Alternator questions

[sooprvylyn]

Ok, so I have 2 questions:

1. I need to run a fuse between battery and alternator on a 1993 k1100rs.  I have read that the alternators on these were once 32wattamp but got upgraded to 50wattamp. Is there a way to tell which one is on my bike?  That said, is a 550amp fuse appropriate(on a 50 watt alternator)? The math says there should be about 4.17 amps running thru the wire, so 5amps makes sense right?Or would I be better off with a 55 amp or 60 amp fuse?

2. In my readings I've read that I need a 3ohm resistor to replace the bulb that was connected to the exciter wire on the alternator. I've been running this bike since Nov last year w/o any bulb or resistor, and haven't had any issues with battery not charging. What's the deal, is this load on the exciter wire actually needed?

Apparently I been riding around on borrowed time or something

Edit:corrected above figures for posterity...in case another needs this info

[Laitch]

Is there a way to tell which one is on my bike? 

[Ingo]

Ahem, the alternators are NOT 35 W vs 50 W, they're 35 Amp vs 50 Amp. so the fuse would be accordingly a 35 A or 50A fuse...  And why anyways?
BTW, the original headlight alone draws 55 /60 Watt, not even counting any of the other electrical stuff...
And: i have NO idea how come your battery isn't long dead if you're running without the alternator's excitation circuit.

[sooprvylyn]

Ahem, the alternators are NOT 35 W vs 50 W, they're 35 Amp vs 50 Amp. so the fuse would be accordingly a 35 A or 50A fuse...  And why anyways?
BTW, the original headlight alone draws 55 /60 Watt, not even counting any of the other electrical stuff...
And: i have NO idea how come your battery isn't long dead if you're running without the alternator's excitation circuit.

Sorry, I actually had 32-50amp in there before I 2nd guessed myself at near midnight. Need to stop doing late night posts. 

The reason I need to know is that I reworked my entire stock wiring to run an m.unit and need to fuse my rectifier regulator which is in the alternator...the m.unit fuses literally everything else on my bike but documentation says fusing the rectifier/regulator is required. Wouldn't a 35amp or 50amp(depending on alternator size) fuse actually be slightly too small? Shouldn't I need to go slightly higher? There were no fuses higher than 15amp on the oem setup, so what was the oem protection on this?

Also, all my lights are led now, so my current headlight draws about 40w which is just over 3amps. My battery has never dropped below about 13v without that exciter wire...tho I'm currently rewiring, hence the questions, and will add in a 3amp draw on that exciter wire moving forward

[sooprvylyn]

In case this label here is why you replied this way...you can see mine is not legible at all

Based on images I can find online this is a 50 amp..sound right?

[sooprvylyn]

Last question I promise: will a 25v 3ohm resistor work for the exciter wire load, or do I need a higher voltage resistor? As I'm replacing the load from a small oem bulb I assume that 25v is well more than enough. Am I overthinking this since it's a 12v system?  Just trying to make sure I fix this system right so I don't destroy my bike or my house. As you can tell, electric ain't my strong suit

[Laitch]

In case this label here is why you replied this way...you can see mine is not legible at all

My reply was a symbol of incredulity when encountering a paucity of resourcefulness.    I don't understand the reason for the fuse. There seems to have been alternating currents of not thinking and overthinking about this. I'm still unsure of whether that's happening here or there.

The exterior differences between 30A and 50A alternators dedicated to Bricks are stark; I know you'll determine them from photos of the components.

Often 50A Brick alternators will have sufficient residual magnetism to start charging without bulb resistance wired into them. It's just one of those submagical things. There are a few discussions about this here and elsewhere. The drawback of that is if the battery goes south, you won't know it if you don't have a bulb somewhere. You could be stranded at the diner after there wasn't enough residual magnetism to start charging when the Brick was taken out on a long haul; regardless, your house will be okay, if it was okay in the first place.

12V LED have built in resistors; otherwise, wire a 3W incandescent bulb under a stylish globe somewhere, be done with this; instead, devote yourself to making your world traveling more complicated than it needs to be.

[sooprvylyn]

That reply is a symbol of incredulity when encountering a paucity of resourcefulness.    I don't understand the reason for the fuse. There seems to have been alternating currents of not thinking and overthinking about this. I'm still unsure of whether that's happening here or there.

The exterior differences between 30A and 50A alternators dedicated to Bricks are stark; I know you'll determine them from photos of the components.

Often 50A Brick alternators will have sufficient residual magnetism to start charging without bulb resistance wired into them. It's just one of those submagical things. There are a few discussions about this here and elsewhere. The drawback of that is if the battery goes south, you won't know it if you don't have a bulb somewhere to tell you. You also could be stranded at the diner if there wasn't enough residual magnetism to start charging next time the Brick was riding on a long haul; regardless, your house will be okay, if it was okay in the first place.

12V LED have built in resistors; otherwise, wire a 3W incandescent bulb under a stylish globe somewhere, be done with this; instead, devote yourself to making your world traveling more complicated than it needs to be.

Thanks Laitch, I know it's maybe a little obnoxious that I asked what may seem obvious questions, but I gotta learn somehow right?

