Mazda Repair Questions? Ask Mechanics for Answers ASAP
If the alternator is overcharging, with the engine running unplug the connector at the alternator and see if it continues to overcharge, if so, the alternator is faulty. The alternator should not charge at all when it is unplugged.
It's possible that the PCM is bad, but it's not common. There have been a lot of issues with rebuilt alternators and very few cases of a faulty PCM. With the alternator unplugged and the engine running, one of the wires from the PCM will have a 12 volt reference on it and when plugged in that circuit turns to about a 1.5 volt square wave signal, this is the command from the PCM to tell the alternator how much to charge. The other wire at the alternator will show very little voltage when the alternator is unplugged and the engine is running and then turns to about a 10 volt square wave when the alternator is plugged in. This is a feedback to the ECM to indicate how the alternator is charging. If you test it with key on and engine off, neither wire will have a voltage reading, the engine has to be running.
Also, check for burned terminals in the C-08 connector at the engine compartment fuse box. This is a connector in the center of the box with a lever style latch. If this connector burns and is damaged it can send a signal to the alternator to charge at full output. If there is any damage to this connector you will have to repair or replace it.
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