Category: How Things Work

On today's newer cars and trucks, the generator (ALternator) provides DC voltage to operate the vehicle's electrical system and to charge the battery. A magnetic field is created when current flows through the rotor windings. The rotor is belt driven by the engine creating an AC voltage as this field rotates within the stator windings.

The AC voltage is converted to DC voltage by the rectifier bridge and is then supplied to the electrical system at the generator battery terminal.

When the ignition switch is on, voltage is supplied to terminal L by the Powertrain Control Module (PCM), turning on the regulator. The generator's voltage regulator controls current to the rotor, thereby controlling the output voltage. The rotor current is proportional to the electrical pulse width supplied by the regulator. When the engine is started, the regulator senses generator rotation by detecting AC voltage at the stator through an internal wire. Once the engine is running, the regulator varies the field current by controlling the pulse width modulation. This regulates the generator output voltage for proper battery charging and electrical system operation. The generator terminal S is connected to B+ voltage.

When the ignition switch is turned to [RUN], the charge indicator turns on (bulb check), then turns off after the engine has been started and the PCM has checked the Charge Indicator Lamp circuit for any problems.

The PCM commands the charge indicator on by grounding the charge indicator control circuit when the PCM detects a charging system problem.

More Auto Repair Help


    • Guest - Vicente

      I have 2002 Chevy Tahoe. My question is when I step in the throttle the battery gauge goes down and a couple of seconds later comes back to normal.

    • Guest - TechHeader

      In reply to: Guest - Vicente

      I have 2002 chevy tahoe my question is when <br />I step in the throle the battery Gage goes down <br />En cople seconds later comes back to normal
      <br />Sounds like a loose wire that is getting pulled when the engine moves when you hit the gas on your Chevy Tahoe. Pull on the wires at the alternator and follow them while pulling to see if they act up. Also check for a loose or broken ground on the rear of the passenger side cylinder head.


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