Today when out motoring I noticed that while stopped at a light my '04 MCS was idling higher than normal - between roughly 1500 and 2000 rpm. By putting the car in gear, applying the parking brake, and gradually engaging the transmission I could force the idle speed down to something more appropriate, like 900-1000 rpm. However, at that level the battery light would come. Rev the engine and the light would go out. Any suggestions as to what is going on? Battery and alternator were new last year, about 3,000 miles ago. TIA

 

You can hook up a voltmeter to the power outlet, then determine if voltage is actually low when light comes on.

I also would be cleaning connections & grounds, & looking for suspect wire insulation breaks.

Get the codes read too, that may give you a clue.

 

most chain auto part stores will do a charging system check for free ... it sounds like an output issue and they can verify this quickly . . .

 

I need to revisit this thread. 155K. Car sat idle for last two weeks while we were away. Today it started up, suggesting that the battery is OK, but the charging light still comes on when idle is less than 1200 RPM or so. Figured I could still drive it to run a brief errand, but about a mile later a succession of lights came on: ABS; tire pressure; air bag; overheating warning light (although temp needle did not move); then radio, clock, instrument cluster illumination, and power steering all stopped working; charging light was now continuously on regardless of engine speed. Quickly turned around and got home, by which time the engine seemed to be misfiring. Crank damper was replaced with an ATI by Way at the Dragon about 10 years/80K ago; supercharger was serviced last year; water pump, a/c compressor, idler, tensioner, belt and alternator were also replaced within the past year and very few miles. (Of all those parts only the compressor and alternator required replacement; rest were preventive maintenance.) HELP!

 

I removed the battery and took it to be checked. Checked out OK. Fully charged it and put back in car. Battery charge light still comes on at engine speeds below about 1200 RPM; all other lights are off. Is there any possible explanation other than a failed or failing alternator? TIA.

 

Probably alternator. Check all the connections. Put a volt meter on the battery terminals with the engine running. You should get at least 14 volts if the alternator is working properly.

 

Took the alternator out and had it tested. Failed. It was a remanufactured Bosch; probably less than 1,000 miles.