Regarding the fuse...I havent been running one, and didnt know I was supposed to have one until I happened upon it while researching my m.unit(which replaced the bulk of my relays and fuse box).  It says a main fuse is required between the battery and the regulator rectifier, which my research said is built into the alternator, because the m.uunit doesnt protect this circuit.  That said, the m.unit literature says max 40amps for this main fuse, which would conflict with what I assume is my 50amp alternator(based on photos). Being that the m.unit wont be in line with that fuse in any way I figured a 50amp would be ok.  Do you think this is an unnecessary addition to this circuit?

I did actually find info on motobrick about this lightbulb thing when researching about k1100 alternator amperage(unsuccessfully it seems), but it didn't jive with my actual experience not having any load on that exciter terminal for months and months, so I wanted to confirm.  I will try to go the bulb route, but will need to find a good mounting location.  Should the bulb light when the alternator works, or when it doesnt?

[Laitch]

I know it's maybe a little obnoxious that I asked what may seem obvious questions, but I gotta learn somehow right?

You aren't asserting you can't teach yourself, are you? Look, finding resources on the Web is like toilet training. Once you know what a toilet does and what your role with it is, that's pretty much all you need. Think of the Web as the toilet in that scenario except your role in learning is to extract from it rather than deposit into it.

Loathe as I am to involve myself with the m-unit, the m-unit diagram is clear—to me, anyway. If this were my morass, I'd put the blasted recommend fuse into the line between the alternator (regulator in the drawing) and the battery positive connection. I'd wire a bulb to a wire from the D+ alternator connection and ground it somewhere. When the key is turned to On the bulb should light; when the engine is started and revved, the light should shut off. If it didn't work, I'd rip out the m-unit, run it through a shredder, sell the Brick and buy a second-hand Ural Gear Up outfit.

[sooprvylyn]

You aren't asserting you can't teach yourself, are you? Look, finding resources on the Web is like toilet training. Once you know what a toilet does and what your role with it is, that's pretty much all you need. Think of the Web as the toilet in that scenario except your role in learning is to extract from it rather than deposit into it.

Loathe as I am to involve myself with the m-unit, the m-unit diagram is clear—to me, anyway. If this were my morass, I'd put the blasted recommend fuse into the line between the alternator (regulator in the drawing) and the battery positive connection. I'd wire a bulb to a wire from the D+ alternator connection and ground it somewhere. When the key is turned to On the bulb should light; when the engine is started and revved, the light should shut off. If it didn't work, I'd rip out the m-unit, run it through a shredder, sell the Brick and buy a second-hand Ural Gear Up outfit.

I'm in fact not saying that.  In fact, what you describe is EXACTLY what my plan was aftwr the research i had done(w the exception of a resistor over a light, but youve convinced me to use a light). I posted here to get some confirmation and opinion from brick guys because this IS my first complete rewiring rodeo and you guys have centuries of combined knowledge about these bikes(and probably others too). IMO that's a fairly smart use of a avaulable resource for a newb. Oddly there is also conflicting info on the Web about some of this, who'd have thunk.

I do expect some of this type of derision here tho, because it's a manly sub forum, on a manly website, for bmw bikes, frequented by purists and hobbiests. It's like the perfect storm of...  I'll brave the storm to get info tho...and I'm sure I've been guilty of the same snark at times.

[Laitch]

It's okay; pith is often confused with snark. Often it depends upon whether you're the giver or the receiver. 

[sooprvylyn]

communication is the response it elicits

[Laitch]

Hopefully the lunches were good at the seminar where you picked up that one.

[sooprvylyn]

delicious

[Ingo]

OK, the fuse is there to protect the exciter circuit and has nothing to do with m-unit or original wiring. A 3W bulb would draw approximately .25A, so a pretty small current. I personally don't see the need, if the bulb is wired correctly and working it would limit the exciter current to 0.25 A. If the bulb is burned out the current is going to be zero. Wire from Batt+ thru bulb to terminal D+, the negative is provided thru housing (ground). An LED with proper resistance would limit current to something like 0.01 - 0.02 A and probably not work for the alternator. The residual magnetism in the rotor CAN be sufficient to make the alternator work, but this residual magnetism is easily lost when shut down in hot condition. I've had many generators where this happened. Had to advise customers to let genny cool before shut-down. Anyway, the bulb provides resistance to limit the exciter current and ensure proper boot-up of alternator. With engine not running the regulator draws current from battery, once engine runs sufficiently fast the generated electricity is used to control excitation and the regulator quits drawing battery power. Therefore the light is ON when ignition is on and engine NOT running. Once engine runs (maybe after a bit of throttle) the light goes OUT and stays out, unless something goes wrong. I hope this helps...

[daveson]

1. I need to run a fuse between battery and alternator on a 1993 k1100rs.  I have read that the alternators on these were once 32wattamp but got upgraded to 50wattamp. Is there a way to tell which one is on my bike?  That said, is a 550amp fuse appropriate(on a 50 watt alternator)? The math says there should be about 4.17 amps running thru the wire, so 5amps makes sense right?Or would I be better off with a 55 amp or 60 amp fuse?

