Electrical Problem

[martinb]

I have a 1979 XL250S (6volt) with non-charging battery but headlight (which uses alternating current while the engine runs) is at over 12 volts and blows the bulb. It has a CDI unit (not points) and a regulator/rectifier. The wiring is old and not very clean so I'd like to think it's a wiring problem rather than a component, have you any clues as to which circuit might have the problem?

[StumpyFingers]

It would more than likely be the charging circuit.
Start by checking the output from the alternator.
This should be stablized by the reg /rec to produce the correct voltage at a steady flow.

[martinb]

More clues for the autoelectricians out there (I can't find any poor connections). Voltage across the battery is a steady 6.5V when the motor is revved, suggests a rectifier/regulator problem.

But...Haynes says that between pink and yellow alternator output wires there should be 0.2 to 1.0 ohms, I've got 65 ohms if my multimeter is to be believed. I'm also getting 18 AC volts at the headlamp bulb holder which suggests to me the alternator is fine but the resistance readings are wrong.

What to buy first, rectifier/regulator or alternator coils?

[alanvendy]

don't know the bike but try this.

take the alternator cover off before buying anything and inspect the coils. if the alternator coils are wet they can normally be dried out (either leave somewhere warm & dry for several days or put in the oven on a low heat).and check connections under the cover.
good luck.