* Alternator: This is the most common cause. When you accelerate, the engine's RPMs increase, putting more strain on a failing alternator. If it's struggling to keep up, the battery light will come on because the alternator isn't charging the battery sufficiently. Possible alternator problems include:
* Worn brushes: These wear down over time and reduce the alternator's ability to generate electricity.
* Faulty diodes: Diodes rectify the alternating current (AC) produced by the alternator into direct current (DC) needed by the vehicle's electrical system. A faulty diode can prevent proper charging.
* Bad bearings: Worn bearings can cause the alternator to spin inefficiently, hindering its charging capabilities.
* Failing voltage regulator: This component controls the alternator's output voltage. A faulty regulator can lead to overcharging or undercharging.
* Battery: While less likely given the symptom (light comes on *during* acceleration), a very weak or failing battery might not be able to hold a charge adequately. Under load (acceleration), the demand for power increases, and a failing battery might cause the light to illuminate. Have the battery tested for capacity and cranking amps.
* Wiring and Connections: Corrosion or loose connections in the wiring between the alternator, battery, and charging system can interrupt the charging circuit. This is especially likely on a vehicle as old as a 1997 Expedition. Inspect all connections for corrosion and tightness.
* Belt Issues: A slipping or worn serpentine belt (the one that drives the alternator) can prevent the alternator from spinning at the proper speed, resulting in insufficient charging.
What to do:
1. Have the alternator and battery tested: A reputable auto parts store or mechanic can perform these tests quickly and relatively inexpensively. This is the most important step.
2. Inspect the serpentine belt: Check for wear, cracks, or slippage. Replace it if necessary.
3. Check all connections: Inspect the wiring and connections to the alternator, battery, and battery terminals for corrosion and looseness. Clean and tighten as needed.
Don't ignore this problem. Continuing to drive with a failing charging system can lead to a completely drained battery, leaving you stranded. The sooner you address this, the better.