I don't think you do need a fuse between the battery and alternator because I don't think the bike has one originally.

[daveson]

That said, the m.unit literature says max 40amps for this main fuse, which would conflict with what I assume is my 50amp alternator(based on photos). Being that the m.unit wont be in line with that fuse in any way I figured a 50amp would be ok.  Do you think this is an unnecessary addition to this circuit?

Yes on the face of it that does seem odd, when you look at it that way, previously I made the same mistake. Yes it is an unnecessary addition, but I think it's a good one, and worth doing. It would be better to have a fuse before the input than a fuse after every output.

The m unit is in line with that fuse in the above diagram.

Notice that the suggestion is not to use a 40amp fuse, but a maximum of 40amps.

I did a similar thing with my K100. I installed a circuit breaker (but didn't include the alternator) to reduce the chances of  previously unprotected wiring from setting the bike on fire in case of a short circuit occurring.

I installed a 40 amp circuit breaker because that was just a bit higher than the total load I measured. I deliberately tested it with a dead short, but the circuit breaker wouldn't trip, because it should be selected based on the maximum load on the wiring, not the maximum load on the bike. Believe it or not the solution was to install a 15 amp circuit breaker. The solution, and the reasons, were given to me in a thread called "Quick fix, circuit breaker, save your brick from burning" in the Project Classic Motobricks section of this forum.

[Ingo]

Also: the wiring diagram does NOT show the B+ connection from alternator to batt, this would be the heavier wire that carries the generated power to the battery for charging and supplying 'lectric to bike... This is usually not fused...

[sooprvylyn]

Exactly the info I was looking for. Thank you, gentlemen.

I will probably take the advice to add a small fuse around 15--20amps in line w the m-unit, even though it's probably not necessary.

I purchased an LED battery indicator for the B+ to match my oil pressure and water temp indicators, and will add a small  resistor(50 Ohm because a 3w bulb on a 12v circuit draws 48 Ohms) in serial w the battery indicator light to get sufficient load to excite the alternator.

I'll also give that referenced post a read through.

Much appreciation!!!

Edit: After reading through the "Quick fix, circuit breaker, save your brick from burning post", I see that the primary concern people have is old wiring, or wiring that has been rubbing against various parts of the bike.  Im less worried about a fuse now as ALL of my wiring to the components mentioned in the post are brand new, sheathed in added wire protection, and fixed such that they cannot rub in any significant way that they are likely to suffer any physical damage.  My wiring is also fairly simple compared to the oem system and also pretty exposed and easy to inspect for wear.  That said, it is the starter relay that seemed to the of highest concern in that post, and a fuse in that line is less ideal than a manual breaker. The problem with starter relays(stuck/welding closed) appear to be caused by low voltage, and thus amperes generated by low voltage wouldnt likely set off any fuse big enough to withstand actually starting the bike.  I will probably do the manual cut-off to the starter option since it appears to be the more logical thing to do.  THe m.unit itself, per the literature, doesnt require any fusing as it has all fusing built in, so fusing before the m.unit seems pointless...and wouldnt protect the starter system at all in the event of a stuck relay since the m.unit only feeds the electromagnet to trigger the relay, but wont stop the load passing through a stuck switch. 

If someone sees something wrong with this logic, please do alert me to my mistake.

Edit 2: Per ChatGPT, the 50Ohm resistor must be wired in parallel in order to work with an led bulb to generate sufficient load on the alternator exciter circuit.

[daveson]

 I will probably do the manual cut-off to the starter option since it appears to be the more logical thing to do.  THe m.unit itself, per the literature, doesnt require any fusing as it has all fusing built in, so fusing before the m.unit seems pointless...and wouldnt protect the starter system at all in the event of a stuck relay since the m.unit only feeds the electromagnet to trigger the relay, but wont stop the load passing through a stuck switch. 

I don't think of it so much as an either-or situation, but more like two separate problems with two separate solutions. It's a good idea to have a battery isolation switch in case of a stuck starter relay as well as for a number of other reasons.

It's also good to have a main fuse, or circuit breaker, to prevent damage in case of a short, even if the wiring is new, that's why they came with fuses from new. Spose for example you're involved in an accident, the ignition is on, you're on the ground, there's leaking fuel, and a short, that's not looking good. I've been reading two brick forums for about five years and I can think of seven cases of wiring melting caused by a short, sometimes resulting in a fire. Haven't heard about one for a long time, maybe we're due for another brick on fire. Pretty rare to hear about fires in modern vehicles, but heard about heaps back in the days.

Dunno nix about m units, so I usually avoid threads which mention them. Since they recommend to use a fuse, I think you should. If it has built-in fuses, I'd be thinking of having fuses in the outputs as well, if the built-in fuses are hard to replace.

[The Mighty Gryphon]

Don't fuse the cable from the alternator to the battery.  There is the possibility of a current surge blowing that fuse.  When that happens the next thing to blow will be the diodes in the alternator.  You don't want that to happen, $$$.

[Ingo]

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