A few months ago the volt meter started behaving differently than it had always done. On startup it would sometimes read above the 13 volt center mark where it should and sometimes below. It would also move from above to below while driving (no relationship to accessory or headlight use) When it is below the "gen" light does not come on.

Sometimes the battery would be too low to start the car so I would put it on a charger or use one of those booster pack things to be sure the car would start.

I was trying to determine if the issue was with the alternator or the battery when the other day the car wouldn't start (slow cranking) even after I'd had the battery on the charger for several hours so I figured the battery was the issue and bought a new one. All seemed good for a few days and then the volt meter started doing its read below 13 thing again even though I was driving and didn't turn on any accessories or the headlights. Again the gen light was not on (it does work as it lights with ignition on but car not running as it should).

Any ideas what is going on? I thought if the alternator was bad that the gen light would come on indicating no charging but that doesn't happen. It's just the volt meter randomly moving to above and below the 13 volt mark. So far the new battery hasn't failed to start the car.

 

Check the voltage across the battery while the car is running with a DC meter (DVM). Turn on all the lights, AC. It should read around 13.8 v, indicating that the battery is being charged. Compare with what your dash meter is showing.

If the DVM shows low charging voltage, replace the alternator, this is after making sure the belt is not slipping!!!

 

Had the car out the other day and the volt meter did its low reading thing again. I checked with a DVM and the reading was 12.83 volts across the battery so I guess the alternator is intermittently bad. I also checked that the belt wasn't slipping.

So if the alternator isn't charging why doesn't the GEN light come on indicating no charging?