Hello everyone, I'm so please to have found this forum and to be accepted as a member.
I have recently aquired my first british bike a BSA C15F 1965
If anyone can please help me with my problem I would be very grateful, the bike seems to be overcharging when idling - Im getting around 8-9v on the battery (its a 6v wired bike).
Some details about the bike, new wiring loom, new stator, coil, reg /rec and battery (sealed 6N63B Powerline) all fitted by previous owner.
There is one wire from the reg /rec that is not connected to anything and it the black one and has 6.5v with ignition on.
There are 5 wires on the reg /rec altogether and are as follows
Red is wired to frame (Positive earth)
Pink is wired to 2x Black / Green wires on loom
Green is wired to 2x White / Brown and there is a 6v on these even with ignition off
Yellow is wired to White / Green
Black not connected.
So my main question is what does the black wire do as I cant see any other reg / recs that have 5 wires inc a pink one?
secondly, is 8-9v too high on idle? it does go down a bit with the lights on though. The ammeter is also to the far right on idle too.
Thanks for any any help anyway
Steve
How many wires are coming from the stator , should be 3.
Is there a name on the reg rec or invoice from the PO
Certainly , your 8 or 9 V are way too high , it will kill the battery.
Try to find the wiring diagram for your reg rec and post it please.
The alternator coils can be wired in parallel or series or a mix of both to vary the voltage and output power. The problem is that modern replacements don't' always have the same wire colours or number of wires as the originals so it may take a bit of working out to match them to the 1960's circuit diagram. Do you have any identification on the alternator stator?
Most regulators have 4 wires, 2 connecting to the alternator (often both yellow) and 2 output wires, usually red and black connecting to the positive and negative bike connections (earth and battery) Some have a wire loop that gets cut to change from 6V to 12V, or maybe that's the other way round.
A 2 wire alternator connection is simple but a 3 wire alternator requires 2 of the alternator wires to be connected together, either green/yellow and green/black OR green/yellow and dark green depending on the alternator. These are connected to one of the regulator input wires and the green/white from the alternator to the other one.
However your alternator appears to have 5 wires; 2 x black/green, 2 x white/brown and a white/green.
More information about both the alternator and the regulator would help.
The only rectifier/regulator that I can find with wires as described are replacements for a Honda MTX125 (shown below).
I'd bin that for start and get a known quality one. 6V reg/rectifiers are hard to come by, most are 12V but this is one : https://rexs-speedshop.com/product/6-volt-solid-state-regulator-rectifier/ (https://rexs-speedshop.com/product/6-volt-solid-state-regulator-rectifier/)
My understanding is that a C15 would not have a regulator, I don't recall one on my B40.
Thanks for helping everyone, i work nights so dont always reply during living hours.
OK, stator wires there are only 2 - green/black and white / green - no info on the make or wiring for reg / rec but it looks new. in fact does look like the one posted by derek above.
Not sure what stator it is but its new and a lucas with new rotor, just 2 wires as i mentioned.
Ill try to get info on what I have - im inclined to replace the reg with a more standard one.
Thanks Steve
this is the stator and rotor -(https://i.ebayimg.com/images/g/VWUAAeSwR~BpoG~Y/s-l1600.jpg)
Hi,
C15s and B40s along with many British bikes had rectifiers fitted as standard. I think they are referred as 'bridge rectifiers'.
The modern equivalent is the rectifier-regulator which seems to do a good job.
Check out Paul Goff for a 6v one.
Dave.
Yes good point, the C15 / B 4 had a rectifier and no regulator. Later you get the Zenner.
The original 6v wiring is complicated, with different coils coming it with different switch positions etc. I did understand it for about fifteen minutes about six months back.
Looking at your picture, that certainly isn't an original 6 volt alternator. I think it highly likely that it is a 12v alternator. (Two wire stators are 12v. Three wire are 6V unless two are joined...)
If the bike has been converted to 12V then the presence of a rectifier/regulator makes sense.
I'm fairly certain that you cannot just bung a 12v stator into a standard 6V loom and simply stick a 6v regulator rectifier in. I had to make my own loom - but I was converting the electrics to 12v.
Has the bike run successfully as configured for a good while? Or did you buy it with a known electrical issue. (i.e. 'just needs the wiring finishing...?')
If the previous owner made up his own loom, did he provide a wiring diagram?
It's a new Lucas loom and was sold as a runner. Like I said everything is new, reg l, coil, battery, stator and rotor...so instead of replacing the new stator for 6v, am I better off going for 12v system ?
