MK III Ignition Light

I have a 75 MK III, Commando. I am running the original charging stator and a regulator/rectifier. The red ignition light has always come on at idle and gone out just above idle. Turning the lights on seems to make it go out quicker off of idle. I also noticed the light comes back on dimly as I climbed over 3,000 RPM. I have checked the system and it seems to be charing okay and I have checked the ground wires as best I can. Up to now I have just ignored it since the battery is maintaining sufficient charge.

Any thoughts as to an issue and how to stop this?

calbigbird said:

I also noticed the light comes back on dimly as I climbed over 3,000 RPM. I have checked the system and it seems to be charing okay and I have checked the ground wires as best I can. Up to now I have just ignored it since the battery is maintaining sufficient charge.

Any thoughts as to an issue and how to stop this?

Click to expand...

I think your regulator/rectifier probably uses a different method of voltage regulation to the original twin Zener system which may be altering the output characteristics of the AC supply fed to the MkIII type assimilator-causing the warning lamp to glow?



One way to stop it would be to replace the "AC output sensitive" assimilator with an LVS unit

Hi Cal,

The Mk3 OEM assimilator uses a single transistor as a crude comparitor, comparing the battery voltage to the rectified and filtered output voltages (there are two output lines) of the alternator with respect to ground. When the battery voltage exceeds this alternator voltage, the transistor is on and the warning light glows. when the converse is true, the transistor is cut off, and the light extinguishes. If your battery is fully charged, say 12.5-13 volts, then the fact that the light turns off above idle shows the alternator to be working correctly. BTW, a feeble battery and a feeble alternator can also give the same result. At idle, the alternator is providing 10 Hz or so of signal to the assimilator circuit. At 3000 rpm, this frequency rises to 50 Hz, which may be a bit high to be properly filtered by degraded filtering components; the assimilator is, after all, 40 years old. This would cause a false indication of low alternator output, and show up as a flickering lamp at speed. A new assimilator, either OEM or aftermarket, would settle the issue for 50 bucks or so. Many aftermarket units avoid the battery reference entirely, and simply compare the filtered alternator output to a stable voltage reference, say 14.5 vdc, and avoid the weak-battery ambiguity.

Sorry for the verbosity; I've got way too much time on my hands today.

Thanks to both of you for the input. This bike is an ebay escapee that was close to being a basket case when I got it. In the course of the resurrection, I replaced the assimililator (from Baxters) and put on a Podtronics regulator/rectifer. Since there was originally no dash and idiot lights I do not know what was going on before that. I suppose I could just continue to ignore the light and leave things as they are, but I hate doing it that way. Is there a consensus? Should I replace the assimilator with a newer version and see what happens? Should I cut one of the wires to the ignition light? (just kidding)

I've had the same issue with my MK III.New Battery,asimilator,sparx 3ph rotor,stator and rectifier.Light is bright at low rpms and goes out above 2000 rpms.Every now and again the light gets bright for no apparent reason more noticeable at night time than during the day.Its annoying but it happens.