Hi, Debbie it could be your idle rpm's are set to low if the light goes out when the rpm's are increased, otherwise you may have a charging issue, so before testing any electrical component in the Charging System it is "IMPERATIVE" that you have a fully charged battery of 12.5 volts or more and be able to pass a proper "LOAD" test if necessary, you may have a preliminary reading of 12.5 volts or more but little or zero amperage, the battery is faulty and must be replaced. AGM type batteries fall into this scenario more so than lead-acid batteries.
1. Check battery terminals for damage or corrosion, check the battery cables at "BOTH" ends for loose, corroded, or broken connectors, "INSIDE" and outside the cable harness, perform connector wiggle test and check cables with an ohmmeter if necessary.
2. To check the regulator unplug it from the stator. Take a test light and clip it to the negative terminal of the battery and then touch first one pin and then the other on the plug that goes to the regulator. If you get even the slightest amount of light from the test light the regulator is toast.
To do this with a meter: black lead to battery ground, red lead to each pin on the plug, start with the voltage scale higher than 12vdc and move voltage scale down in steps for each pin. Any voltage is a bad regulator.
3. On the other part of the disconnected regulator plug. Set the multimeter for Ohms x1 scale and measure for resistance across the pins of the stator. You should read something around 0.1 to 0.2 ohms for a 32 amp system.
4. Then check for continuity between each pin on the plug and frame/engine ground. The meter needle should not move (infinite resistance)(digitals will show infinite resistance) if the meter needle does move (indicating continuity)(digitals will show some resistance), recheck very carefully. If the meter still shows continuity to ground the stator is shorted (bad).
5. Set the meter to read A/C volts higher than 30 volts (the scale setting for voltage should always be higher than the highest voltage you expect or you may fry the meter). Start the bike, and measure from one pin to the other on the plug (DO NOT cross the multimeter probes! - touch them to each other). You should read roughly 16-20 vac per 1,000 rpm.
6. If the battery was good under load test, if the stator is NOT shorted to ground, and the stator is putting out A/C voltage, then the regulator is bad (most likely even if passed step 2)
For more information about your question and valuable "FREE" downloads that you will need for viewing or printing please click on the blue links below. Good luck and have a wonderful day.
Battery light comes on when idling and goes away when accelerating...
BMW 1150 GS Repair Manual
BMW Parts Microfiche
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