Q: Battery light comes on after starting and stays on running the battery down

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I changed alternator and PCM and the problem still exists. I checked the wires and continuity from the PCM to the alternator and they are good. alternator will not charge even though it is new. I have been told there is a bad wire or a shorted wire. All fuses are good. ???????

My car has 216000 miles.
My car has an automatic transmission.

You need to have the alternator’s large red terminal from alternator to the starter tested for proper battery voltage. The orange wire coming out of the alternator should have battery voltage at all times. This is the volt sense wire and is fused by a 10amp fuse in the engine fuse box. The last alternator wire , a small red one, goes to the PCM and this is the wire that the PCM uses to control the alternator output. The PCM should be scanned as well as the data from the instrument cluster to see if the instrument cluster is causing the problem by not giving feedback voltage to the PCM.

You may have a defective cluster or you’ll need to disconnect the connections at cluster and PCM. Then add dielectric lube to connector pins and seals. This will help stop fretting corrosion in connectors. This can potentially cause high resistance in connections and cause the problem you’re having.

If you need some help with these checks, consider YourMechanic, as a certified technician can tests these components and diagnose the issue with your Battery Light.

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