Wiring and Magneto Notes, Observations and Assumptions on My R12

My 1941 R12 was a war bike, built in March of 1941 and delivered directly to the German army. In 1996 this bike was in private use in South Africa, when it was consigned to Cascade BMW in Seattle on spec, and I bought it. That's the history I know of this bike.

As a war bike, it would have been painted a flat medium gray, and been delivered with a magneto ignition and a solo carburetor. As it arrived in my hands, it had been painted in civilian livery (gloss black and white pinstripes). It also had dual carbs of an unknown origin (no maker's mark on them), but resembling later Bing slide carbs.

The headlight shell (or bucket) has a charge lamp on the left of the key, and a fuse socket on the right. This R12 is by far my oldest bike, but this bucket looks a lot like the one on my 1953 R51/3, so I thought nothing of this arrangement. However, it turns out that buckets like this were only used for battery ignition R12s, not the magneto ignition with which my bike is equipped.

The reason is that Bosch supplied complete systems to BMW, including the generator unit and bucket, and an external coil for the battery ignition systems. The generator units are different between the magneto and battery ignition systems, and have different outward appearances as well. Only the battery system generator has a terminal to provide one side of the charge lamp. (I don't know why this is, it just is. It is the case that the condition of the battery is of greater consequence to the rider of a battery ignition bike than a magneto ignition bike. Magnetos don't rely on any battery power to operate, which is why they still are used in light aircraft.)

When I started investigating my bike's electrical system in more detail, one thing I noticed was that my generator had this small stud (red circle in picture) with a wire leading into the bucket. An equivalent wire wasn't shown on the wiring diagram. I kind of hoped that this was the terminal for driving the charge lamp.

The wiring on the bike was all home made, and not very pretty. I looked to the usual sources and found that a repro wiring harness is available, although it's for the battery ignition bike. After thinking about it, I decided to order one, hoping that I could use it on this bike by reusing some of the wires for different purposes.

At this point, I'm going to provide a quick summary of the DIN wiring codes, which I will use for the rest of this page. DIN (Deutsche Industrie Normale, or German Industry Standard) provides numeric codes for nearly any wiring connection in a vehicle. Some of them are:

Partial Table of DIN Wiring Terminal Codes

DIN Number	Purpose
2	Magneto Ground - shorts the magneto and keeps it from providing a spark.
15	Switched Power - a connection to power controlled by the key.
30	Battery Positive
31	Battery Negative or Ground
51	Generator and/or Battery Positive
54	Horn Power
56	Headlight Power - 56a is low beam and 56b is high beam.
57	Parking Light Power
58	Taillight Power
61	Generator Positive

Here's a little aside: the way the charge light works is to put 51 on one side and 61 on the other. Because 51 includes both the battery and generator output, while 61 has only the generator output, current will flow if one is stronger than the other. There are three cases when this happens:

1. Generator stopped. When the ignition is on but the motor isn't running, 61 is a ground, but 51 still has the battery (which is discharging), so current flows from 51 to 61 and the light is bright.
2. The generator is turning slowly. At idle, the generator doesn't spin fast enough to create the full power it's rated for, so a small amount of current flows from 51 to 61, and the light glows more dimly.
3. The voltage (and current) regulator isn't working, and the generator is cooking the battery. In this case, the generator is working full blast and feeding too much current to the battery, so current flows from 61 to 51 and lights the lamp backwards. (The lamp is just a resistor and doesn't care what direction it is impeding the electron flow.)

When I opened up the bucket, this is what I saw (roughly; I added the tape flags as part of figuring out what went where). It was immediately clear to me that the terminal board was not like any other Bosch/BMW unit I'd seen before. The flat blade quick disconnect electrical connectors showed that the board was much more modern than the rest of the bike. Unfortunately, none of the terminals was labelled.

If I had the board in my hand, it wouldn't take long to decipher its circuits. But I am reluctant to remove the board from the bucket because it is held on by four metal tabs that slide through the board and are then bent. This holds the board in place against the pressure of a spring between the middle of the board and the bucket, which also keeps the pieces that make up the headlight/parking light switch under pressure and in contact. The tabs are a part of the bucket, and only have a certain number of bends in them before they break off. Of course, I don't know how many times mine have already been bent. Repairing them when they break is a big headache.

So I spent parts of two days with a meter and the battery figuring out the circuits. The picture at left diagrams the board as I can see it. The hole in the center of the board, with the three copper "fingers" under it, is the ignition switch. Before the key is inserted, the top two fingers are in contact. This grounds the 15/54 contact when the key is removed. When the key is pushed down, it breaks the contact between the top two fingers and makes contact between the middle finger and the bottom finger. This switches terminal 15/54 on and makes it hot.

The charge lamp is also kind of interesting. In the photo above, you can see that a bit of copper has been soldered to the side of lamp's base, and it arches up the side of the glass. This modification was necessary so that it can contact a metal strip rivetted on the top of the board.

This is the actual circuit that the board implements. The red circuit line corresponds to the red wire in the photo and the schematic board drawing. The switch on the right side of the red lead is the ignition switch. The rotary switch depicted above terminals 56 and 57 is the headlight/parking light switch. The actual implementation of this switch is on the top side of the board and not visible.

This board is set up for battery ignition. There's no provision for terminal 2 (magneto ground), which leaves the engine ready to fire at any time, and also means it must be stopped by some other means than the key. The only switched connection (15/54) is hot when the key is pressed, and that power also interferes with the magneto operation.

In order to get the switched connection I needed for terminal 2, I had to disconnect the red wire, and thus the hot side of the ignition switch. This meant that there could never be any power to the charge lamp, but since my generator doesn't have a terminal 61, it didn't matter anyway.

The one other ramification was that I no longer had a switched hot lead (15/54). This lead powers the horn (and with other changes, the brake light). I moved them to the terminal labelled "hot, fused", figuring that they probably won't run down the battery since both loads are controlled by momentary contact switches.

The resulting board and wiring drawings look like:

As I mentioned quite a ways up above, I bought a new wiring harness. It did, indeed, have enough (and more!) wires to support the magneto ignition needs of my bike. But the layout is a bit different than the original wiring diagram, so I created this new diagram.

The harness came with leads for a brake switch, and I modified the tail lamp to use a dual filament bulb. I bought a cheap (JC Whitney) brake switch that uses a spring clamped to the rear brake rod to actuate it. Fortunately, the R12's pressed steel frame channel is big enough to swallow up the switch and hide it from casual observation.

There are a couple aspects that are different from the original wiring plan. One is how grounding is handled. Originally, the battery negative terminal was wired to the back of the transmission, and a ground lead from the motor was brought into the bucket. That second lead was missing when I got the bike, and it seems that ground was brought into the headlight via the metal sheathed speedometer cable. Now, the battery negative is brought directly to the bucket, and then brought back to the engine. Another back-and-forth occurs with the battery positive. Originally it ran directly to the generator, and then terminal 51 brought power to the bucket. Now battery positive goes to the bucket, but immediately turns around and heads back to the generator so that 51 can bring power back to the bucket.