Btw...the previous owner has said it's done around 100 miles since he restored it
I'm more familiar with the C11G -also 6v alternator. There were some quirks to the wiring:
1 The erg setting on the ignition switch that fed the coil straight off the alternator so you could start with a flat battery.
2 One of the alternator coils routed though the light switch so you got a higher output when the lights were on.
What I do remember is that the battery was either flat or over charged so I'm not sure the system ever worked that well.
There are better ways to wire up the system now.
Looks like you got lucky with that alt.
Almost certainly will produce enough juice for a 12V conversion, which makes the wiring a lot less complex. Buy a recommended 12V reg/rect, connect the output to earth and battery and you're good to go.
(Over-simplified I know, but that's the essence of it)
Hi,
12v is the way to go. The standard 6V stator can be used for 12v, unless you want lots of output, as it is the battery, reg/rec. that make the difference.
Neg. earth could be done at the same time.
It looks like there is a 10A on the stator so is it 10A output?
Dave.
If its a 2wire stator then it highly likely already 12v , hence your high idle charging voltage.
There are plenty of decent 4 wire reg rec available to match.
Maybe look at your ht coil to see if it has any markings indicating 6 or 12v
Finally , I can't remember what battery you said you had as any conversion to 12v will dictate a new battery potentially
And bulbs of course , led?
It sounds like the previous owner has tried to use 6V regulator from a Japanese bike. Many of these use a sensing wire from the switched side of the igntion, this is used to help regulate the voltage. Without it the output is usually too high.
The better solution, as suggested by the others, is to use a modern regulator/rectifier, upgrading to 12V at the same time.
The attached drawing shows the basic circuit for a 3 wire alternator with +ve earth. For the 2 wire alternator simply connect the 2 alternator wires to the 2 yellow input wires of the regulator and the regulator's output wires directly to their corresponding battery terminals.
You can ignore all the wires used to regulate the charge that go to the lighting switch, leaving them disconnected.
Thanks for all the advice,
Im confused that there can be 6 and 12v stators when they output AV volts though?
So is it the stator that is creating the overcharge then, due to the fact that everything else is 6v ? I have checked and bulbs are 6v too, there are no markings on the reg/rec or but the coil is definately 6v lucas.
So how much do i need to replace to make this 12v, is it the reg / rec, coil, battery and bulbs ?
Thanks again for all your help
Steve
2 wire stator are 12v .
If everything else is marked 6v , then cost out a replacement 6v stator , from one of the reliable suppliers .
( when I bought my 12v , 2 wire stator to convert my C15 to 12v it was about £90).
12v Ht coils are around d 20 to 25
A 12v reg rec is roughly 30 to 50
Bulbs , well 5 each give or take.
A 6v hooter will work on 12v , just screams a bit
It may be cheaper to convert to 12v.
Finally , get your multimeter on AC volts , disconnect the 2 wires from it to the bike , start it up and check the AC output whilst running, idling , no more than a fast idle will give you 30 or 40 v AC
See what you've got , before spending cash
The big advantage of going 12V - to get a decent headlight, is less now that there are very good 6V LED bulbs. If yu can get the 6V working as it should then with LED bulbs there's no rush to go to 12V.
OK, just started it up and have 35vac on the stator wires - so does this indicate a good stator - just dont see why this can be defined as either 6 or 12v if they both output 30-40vac then surely any one can be used and its the reg/rec that is perhaps wrong. I dont think im understanding all this correctly :-)
Steve
They are 6V or 12V depending on the spec of the stator winding. Obviously either/both will output over 12V in use, hence the regulator.
IMO trying to revert to a 6V system would be a bit of a retrograde step; 12V batteries and bulbs are more readily available.
I dont particularly want to change the stator, it seems new and is outputting the correct voltage according rhyatt post above.
so at the moment heres what i know -
stator is 12v and good (i have no guarantee its 12v but a previous post sugests that 2 wire is 12v)
Reg / rec is unknown, 5 wires which is odd
Battery, bulbs and coil are all definatly 6v
so is switching to 12v as simple as replacing battery bulbs and coil?
Pretty much yes, I've got one bike on 6v and swapped the other to 12v. Its really depends on what components you have to hand. 12v coils and bulb are easier to get hold of. Get an ID on that regulator, it can probably be wired for 6v or 12v. Get that right and the rest is easy.
To be honest, going to 12v is a sensible move.
