Scratching my head over this electrical gremlin

[rwingsfan]

Ok, back story is my 81 bobber with a Hugh's handbuilt 277 rephaser. Had some running issues that have a least been somewhat rectified and is running acceptable now. Was not charging before and ended up changing out all my wiring, checking and double checking ground. Now everything is good except when I plug in a headlight. When I do it seems to even refuse to charge. I can unplug the headlight and can check anywhere in my system, even at the headlight plug and it is solid voltage and shows charge on the voltmeter. Soon as you plug it in voltage drops badly and won't charge. I have tried a couple different bulbs with no change. I can even put the voltmeter leads in the headlight plug and it is solid with charge, stick a bulb in and all goes to hell. All my other curcuits are good (tail, brake, ignition) This one has me a bit stumped. I have a good solid ground, it is grounded directly to the motor. I guess the only thing I have not tried is running the ground all the way back to the battery, should not have to, should I? Any direction is appreciated.

 

[littlebill31]

Not sure, but mine was kinda doing the same thing. I'd plug the light in and the tail light dimmed. I had another headlight plug, switched it, and then it was fine. I looked at the plug and the connectors inside were really loose and old. Might want to check it.

 

[Purplezinger]

It sounds like a loose or corroded connection. This could be anywhere, harness, connectors, plugs, etc.

 

[pamcopete]

rwingsfan,

Well, you don't mention at what RPM you are measuring the voltage. At idle, the voltage will indeed drop when you connect the headlight. Try it at 3,000 RPM. If it still drops, then I would start by checking the brushes on the alternator, and the slip rings. Brushes at least 3/8 " long. Slip rings copper clean. Next would be to check the resistance of your rotor for 5 Ohms. If it is, like, 3 or 4, it will still produce an output, but will not be able to support the load of the headlight.

 

[rwingsfan]

Sorry guess I was not clear enough. Like I said wiring is all new and simple. Have a pma conversion also. Nothing even close to stock. It also stops charging when the headlight is connected but charges perfect when disconnected.

 

[Patches]

Is the headlight wired directly to the battery on a switch? Or is it tied into the wiring for the charging circuit? If so, that could be your problem. What is your amp draw for that specific light? Have you checked to see if it charges at a higher RPM? What size battery are you running? Is this a stock charging system. Many won't charge at idle.

 

[KeithB]

It also stops charging when the headlight is connected but charges perfect when disconnected.

You don't specify at what RPM you are making these measurements.
Connect the volt meter across the battery and, with the headlight connected, rev the engine up to 3000 and check voltage.

 

[member3480]

I have not used one of those new ALT setups but sounds like a grounding issue. Some head light(aftermarket) are not BOLT GROUNDED you have to add a seperate ground wire. Had a simular problem with aftermarket lite. Bike shut off when light put on. If you have a volt ohm meter hook it to the battery and put it on 24volts dc. Check your voltage before you start bike then turn key on see if voltage drops a lot or a little. I run two fuses so i can take out coils or charging system so i can check which is pulling voltage. If you don't no problem without bike running turn light on voltage should drop but not down to like 6 volts DC if it does check your grounds. I have had a lot of these with bad grounds and they looked good. Also take a wire and ground it and touch the light see if voltage goes up. Bike running at idle will run at 12 volts DC and when you rev it most go to 13 1/2 volts DC or higher.