Faulty Alternator

The vehicle is a 2004 Ford Explorer with a 4.6L V8 engine and 4x4 automatic transmission (VIN JL6DGJ1E77K007152).

Jan. 1, 2020

2 min read

Dear Roy,The vehicle is a 2004 Ford Explorer with a 4.6L V8 engine and 4x4 automatic transmission (VIN JL6DGJ1E77K007152).

The truck had a no-charge condition, and I confirmed that the alternator was not working. I installed an aftermarket unit, and it put out 14.2 volts and 13 amps at idle. All was good, but I still had a charge light on.

I thought that I had a bad unit and changed it. Same problem.

I talked with some friends, and they told me that I needed to use a factory remanufactured unit or the light will not go off. When I got a factory remanufactured alternator, the counterman told me the same information but could not tell me why. He said that all aftermarket reman [units] will charge but will not turn the light off.

Can you help me understand why? Can you help me turn on a light in my head and off on the dash? Mike Banovich, president USA Auto Repair, BayShore, NY

Dear Mr. Banovich After the engine is started, voltage is applied through the warning lamp circuit from the powertrain control module (PCM) to the voltage regulator. This turns on the regulator and allows current to flow from the battery sense circuit to the field coil. This circuit also is used by the PCM to determine whether or not to send a message over the controller area network (CAN) communication link to the instrument cluster to operate the warning lamp.

If there is a malfunction in charging system operation, the regulator will ground this circuit, which sends a signal over the CAN link to the PCM to illuminate the lamp.

I would venture to guess that your rebuilder is either using the wrong regulator, or it is one of suspect manufacture that is unable to communicate with the PCM.

About the Author

Roy Ripple

Roy Ripple passed away in September 2021 at the age of 84. He was an automotive technician his entire life. He owned his own shop for years before his first and only venture into automotive journalism, with Motor Age magazine. In his youth, he was an avid drag racer and a good friend of Don Garlits. He raised four children, rode motorcycles, hunted deer, and loved to shoot trap. His son, Roy Dennis Ripple, now writes for Motor Age. You can see his work here.