1959 Model 50. The bike is 12v positive earth. It has a new RM23 alternator fitted. Voltage recorded at the battery is about 12.5 at rest and over 14 when running the engine at around 3000 rpm. I think this proves the alternator is working. However, the bike's ammeter never shows a charge. It flicks to negative amps when the ignition is switched on and doesn't move from there when the engine is revved. If the headlight is switched on it shows a negative 6 amps.
The Lucas Regulator/Rectifier was purchased from Paul Goff at the same time as the alternator and, when installed, did work as it should with the ammeter showing a positive reading when the engine was revved and totally compensating for the drain of the headlamp. All wiring checked. So, what has gone wrong ? Since the installation I have inadvertently completely flattened the batterry twice by leaving the ignition on. Can this damage the rectifier ?
How can I check the rectifier is undamaged, possibly using a multi-meter ? I'd prefer a check before purchasing a new rectifier and then finding the problem lies elsewhere !
Mike Haworth
Could be different causes…
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If the battery voltage rises…
If the battery voltage rises to 14v when running then without any doubt the system is charging .If its not showing a charge on the ampmeter that's usually either the regulator doing a good job and the battery is charged or incorrect wiring. A really good battery will eventually rise to about 14.4 volts under charge, but drop to 12.6 or thereabouts if left for a week ot two. They are supposed to be fully charged and will sulphate and become weak if left in a low state of charge. Showing 14v means the rectifier must be working too.If all is correct when you leave the lights on for a spell and start the motor you should see a charge till the battery picks up enough charge to bring the voltage up to the point when the regulator (or Zener) says enough ,hopefully at about 14.4v. Its possible that you have a reduced output so the battery is not getting enough amps to fully charge it ,
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As Robert says,
both the alternator and regulator seem to be working given the 14 volts showing. If nothing has changed (eg wiring connections) then the ammeter may be at fault - pattern items are of pretty poor quality.
If your digital multimeter has a 10 amp ammeter option, connect this in series between the battery live lead and battery terminal (obviously disconnecting this first), then switch on and start the engine. You should see a current in one direction before you start then this should change to the other direction proving charge is occurring.
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Could be different causes. Likely the emptied battery has probably died. I've never figured out the failiure mechanism of a battery. They fail in different ways. In my experience, a correct showing motorcycle ammeter is almost as rare as hens teeth. You didn't say if you have a new rotor. If the battery don't take charge, a healthy alternator will heat the reg/rec more. If not sufficiently cooled, it could be damaged. I should try with charging the battery with a battery charger. When done. check battery voltage after an hour. They usually show more than 13.1V directly after disconnecting the charger. After about an hour, expect more than 12.8V. A couple of days later, voltage should not have dropped significantly..