I think you might be due for a load test on the charging system. Besides voltage, (electrical pressure), which is what is detected to determine when to turn on the warning light, the alternator must be able to provide the necessary current, (volume or flow). There have been a lot of problem with wear of the brushes inside the alternator that leads to it being dead and the warning light turns on. That always starts out being intermittent and can stay that way for weeks. The part is very inexpensive and fairly easy to replace, but most people just buy a rebuilt alternator.
A less-common problem is one of the six internal diodes shorts. That will show up on a load test as it can only deliver very close to one third of the rated output. A 90 amp alternator is one of the common sizes. If it has one bad diode, all it will be able to produce is around 30 amps. It takes around 10 amps to run the fuel pump, 10 amps to run a pair of head lights, then you still have the tail lights, radio, heater fan, fuel injection and ignition systems, so there isn't any spare capacity left over. The rear defroster also only draws around 10 amps so that is insignificant, ... IF the alternator is working properly. You might have stumbled onto a clue that there is a charging problem before something worse happens.
Please consider a to help us answer more questions.
Tuesday, May 10th, 2011 AT 10:45 PM