AJS 31 battery charging issues

[Geoff Sharpe]

Hi,
I would be so grateful for any help and advice with an electrical issue on my 1962 registered Model 31, which definitely has a charging problem. It has let me down on several occasions now, not too far from home resulting in sweaty work-outs to push it back. The battery is in good order when I set out but is flat when I get it back home. I really want to enjoy it on the road but it needs fixing first.

It has a battery/coil ignition system with charging (or not) provided by an alternator. It has a 12 volt conversion with a Boyer Bransden Power Box doing the rectification. It does not have electronic ignition.

When the engine is running the ammeter never goes into the positive side, and I'm gaining more confidence that the ammeter is telling the truth.

The alternator appears to be working to an extent. I don't know whether the output from it is sufficient but I've measured 10 Amps AC in the output wires with the engine running.

I've recorded a whole load of other measurements but the thing that intrigues me most is this one:
Engine not running – ignition switched on:
DC Current measured in the charging circuit = -2.3 Amps (initially -3.1 Amps)
Is this normal behaviour? It seems to me to be a lot of negative current to be offset by the charging system even before starting the engine.
Obviously it's worse with lights on - DC Current measured in the charging circuit = -5.5 Amps

All ideas and opinions will be gratefully received.

[clive]

Cannot help much except to say you can test the ammeter by seeing if it goes negative when you turn on the headlight, if it shows about 4 amps discharge its probably to be relied on although they do have a habit of flapping from positive to negative as the vibrations rise. However to help those who do know about alternators, power boxes and coil igntion etc it would help to know has this fault just started, or have you only recently bought or got the bike running. My thinking is that the power box or alternator may be wrongly wired but if you know it was working before then one of the items failing is more likely.

[Geoff Sharpe]

Hi Clive,
Thanks for responding. As I've said, it goes even more negative when the lights are switched on, more like 5.5 Amps.
I've only owned the bike for just over two years, and I think it has always had this issue, but it seems to be getting worse in terms of not being able to go very far before the battery gets drained.
It has been off the road for about the last 12 months due to it suffering a more serious breakdown which included blown head gasket, but work commitments meant I could only devote time to fixing it recently. I'm now semi retired.
I've thoroughly enjoyed doing all the work so far myself and it's starting first kick every time and running well while the battery permits. This is my intro to classic bikes so I'm learning all the time. I've only been a member of this club for a few months.
Just looking forward to the days when it can go a bit further.
The full story of the measurements I have made are as follows:

Engine not running - Voltage across battery terminals (fully charged) = 13.13v
Engine running - Voltage across battery terminals = 12.4v
Engine running - Voltage across battery terminals with lights on = 11.84v
Engine running - Voltage across Power Box output wires = 15.19v

Engine not running – ignition switched on:
Current measured in the charging circuit = -2.3 Amps (initially -3.1 Amps)
Is this normal? It's quite a lot of negative current to be offset by the charging system.

Lights on - Current measured in the charging circuit = -5.5 Amps

Engine running - Current measured in the charging circuit = -1.5 Amps
Lights on - Current measured in the charging circuit = -3.9 Amps
Full beam - Current measured in the charging circuit = -4.3 Amps

I hope I haven't bored too many people. Sorry if I have.
My dilemma is, should I start shelling out my hard earned on replacement parts for the alternator or could it be caused by a fault somewhere in the wiring.

[dave16mct]

Very difficult diagnosing electrical problems from afar but could you give more info:
what alternator have you got? eg is it encapsulated or exposed coils? 2 or 3 wires? What are you testing with , Multimeter? I've used a Boyer Powerbox for some time with no problems and it has its own internal capacitor, so it can be used instead of a battery (once charged up) So if your battery went flat I think you could have removed the fuse and started the bike up to get you home.
Did you do the conversion to 12volts? If your alternator has 3 wires, did you join the correct 2 together to give full output? Did you rewire or did you adapt the old loom?
Dave.

[Expat]

Hi Geoff,
Afraid I can't be much help other than saying with my recent issues I found that with ignition on, engine not running, you can expect to see around a 2amp discharge when CB points are closed and zero discharge when they are open.

Have you tried turning engine over slowly to open CB points and observe the above?

These numbers are from my 6 volt G3 so will prob be different for your 12 volt system.

Good luck

Steve

[Geoff Sharpe]

Very difficult diagnosing electrical problems from afar but could you give more info:
what alternator have you got? eg is it encapsulated or exposed coils? 2 or 3 wires? What are you testing with , Multimeter? I've used a Boyer Powerbox for some time with no problems and it has its own internal capacitor, so it can be used instead of a battery (once charged up) So if your battery went flat I think you could have removed the fuse and started the bike up to get you home.
Did you do the conversion to 12volts? If your alternator has 3 wires, did you join the correct 2 together to give full output? Did you rewire or did you adapt the old loom?
Dave.

Hi Dave,
The conversion to 12 volts was done before I bought it. It is a 2 wire alternator. When I had the primary chain case off I think I noticed it said Wassell on it. The coils are encapsulated. Photo attached.
I'm testing with a Signstek clamp type multimeter bought from Amazon. It can measure AC and DC Amps without breaking into the wiring. It seems to work well, and appears to prove the ammeter readings.
I only learnt a few days ago of the trick of removing the fuse to start the bike. I tested that on the drive, but still not sure if you'd then ride home with fuse removed.
Thanks for your interest.

IMG_2783.JPG

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[Geoff Sharpe]

Hi Geoff,
Afraid I can't be much help other than saying with my recent issues I found that with ignition on, engine not running, you can expect to see around a 2amp discharge when CB points are closed and zero discharge when they are open.

Have you tried turning engine over slowly to open CB points and observe the above?

These numbers are from my 6 volt G3 so will prob be different for your 12 volt system.

Good luck

Steve

Hi Steve,
Thanks for that, very interesting.
I'm not at home now for the next couple of days so can't check, but I'm pretty sure the 2.5 Amp discharge is constant when cranked over slowly. The bike is so eager to start since my work on it that it's not an easy test to do. Perhaps I need to take the plugs out?


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[Duncan]

Geoff,

Off at a bit of a tangent, looking at your picture there should be a plain clutch plate surface under the clutch pressure plate, depending on the clutch type/year this will either be a bonded one side or completely plain plate.

[JohnHewes]

I would think that if you have 15v at the Boyer out feed then the alternator and Boyer are working OK. For some reason this is not reaching your battery and that is where you have to find the fault.
All your current negative measurements prove is that the amp meter is connected the right way round.
You need to check against the wiring diagram for an alternator bike, substituting your Boyer output for the rectifier output.
It will be a balls aching job! Get the tank off and the headlamp reflector off and try tracing the wires and comparing them with the diagram, not easy but I feel sure that is where your problem lies.
My G3 has a by similar arrangement and it works really well, very pleased with it, but it also took some sorting out at first, traced to a bad connection I had made in the headlight.
Good luck! John

[Reynard24]

The symptoms you describe are very similar to a power box failure I saw on a friend's lightweight although I never measured the current draw just the voltage across the battery, alternator and power box outputs.
On that bike the power box output off-load i.e. open circuit was around 15v but this dropped to 12V when the box was reconnected. As far as I can remember the alternator output was around 35VAC open circuit. I had a serviceable power box to substitute and this cured the problem and is still working.
Boyer used to offer a testing service or you could try one of the A-Reg boxes as supplied by the spares scheme which are about half the price of a Boyer but work just as well - I've used two of them on my bikes.