Fuse question - R51/3

[Micha]

I have installed the old powerdynamo alternator back into the bike.
Unfortunately I also do not remember what fuse was installed, if at all:
The installation instructions state at least twice that a 16 amp fuse must be installed on the battery outlet (+)

https://www.powerdynamo.biz/wiredias/g-only.pdf

I do not know such values; afaik, the values ​​are at intervals of 5 at a time, i.e. 10, 15, 20, etc.
Have you ever seen a 16 amp fuse?
Can I assemble 15 instead of worrying?

Does this means I can also skip the 8 amp fuse inside the headlamp bucket, which is installed there as an original requirement?

[312Icarus]

Fuses protect the smallest wiring in the circuit from over heating due to over load. (At least in general). A fuse a couple amps bigger than spec is probably not going to cause a big issues as the ampacity of the wire is pretty liberal. That said, old wire, with deteriorated insulation and bit over fused might/will hasten the deterioration if the wire runs hotter.

Consider that the /2s had no fuses what so ever, and most haven’t burned up. (I nearly burned mine up when I ran it with the speedo cable disconnected and it arced in the headlight bucket. I now have a system fuse at the battery so I can cut off all current even if something else is on. The battery has enough stored energy to start things on fire even though it is pint sized.

Icarus

[wa1nca]

Yes a 15 amp fuse would be fine
As far as the 8 amp fuse it is only inline with the high/low switch wires and the 2 wires going to the headlight
if the voltage now is 12 volts then that fuse should be reduced to a 4 amps
Ps
The headlight fuse was eliminated on the r50-r69s series from 1955-1969
Tommy

[312Icarus]

If a fuse is only in circuit to protect the wiring...the voltage doesn’t matter. Amps are amps regardless of voltage. If the fuse is there to protect the device in the circuit, then the voltage may matter.

Icarus

[Micha]

Yes a 15 amp fuse would be fine
As far as the 8 amp fuse it is only inline with the high/low switch wires and the 2 wires going to the headlight
if the voltage now is 12 volts then that fuse should be reduced to a 4 amps
Ps
The headlight fuse was eliminated on the r50-r69s series from 1955-1969
Tommy

Tommy (and all other friends here),

Duane Ausherman's drawing (https://w6rec.com/r513-electric-diagram/) shows the 8 amp fuse (that you suggest should be changed now to 4).
While Salis does not (https://www.bmwclassicmotorcycles.com/p ... ake-light/).
Do you have any clue why it is not shown at salis?

Another question: To my untrained eyes, this fuse should be installed between 51 and the light switch mechanism itself.
I unscrewed a bit the complete Ignition switch plate and could not find any fuse holder or so.
Will I be ok with fitting this 4 amp. fuse after the switch - on the wires that go to headlamp terminals 56, 57, 58 and to the rear light?
If yes, can these wires all be taken through the same fuse?

BTW, you said that "The headlight fuse was eliminated on the r50-r69s series from 1955-1969" - that's interesting: do you know the reason?
Thanks a lot for your help!

[schrader7032]

The R51/3 diagram shown on Duane's site is not an OEM drawing. Duane attributes the diagram to Mark Millard who redrew it and put all the pretty colors on it, etc. Mark also added the fuse for whatever reason, maybe suggesting where he put a fuse on his bike. Who knows.

Maybe he was modeling the R51/2 diagram. In Roland Slabon's book, he indicates that the R51/2 board was fitted with a fuse and says "something that BMW did away with in 1954".

[wa1nca]

The R51/3 diagram shown on Duane's site is not an OEM drawing. Duane attributes the diagram to Mark Millard who redrew it and put all the pretty colors on it, etc. Mark also added the fuse for whatever reason, maybe suggesting where he put a fuse on his bike. Who knows.

Maybe he was modeling the R51/2 diagram. In Roland Slabon's book, he indicates that the R51/2 board was fitted with a fuse and says "something that BMW did away with in 1954".

My 1954 r51/3 has no fuse
Tommy

[jwonder]

All, I wanted to weigh in here a bit. I do not have a degree in EE, but I have been engineering, wiring and building harnesses for race cars (old and new) for the past 35+ years in which I learned a lot about wiring in the real world.

1) Icarus is 100% correct that fuses are there to protect the system from damage. It is much better to get rid of the power source than damage the system, or worse have a fire.

2) Tommy is right that in the 12 volt system that you half the fuse amount (8 to 4). If your wiring is good and you cannot find a 4 amp fuse, I would just keep the 8 amp fuse in. A short will blow both readily.

3) Tommy is right that the 8 amp fuse is there to protect the headlight and parking light only. I went to our library (shameless plug) and found the R51/3 owners manual at https://vintagebmw.org/library_data/R51 ... h_1951.pdf and the wiring diagram is at the back. You will see it gets V+ from terminal 51 and distributes it to terminals 56 or 57 based on the key position. 57 feeds the parking light and 56 feeds both the speedometer light and the headlight switch on the handlebars (hi/low switch).

4) I always wire a fuse in the circuit at the battery V+ wire and for the most part in our systems this is there to protect the battery while running and any short that might occur when the motorcycle is NOT running. See below why this fuse does not really matter.

5) It has not been stated yet that on the BMW models that have magnetos, the ignition circuit is separate from the chassis circuit. If the fuse that leads to the battery V+ is blown the motor will still run and the generator will still run the electrics even if that fuse is blown. This is not ideal if there is a short in the system as you will not know it because your happily motoring down the highway, and likely why BMW did not put a fuse there. A good example of this is a short I had in my horn that blew the fuse at the battery, yet when I was running the motorcycle down the road I could get a very sick sound of out of the horn. Thankfully I realized there was a problem and stopped using the horn and rebuilt it when I found the issue. Not ideal electrical engineering and to really protect the whole chassis system there should be a fuse on the output side of the generator. That fuse would best be put inline to terminal 51. Thinking of it more I might just do this.

Best,