This sounds like you have an alternator going out here is a guide to help you see what you are in for when doing the job.
Here are some diagrams of how it is done on your car (Below)
Computers are very sensitive to system voltage. The engine computer controls the charging system and monitors system voltage. The built-in voltage regulator switches the alternator on and off hundreds of times per second. When it detects low system voltage, it instructs the instrument cluster to turn on the battery light. The cause could be as simple as very low engine speed while idling at a stop light accompanied by an unusually high current draw. High current items include the rear window defogger, heater fan, and to some extent, the wipers, power seats, and power windows. The faster the alternator spins, the more current it is capable of producing. Higher voltage (electrical pressure) is needed to produce higher current (electrical flow). Both current and voltage from the alternator drop at idle. The temporary demand for current in various systems around the vehicle is made up by the battery, but after prolonged current draw, the battery voltage starts to go down. This is one thing the engine computer detects.
In addition, since the computer is switching the alternator on and off very quickly, it monitors the small current it is controlling. If it doesn't see the current it expects, it will memorize a diagnostic fault code, ("alternator field not switching properly"), and will eventually turn on the "Check Engine" light. That light is turned on whenever a problem is detected that will adversely affect tail pipe emissions. Low voltage will reduce ignition coil output so spark will be weak, and fuel injectors will turn on late and will not respond as the computer expects, so incorrect fuel metering will result.
The next time you see the battery light turn on, make some quick observations that will help your mechanic. If it only occurs at idle, try increasing engine speed just a little. If the light goes out, suspect low engine speed coupled with a lot of stuff turned on. If the light stays on, the next most likely suspect would be worn brushes in the alternator. 75,000 miles is way too soon for this to happen unless you drive in dusty conditions a lot. If you have the more common Nippendenso alternator, the brushes can often be replaced without removing the unit from the engine. Most people just replace the entire alternator. Rather than blindly throwing parts at the problem in hopes of fixing something that's intermittent, a few simple voltage measurements will find the cause of a charging system that's malfunctioning. You need to wait for the problem to occur though before making the measurements. If it is indeed worn brushes, the problem will become more prevalent over the next few months.
Images (Click to enlarge)
Wednesday, December 2nd, 2009 AT 2:13 AM