Charging light

after sitting for a month, I started the car and the alternator light came on very bright. After a bit of rev, the light dimmed but did not go out. It stayed lit for a 15 min drive. All was working correctly before being parked

The battery was on a NOCO maintainer model G 3500 for the month if that would make any difference

the alternator was rebuilt a year ago and VR replaced at the same time

Where do I start trouble shooting this problem?

The charging light would indicate that the alternator is not producing the needed charging volts (I don't know the exact number, but it would be between 13-14 volts) so you'll need to test that first with a volt-meter with the engine running. Swapping the VR is easy.....having to R&R the alternator, not so much. Don't take comfort in new or rebuilt parts...they often fail unfortunately. No harm either in making sure that all your battery connections and ground points are bright and shiny.

where do I check the alt output for 13-14 volts?

Check running voltage the battery. It must be over 12.8. Anything less and the alternator is not up to snuff. A fully charged battery is 12.6 with engine off.

The alternator 'idiot light' on Bosch systems is very important. When the dashboard light lamp burns out the charging circuit is disabled and won't work. The light is integrated into the system.

1st make sure the battery connections and the ground strap at the transmission are absolutely clean and tight. Likely they are or the starter would balk.

Remember if you run the engine and the connections to the battery are lost the diodes in the alternator are ruined immediately.


Changing out an alternator is not too difficult. Be sure to make many photos of the connections BEFORE you disconnect the alternator.