1991 BMW 735iL Alternator Voltage Regulator Replacement Cost

The alternator voltage regulator is a component of the electrical system on a car, truck, or SUV. More specifically, the voltage regulator is part of the charging system responsible for managing the output of the alternator and maintaining a full charge on the battery . It is either housed inside or attached to the alternator.

The voltage regulator works with the alternator to make sure that electrical output stays within 13.5-14.5 volts and that the battery stays charged. A vehicle's battery is used to provide power to start the engine, and it is used to smooth out the supply of electricity when the vehicle is running. In time, a battery would lose its charge and die if it had to run all of the systems on a vehicle and if it was not recharged regularly. The alternator is responsible for taking over once the engine starts. The alternator provides power for all of the vehicle electrical systems (with help from the battery in the case of a load spike) and is responsible for keeping the battery in a state of full charge. If the battery gets too weak, the alternator charges the battery. Once the battery is fully charged, the alternator stops charging. The voltage regulator monitors the battery's state of charge and signals to the alternator when it is time to charge and time to stop charging.

Symptoms of a bad voltage regulator might include a dead battery, dim or pulsing dash lights, or erratic engine performance. Each of these symptoms can also be attributed to other causes, but if the voltage regulator is failing, it can prevent the battery from charging properly or the alternator from putting out the correct voltage for vehicle operation. If the voltage regulator is not signaling to the alternator that it is time to generate a charge, the battery will discharge. A weak or dead battery will not be able to start the vehicle. If the battery drains while the vehicle is running (because the alternator is not putting out a charge), the headlights may dim, the dash lights may dim or pulse, and the vehicle could stall, leaving the steering system powerless and leading to difficulty braking. If the voltage regulator is faulty, a warning light on the dashboard should illuminate.

Any time problems occur with the vehicle charging system, immediate attention is required. The voltage regulator is an important component of the charging system. If the symptoms of a bad voltage regulator occur, the vehicle should either be driven to the nearest qualified repair facility, or should be parked and towed. A comprehensive inspection and evaluation of the starting and charging system should be performed.

The alternator and voltage regulator are not maintenance items that have replacement intervals. A factory alternator (with the voltage regulator inside) may last 100k miles or more, however, it is not unusual for an alternator to fail closer to 80k miles due to its moving parts, heat, and internal components that wear.

The symptoms of a failing voltage regulator are similar to those of a weak or dead battery or a bad alternator. To diagnose the voltage regulator, a technician will first rule out a dead battery by using a multimeter to check for correct resting voltage (12.6 volts), and possibly conducting a load test to make sure the battery is capable of delivering proper voltage under a load. If the battery is in good shape, the technician will start the vehicle and test the system for operating voltage. A car, truck, or SUV should run with voltage between 13.5-14.5 volts. The voltage regulator is designed to keep the voltage in this range. If the alternator is putting out too much or too little voltage, either the alternator or the voltage regulator is likely at fault.

In most cases, the voltage regulator (which is located inside the alternator housing) is replaced in conjunction with the alternator. It is possible to replace only the voltage regulator on some vehicles - usually by disassembling the alternator to gain access - but because the other alternator components can wear out over time also, the alternator is commonly replaced. Whether the alternator is to be replaced or only the voltage regulator, the alternator needs to be removed from the vehicle. In order to do this, a technician will need to locate the alternator and remove any components that are in the way. Depending on the vehicle make and model, this could be a significant amount of work; some vehicles have several components that block access to the alternator. In some cases, the vehicle will need to be raised and supported off the ground for underbody access. With the alternator accessible, the technician will remove the serpentine belt, alternator electrical connections, and mounting hardware, taking care not to damage the radiator and other nearby components. Once the alternator is out of the vehicle it can be disassembled (for replacement of the voltage regulator) or simply replaced.

RepairPal recommends a professional comprehensive inspection of the vehicle starting and charging system to determine the cause of symptoms similar to those of a bad voltage regulator. If the alternator or voltage regulator need to be replaced, it is advised that the serpentine belt, idler pulley, and belt tensioner be inspected and replaced at the same time.

Care must be exercised any time the electrical system is being serviced to prevent injury or damage to sensitive vehicle electronics. Other problems, such as dirty and corroded battery terminals, can produce symptoms similar to those of a faulty voltage regulator.

Access to an alternator can be tricky. There are many surrounding parts that usually need to be removed and there is often very little space to work in. Damage to nearby components - such as the radiator - is possible. Voltage regulator or alternator diagnosis and replacement can be performed on many vehicles by an experienced DIYer, or otherwise left to a repair professional.