1990 GMC S15 Alternator Replacement Cost

The alternator is mounted on the engine, and creates power when it's turned by the serpentine belt or drive belt. As it spins, it creates electricity, and the built-in voltage regulator and rectifier are responsible for ensuring the current is correct for the vehicle. The alternator produces an alternating current (AC), which is changed to usable direct current (DC) by the rectifier diodes within the alternator. 

A weak alternator can result in dimming of the dashboard lights and the headlights.

A failing alternator can cause problems starting the vehicle and may result in a repeated need to jump start the vehicle.

If the alternator fails, the dashboard battery warning light will turn on while you drive. Soon after, all electrical components will begin to lose power, the vehicle will stall, and a jump start won't work. If the alternator's bearing fails, it can cause a grinding noise that changes with engine speed. » More symptoms of a bad alternator

A vehicle with a weak alternator can be driven to a repair shop, but if the alternator has completely failed, the car will need to be towed.

Alternator failure is fairly common, but an alternator can last well over 100,000 miles without incident. Harsh driving conditions and custom power accessories can shorten the life of an alternator.

When there is any failure in the starting and charging system, the entire system must be tested. Repair shops use professional test equipment that checks every component simultaneously and definitively. If the alternator is the problem, it will be identified.

If your battery cannot deliver 10 volts or more during a battery load test, the vehicle may continue to experience starting problems if the battery is not replaced.

When replacing the alternator, the technician must remove the battery cables to prevent electric shock. The accessory drive belts or serpentine belts must be removed from the alternator, along with electrical connectors and several bolts. Once the bolts are removed, the alternator is free from the alternator brackets, and can be removed. Replacement is in reverse order, and proper torque must be applied to prevent damage. The vehicle will then be retested.