Symptoms of a Bad Voltage Regulator. Automotive voltage regulators were once bulky assemblies full of contact switches and wire coils, boxed up and bolted down under the hood. Modern regulators are small, solid-state components, typically tucked away inside an alternator. The purpose remains the same regardless of location or construction. The regulator allows voltage to fluctuate slightly within a certain range, much like the way your home thermostat helps keep the rooms at a select temperature. You might get upset, or even ill, if room temperatures went wild, and a bad voltage regulator can create similar havoc for a vehicle.
Several symptoms can develop if the regulator prevents sufficient voltage from being delivered. A battery may weaken slowly, or other symptoms can appear suddenly, if the regulator fails while driving. The dash indicator light might not warn you until the battery falls below a certain level, but engine performance can falter in the meantime. Computer controls, ignition systems and fuel pumps all rely on a constant power supply to function properly. Engine performance might dim slowly as voltage drops, or hesitate greatly if headlights or other accessories are turned on. Lights on the dash or elsewhere can dim or pulsate in brightness. Eventually, the vehicle stalls and the battery may be too weak to crank the engine. Most warning systems alert the driver in advance of severe failures.
A voltage regulator can also fail by allowing too much voltage to be generated. Subtle symptoms can occur in this instance whenever the overage is moderate. Headlight bulbs and tail lamps may be casualties of a derelict regulator. Check the charging system if bulbs die often, or in some spectacular way. You should check for excess charging voltage any time the battery electrolyte levels need regular replenishing. Heavy venting can occur if the battery is overcharged, and the acidic vapors can corrode nearby surfaces quickly. You can look for signs of corrosion near the battery, but stay clear of any battery that makes noise, or emits a vapor.
Mechanics used to refer to dashboard warning lights as "idiot lights" because they were not always reliable, but such warnings are often the earliest indication of voltage regulator problems. Cryptic abbreviations, like "GEN" or "BAT" appear on some charging system warning lights, possibly blurring the intended message. Some older import cars are notorious for illuminating every dashboard warning light whenever the alternator fails. Such lights have been abandoned in favor of more reliable and recognizable alarms. However, you must take any and all of these devices seriously to prevent unregulated voltage from harming, or disabling your vehicle.
Charging system warning lights that never illuminate can be an indication of an open in the voltage regulator circuit. You can check the operation of the warning indicator by turning the ignition switch on, without starting the engine. The light should come on, and stay on until the engine starts, and charging begins. The bulb that illuminates the warning could be defective, but it is just as likely a problem in the circuit that serves the voltage regulator. You can safely assume this to be true if the engine cranks slowly at times, or you if need an occasional jump-start.