge before.....hopefully they can chime in.

Bruce

Bigsieuk
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Working backwards

After about 2hrs of trying to fathom the intricacies of the board. I gave up. I've started to work to the board from the destinations and see where we go from here.

I've put the looms back on the shelf and just run my own wires. Anyone need a loom, let me know.

Regards

John

Bigsieuk
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R12 ignition boards

I found a picture on German ebay of a board that looks very similar to the board as shown in the English manual. It might make things easier for those considering buying ignition boards and wiring looms in the future.

Note the difference in the boards that are available from some vendors.

Regards

John

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Darryl.Richman
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You need to be careful about

You need to be careful about what components you have. The R12 came with a magneto version and a battery/coil version, and the parts don't mix. The magneto is far more common (all army bikes and earlier "authority" bikes) had magneto ignition. Only the civilian bikes had battery/coil, and BMW didn't sell many of them because they introduced the R5 and R6, which gave better performance, just a year or two after the R12.

You must make sure that if you have a magneto bike, you have a magneto board. This will have a terminal 2, which is grounded when the key is removed and otherwise disconnected. If you don't have this kind of board for a magneto bike, you won't be able to stop the engine by pulling the key. If you have a battery/coil board for a magneto bike, you can replace it with a /3 board if you can't find a correct replacement.

51 is the generator output, and many boards have a terminal labelled 30/51 (30 is battery positive). The D2B magneto has two screw terminals in the upper rear cap, and 30 (from battery positive) goes to one and 51 runs from the other to the board.

61 (charging status) was not available on the original D2B magnetos, and a correct headlight shell for a magneto R12 will not have a red charge light lens, and a correct board for the magneto R12 won't have a charge bulb. And a stock D2B magneto doesn't have a terminal for 61. However, if the voltage regulator has been changed to a modern electronic version, there probably is a 61 terminal inside.

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Bigsieuk
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RRC Board

Thanks Darryl,

Attached photos of the RRC Magneto board I've bought.

Regards

John

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Darryl.Richman
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Wolfgang Reichenberger at RRC

Wolfgang Reichenberger at RRC is very knowledgeable and helpful. I got some new carb jets when I visited him once.

The board you have looks like /2 board. In the second photo you can see the terminal numbers stamped into the board - 2 at the lower left, 30/51 in the lower right, etc.

There is a German DIN standard for numbering auto/motorcycle wires and connections. We printed a list of them as they apply to older bikes in a past issue of our magazine. BMW in the past had adopted a fairly standard way of coloring these wires, too.

 2 - magneto ground (stops the motor when grounded, otherwise N/C) - black or black with a white tracer
15 - switched power (hot when the ignition is on, otherwise N/C) - black with a green tracer
30 - battery plus - red
31 - battery ground - brown
51 - D+ (generator plus) - red
54 - power for the brake light switch - black with a red tracer
56 - headlight - white
  56a - high beam - white
  56b - low beam - yellow
57 - parking light
58 - taillight - black
61 - D field (used to drive the charge light)

You should be able to make this all work, but you are trying to put together two different systems, so you have to understand them both to make it work.

On a /2 bike, the charge light is separate from the board, so there is no 61 terminal on the board. Also, because the parking light and taillight are always on at the same time, there's no 57 terminal, just a 58. And the headlight lead (56) goes out to a switch on the handlebar before returning as 56a and 56b wires, which go directly to the bulb. A /2 also has a green wire that goes from the neutral light in the headlight bucket to the neutral switch on the back of the transmission.

If I were you, I would identify all of the wires in the harness and put little masking tape flags on them, both ends. If you have a /2 wiring harness to go with your /2 board, you will have a couple of extra wires, like 61 and a brake switch wire (unless your R12 has been adapted to have a brake light, which is a common and not too hard thing to do).

Then I would lay out the loom along the frame and determine where you are going to physically run it. Note how 30 comes up from the battery and goes down at the motor, you will have to get this to the magneto. 51 comes up from the motor at the same point - it will also connect to the magneto - and continues on to the headlight shell. 58 runs from the headlight back to the front of the rear fender, as does the brake switch wire, which goes down to where the brake switch normally is near the rider's right foot. On a /2, there is a set of screw terminals on the front of the rear fender that continue wire runs to the taillight, brake light and ground. You will probably have to adapt something here.

I am sure I'm forgetting something here, hopefully others will be able to jump in and help, too. (I am travelling in Europe right now and this is all from my very fallable memory.)

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Darryl.Richman
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Here's one thing I forgot:

Here's one thing I forgot: the /2 board has no provision for a fuse, but most R12s had a fuse - it's a red-brown bakelite screw plug on one side of the keyhole. This is why I used a /3 board in my r12, it provides for the 8 amp torpedo fuse.

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Bruce Frey
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Here are some things to think

Here are some things to think about as you wire your bike.

If you look at the schematic in the Bosch manual, there is just a single wire from 51 on the mag to 30/51 on the board. Some harnesses I have seen have another wire from +battery to 30/51 on the board.

The Bosch JN5 tail light should have both hot and ground wires as the schematic shows. The ground wire is used instead grounding to the frame (like the horn) because the JN5 was also designed to be de-mounted and used as a hand light. If you have a later JN7 which is not de-mountable, you could probably ground to the fender and omit that wire.

If you have a brake light, I would take power from the tail light (58) terminal.

Make sure -battery is well grounded to the frame. I also like to have a dedicated ground wire from - battery to the 31 terminal on the board as well as a ground connection from board 31 to the headlight shell (otherwise headlight may depend on a ground connection through the headset bearings).

Good luck!

Bruce

Bigsieuk
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Thanks Bruce,

I've run wires as per the Bosch manual back to the board and labelled as you suggested. I've also been studying the 'original board' pictures that were on the ebay site (thanks for those whoever you are).

I've overlayed the switchable connector at the end of the key barrel onto the ebay pictures to indicate which points are connected when the key is turned, I'll then overlay that onto the new board I've bought to see which connectors align correctly and run the new wires accordingly. If I had the skill set I'd manufacture the boards up as per the ebay picture. Might make things easier for mag R12 owners.

I've found this a great help and I'll post these up when I've completed this exercise.

Regards

John

Bruce Frey
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As I re-read what I wrote, I

As I re-read what I wrote, I should have said to make sure -battery is well grounded to the frame AND THE ENGINE/TRANSMISSION. I run a wire from -battery to a transmission bolt.

The - terminal of the generator is its frame. If you need to shim it to adjust chain tension, you must use metal shims to maintain continuity to the engine case.

Bruce

Bigsieuk
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Finally

Thanks Bruce and Darryl,

I went back to basics on the wiring and ignition.

I put the new loom back in the box and ran my own wires as there are so few. With the board again I went back to basics, I disconnected all the wires that come with the loom and overlaid the (what I believe to be) original layout for the wires. and connected accordingly. I ran an earth from the tail light back to the board and connected it to the earth from the battery/frame/gearbox just in case.

The only issue I can see is that you can remove the key from a non-central position and leave live power to the lights accidentally you could run the battery down.

As often is the case there was a lot of fettling to get things to fit properly. It may have been easier if a mag R12 specific board were available but I can understand the reasons for manufacturing a 'catch all' board and loom.

Regards,

John

Darryl.Richman
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Congrats, John! That's what

Congrats, John! That's what I do on these bikes because, as you say, there are so few wires to run. Now you know just how it's all wired and it won't ever be intimidating to you again.

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