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Jared D
Jared D, ASE Certified Technician
Category: Mazda
Satisfied Customers: 1784
Experience:  Shop Supervisor at County Fleet
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My son came home last week in s 2005 4cyl mazda 6 ...battery

Customer Question

hi. my son came home last week in his 2005 4cyl mazda 6 ...battery light was on battery boiled dry....alternator was putting 17 to 19 volts to the battery....we replaced the alternator with a used one with a 30 day warranty....replaced the battery...he drove 5 miles and the battery light came back one and the second alternator was overcharging as well...he had an inverter of some sort plugged into the lighter on the day of the original problem....we have anoter replacement alternator coming tomorrow...will it be ruined as well?
Submitted: 1 year ago.
Category: Mazda
Expert:  Jared D replied 1 year ago.

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.

Expert:  Jared D replied 1 year ago.

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.

Expert:  Jared D replied 1 year ago.

I just wanted to check in and see if you needed any additional info. I would appreciate a positive rating if I have been of assistance. Thanks!