I had an extra thought re the potential conversion, what ignition do you have ? Cb points , condenser etc ? If so, then these will work fine with a new 12v coil.
It is frankly , a simple job, contact someone like AO services for a 12 v reg rec, he sells decent stuff and is helpful if needed.
Take the opportunity to make a nice mounting plate for the reg rec under the seat on the frame.
As mentioned by someone else, join the 2 stator wires to the 2 yellow RR wires and then arrange a fuse in the battery connection and a decent earth connection.
Change the ht coil , and ensure the polarity is correct (- on coil to the points if you choose to go negative earth)
Change the battery of course
Change all bulbs , decide upon led if you wish.
Kick start and check charging voltage around 14v
Anyone and anything but the AO Services item!
I suggest the DVR2 unit, which is far better and more reliable.
I may be wrong , but isn't the DVR2 for dynamos , not alternators?
There are a multitude of reg rec on the market , 4 wires , 2 x yellow 1 x red and 1 x black .
Ranging from 10 or 12 quid to 60 quid at Rex's speed shop.
Wassell unit in the middle for 35 and loads of cheaper ones on ebay.
Plenty from which to choose
You're right!
I think when I see "AO Services" it triggers bad memories.
Never had an reg/rect from him and they may be good, but I wouldn't buy it on principle. ;)
Quote from: limeyrob on 23 May, 2026, 20:31 Pretty much yes, I've got one bike on 6v and swapped the other to 12v. Its really depends on what components you have to hand. 12v coils and bulb are easier to get hold of. Get an ID on that regulator, it can probably be wired for 6v or 12v. Get that right and the rest is easy.
I think he'd be better chucking that regulator in the bin and replacing it with one which has known parameters and connections.
Quote from: Stevieo on 23 May, 2026, 20:02 OK, just started it up and have 35vac on the stator wires - so does this indicate a good stator - just dont see why this can be defined as either 6 or 12v if they both output 30-40vac then surely any one can be used and its the reg/rec that is perhaps wrong. I dont think im understanding all this correctly :-)
Steve
It is the regulator that sets the voltage, not the alternator. The regulator achieves this by burning off the unwanted energy as heat. So it is essential that the regulator is mounted where it will get cool air and preferably attached to something that will act as a heat sink.
Yes, regarding regulators. Dynamos and alternators with brushes and coils in the rotor (like car alternators) can be regulated by varying the current in the armature or rotor so they only produce what is needed. Our alternators are fixed with permanent magnet rotors so its impossible to vary the output other than be changing how the coils are connected. So we have to burn off unwanted electricity which is what the Zener diode does. So there's a lot of heat. It actually makes sense to run with the lights on since that is also burning power off.
I used a small square of aluminium sheet about 4" x 4" and bent a 1" angle on 1 edge.
There's enough bolt holes on the transverse channel the runs between the 2 seat frame tubes and mounted it there with the RR in line with the direction of travel.
Easy run for it's wires there and cool enough
There is a modern trend towards series regulator which work by restricting the current from the alternator and have the advantage that they don't get hot. Shindengen make a range of MOSFET regulators, such as the FH020AA, featuring this and are used on most current bikes from Honda, Yamaha, Ducati, etc.
The attached picture from their website demonstrates the difference. Incidentally, you can use a 3 phase regulator on a single phase system, just leave nothing attached to the 3rd input terminal.
Thanks for the continued support everyone, Ive decided to keep the stator and get new reg/rec coil battery and bulbs all 12v - ill be looking for a standard reg with 2 yellow, a red and a black wire - this seems to be the most common.
I still dont fully understand where the overcharge is coming from but im hoping its the fact that i have a 12v stator being used with a 6v reg? We will see
Well, I for one have learned quite a bit in the course of this discussion so it cuts both ways!
I think all of us who are familiar , experienced, scarred by old British bikes will have learned the lesson: It may look obvious and only have a few moving parts but that does not mean its simple to diagnose a fualt. Quite the opposite. Despite the logic of complexity it only takes a few parts to cause an impenetrable problem. Amal carb, I rest my case. :)
That's the beauty of forums.
Like minded people sharing their knowledge and thoughts.
OK, a small update, i have removed all bulbs, coil, battery & regulator - no id or the regulator btw.
I have all 12v parts on order so will arrive by the end of the week.
