Charging System Voltage
High charging system voltage is usually caused by a stuck closed voltage regulator. Voltage regulators adjust output voltage according to the demands of the electrical system. In late model vehicles this is typically accomplished through a sensing circuit that informs the PCM of battery temperature and electrical system demands.
Voltage output is achieved by changing the amount of current to the field circuit. The field circuit or exciter wire is where current is input into the alternator. The more current into the alternator’s field circuit, the more voltage output the alternator will produce. The increase in current increases the magnetic field created inside the alternator increasing voltage output to the battery terminal.
The older electro-mechanical voltage regulators were more prone to this problem than the newer electronic and solid state regulators. These regulators were placed inside the alternator itself or mounted in the engine compartment. Sometimes the contacts inside the regulator become stuck closed producing a constant and unrestrained voltage output.
Alternators can produce in excess of 250 volts of power and left unchecked will overcharge the battery or worse damage electrical system components. As engine speed increases, so does the alternator’s output, possibly making the vehicle’s headlights brighter during acceleration.