Roadster not charging

How old's the battery?

Hi Paul,

Has anything else changed coincidentally with this situation? have you had the tank off, changed the handlebars, been poking around in the headlamp? sounds like a simple wiring fault to me.

Do you still have your original rectifier and zener diode? might be worth re-fitting them and seeing what transpires. You should get 13 to 14v across the battery at about 2500rpm

Check the stator output. I think you should be getting approx. 15 volts AC when revved, across any two wires. Also check your earth connections.

I recently had a similar problem after replacing the primary on my bike. The alternator output measured OK but there was no output from the Powerbox. I thought it was a faulty Powerbox, but no! Turned out I had trapped an alternator wire, grounding it to the chassis. Worth checking in your case?

David

Use a multimeter (very cheap if you don't have one) to check the voltage output from the power box. Should be approx 14.5 volts. I have just had a similar problem - turned out to be a corroded alrernator connection plus the main feed wire adrift at the light switch - always more fun when 2 things fail at once.

Substitute the battery if you can get or have a spare. Having just installed a new rectifier / regulator, I had to make new a new loom from the alternator to the rectifier as there was so many connections on my MKIII that all seemed to be connected to the alternator output - it was easier to run new than check them all for continuity and corrosion. After much testing and though a lot pointed to the battery, replaced it and now it is astonishing how much effect the old battery was having on the charging circuit.

I believe that the anti-sulpahting style batteries fail by increasing their internal resistance, so despite failing to accept and hold charge they also incease the loading of the whole electric circuit - after all, it is just another component in the circuit and they are most likely to fail in this modern age.

Hi Paul,

The problem with measuring the AC from the alternator is that your voltmeter will be measuring the RMS voltage which is only valid if you have a sine wave. You will not be getting a good sine wave out of your alternator.

Having said that I would expect to see a bit more than 10V AC.

Try disconnecting the alternator and connect a headlight bulb across the output, not a brake light bulb or similar, you need something that will load the system.

Start the bike and the light should be fairly bright at tickover, take the revs up a little and it will get brighter, take it up more and you will have a blown bulb !

If it is dim then your alternator needs investigating. If it is bright then you need to start working your way through the wiring. You say you have swapped the Boyer power box and the battery. it does not leave a lot left.

Regards

Tony

I'm positive (electrical joke) that's too low. As stated previously DC charging voltage is approaching 14 volts. Looks like you need a new stator.

You should be looking for a very low resistance between the two stator wires, ie less than 1 ohm. ( disconnected from the bikes wiring ) There should be no continuity at all between either wire & the metal body.

Having satisfied yourself that all is ok, test with the headlight bulb as Tony has described earlier, then also connect the bulb between one wire & earth, then the other wire & earth, you should get no light at all. If ok, reconnect the stator wiring.

Next step is to disconnect the output from the powerbox & connect in an ammeter, start the engine & you should see the ammeter register a charge, turn on the headlight, rev to about 3000rpm & you should see about 9amp charge for an RM21.

If you see significantly less than 9amps, it would suggest failed stator windings or, more likely, weak rotor magnets. A crude check for this is to see if each magnetic segment will hold the weight of the rotor, use a screwdriver or similar to pick the rotor up, if any segment will not hold itâs weight, itâs almost certainly the root of your problem. Even if it passes this test, if itâs more than a few years old, a new one will invariably improve charging outputs.

Regards, Tim

The only problem I've had with the rotor was the centre becoming loose from the magnets. This was after 38 years and 145,000 miles. Could be wrong, but I reckon it's worth changing the stator first, after checks, before splashing out on a rotor.

Previously simon_ratcliff wrote:

The only problem I've had with the rotor was the centre becoming loose from the magnets. This was after 38 years and 145,000 miles. Could be wrong, but I reckon it's worth changing the stator first, after checks, before splashing out on a rotor.

Worth checking the air gap between the rotor and the stator too. If the stator has been rubbing on the rotor through misalignment it will have worn a part of the stator away and increased the air gap, hence reduced output. Check for bits of black debris in the chain case.

I'll be happy to admit I was wrong and Tim was correct with his diagnosis. Every day's a school day.

Higher output is obviously better but you'll need a compatible regulator/rectifier to suit the stator which could be single or three phase.

Paul, Sounds like youâre on the right track, always good when you can test by substitution. Re high output alternators, as Simon says, if you go three phase you will need to change the powerbox as well & often the higher output single phase units only provide more watts at high RPMâs, at lower RPMâs they may actually provide less than an RM21. Good if you ride at speed/higher RPMâs most of the time but if your riding is mostly pottering around back lanes or in traffic, the RM21 may be a better bet.

I actually run an RM21 on my electric start in conjunction with a high cranking amps battery as I have the bike geared for very relaxed cruising & found that with lights on I had to ride like a lunatic or hold third gear for the RM23 to keep the battery topped up. The RM21 is coping much better as it delivers more watts at lower RPMâs & I use the electric start pretty much all the time. An RM21 in good fettle should be easily capable of looking after the needs of a battery that only has ignition & lights to put a demand on it.

With regard your charge light, what do you have? If you still have the old assimilator, Iâm amazed itâs still doing anything & I wouldnât take too much notice of it, amongst the many thing they donât like are heat & vibration so Norton in their wisdom hung it in a spring right above the cylinder head. Interestingly, Thwaites use exactly the same unit on their dumpers of the period, they bolt them to a substantial steel bulkhead away from any heat & with minimal vibration & Iâve only know one to fail in that application, & that was so old the case had virtually corroded away & the damp had got to it.

Regards, Tim

Paul,

Please measure the diameter of both Rotas. The air gap has only to be slightly enlarged for the charge rate to drop off.