One thing I have read is that now that I'm switching to 12v, the stator can be wired directly to the regulators 2 yellow wires, whereas before, on 6v they were wired into the loom. so this will bypass the emergency circuit and anything else not required for 12v operation. Also, the DC wires on the regulator can also bypass any loom connecters and go straight to the battery terminals. At least I hope this is the case, if anyone has any further knowledge on this then Id love to hear it. Thanks
Steve
*edit* it actually appears that derek996 has already mentioned the direct wiring on the previous page, must have missed it.
pretty much, although if you by pass the loom , you will bypass the ammeter , so you'll need to integrate the reg/rec output via the ammeter wiring (only the red output wire of course, the black goes to earth NEG).
When converting my C15 from 6v to 12v I took my brand new loom apart and altered it substantially to remove all the redundant wires (and to run separate earth's to all components - which is a personal preference).
I am not at all sure whether you can run a 12v system using the standard 6v loom. I guess that redundant wires for the emergency position (and the extra lighting coils) will have no power input, and therefore won't cause any issues - but you would need to check that through very carefully - for every switch position.
Yes with a regulated 12V set up a lot of the complexity of the old style 6V is not needed. They were regulating by switching more coils in for lights and you had "emerg" which implies they were expecting flat batteries. You can wire the regulated 12V like any other modern vehicle with the regulator output going to charge the battery on a single wire.
If the alternator wires are connected directly to the regulator and the regulator output wires are connected directly to the battery and ground then all the wires which previously regulated the charge are effectively disconnected and are redundant.
You are right. That does make the 'conversion' very simple. (As my bike was in 1000 pieces and I was wiring it from scratch, making the loom 'bespoke' was an easy choice. So I never really looked at how you would convert an intact, running bike.)
Thanks again and thanks for posting the 12v wiring diagram too. I was going to disconnect and cap any wires not required on the ignition switch and light switch but it seems is not required as they just won't get used when bypassed
One thing I would urge you to consider is adding a fuse into the wiring. You will find a long debate about where a fuse should go - mine is on the brown/blue wire a couple of inches from the battery....
Just looked and thats exactly where there is a fuse by the previous owner, brown/blue going to battery neg.
thanks
One thing some people do is to replace the ammeter with a voltmeter, this makes a lot more sense with an alternator and some have fitted a small digital voltmeter in an old ammeter case.
Thankyou, good idea about the voltmeter.
Well i have an update and its not good, installed new battery, coil and reg rec, all 12v - no bulbs yet as they didnt arrive.
All wired in as standard except for the stator going directly to the regulator.
There was no spark at all, fitted the old 6v coil albeit in a now 12v system and got a good spark again, and also started.
So have I gone and bought the wrong 12v coil - mine seems to have liquid inside it, oil maybe? and its a fair bit bigger than the old one too, has 2.9 ohms across the terminals, its this one - https://www.ebay.co.uk/itm/356763950400 (https://www.ebay.co.uk/itm/356763950400) - btw the new 12v coil also gets very hot too and def wired up ok.
If anyone had a recomended 12v then id be glad to hear what you think,
thanks
Steve
https://www.ebay.co.uk/itm/331633479355
I spent quite a bit of time researching coils, as my electronic ignition system (pazon) has specific requirements. I ended up with this one - basically what BSA would have been fitting on 12v bikes. (Except that it is now no doubt built in the Far East.) Its working perfectly.
Thanks for that..I've got that ordered now and will post back if it's any better.
Im so sorry for dragging out this thread but im a bit confused, I dont have the new lucas coil just yet but im really beginning to wonder about the coil wiring,
previously in the running 6v system (albeit overcharging) the wiring was as follows-
coil positive - 2 black and white wires, one to ignition and one to condenser / points
coil negative - 2 white wires, one to ignition and one to another switched supply, brake light switch i think.
Now for the 12v wiring when my new coil comes I was planning on the following -
coil positive - 1 black and white wire to the condenser / points
coil negative - still 2 white wires to the same place as the 6v.
the only difference being there is no black/white to ignition as this is for the emergency position (I have capped this off at ignition along with the black and the green/yellow for alternator.
my problem is that google ai keeps saying its recommended to connect the coil positive straight to the frame, how can this work or is it wrong info?
Thanks for anyone that still has an interest, I hope I get to close this soon :-)
Steve
Upon conversion, your coil is pretty much the same wiring as it was.
Correct , you don't need the emergency wire.
If you've also converted to negative earth , then the wire coming from the points and condenser must go to negative terminal on the coil.
The ignition switched supply wire must go to the positive side of the coil.
Hope that helps