Charging system issue

TMFATHFAN06

MEMBER
2004 MERCURY SABLE
3.0L
6 CYL
FWD
AUTOMATIC
77,000 MILES

Back in December we had to replace the alternator and battery since then I've been getting a message on the dashboard "check charging system" and the battery light with it. It dings stays on a few seconds and goes off. It's gotten so annoying I've resorted to hitting the reset button several times which seems to make it stop for a while. The wiring for the battery and alternator have been checked, both the battery and alternator have both been tested. Even bench tested the alternator. It was mentioned to check a fusible link wire but none of us can seem to find one under the hood. Any help with this would be great. Thanks.

1 Reply

KASEKENNY

MECHANIC
18,907 POSTS

The charging system light is indicating that at certain times the voltage produced is below the proper threshold.

However, we need to make sure we tested the alternator as shown in this guide.

https://www.2carpros.com/articles/how-to-check-a-car-alternator

Then we need to make sure the battery passes a load test.

https://www.2carpros.com/articles/car-battery-load-test

If the alternator and battery pass, then we need to check the wiring from the alternator to the PCM. This wiring is responsible for communicating the voltage from the alternator to the PCM so that the PCM can turn the regulator on and off as needed.

If there is resistance in this wiring, then it will not get accurate readings and can turn the charge light on even though the alternator is not the issue.

https://www.2carpros.com/articles/how-to-use-a-voltmeter

If this all checks out, then we need a scan tool to monitor what the PCM is getting as voltage from the alternator because the PCM would be turning the light incorrectly which means the PCM would need to be replaced.

Below is the wiring diagram that will help with this. Please let us know what questions you have. Thanks

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