From:Dave Hennessey
Subject:RE: Troubleshooting electrical issue 1951 125S Date:Sun Apr 22 21:48:31 2018
Response to:22635
First, you need to charge up your battery. The Battery Tender Jr. is a good trickle charger - 1.25 amp is right for motorcycle batteries. Will probably takes 24 hours to get fully charged. $28

Next you need to understand that the generator doesn't charge until the bike is going around 20 mph. So you can't just let it idle, you'll have to rev it up about 1/4 of the way.

Once the battery is charged up, start the bike, run at 1/4 throttle for a couple of minutes, then turn on the lights. The headlight should stay bright. Rev the engine up and down, and the headlight's brightness should vary a little with speed. If so, things are good.

If not, there's one thing to check before it gets complicated. The regulator needs to be grounded. There is a little wire on one side of the rubber mounting, which connects the base of the regulator to center mounting leg. This little wire is often broken or missing. Use an ohmmeter and check from the base of the regulator to a point on the engine or frame. You should get 0 ohms or very close to it.

Now you're going to need an ammeter to determine whether the voltage regulator or generator is at fault. $9

Now look in your Service Manual, Section 5E, for the testing procedures. No service manual? Part# 99444-93, available from your local friendly Harley dealer contains a service manual. $40

Here is the section from the Service Manual on testing.
Let us know what you find out.

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