2004 mazda 3 bad diode in alternator questions?

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Sarah M

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I have a 2004 Mazda 3 with about 52k miles. Just took it in (without symptoms - just for a tranny flush and oil change) and apparently my battery has a dead cell caused by a bad diode in my alternator. I'm not really sure how it all works, other than I know that the alternator keeps the battery charging. Can someone explain it a little better? They gave me a new battery and ordered a new alternator, but I have had no symptoms. The only thing I DID notice was more static on the radio the past week or so before taking it in.

What causes a bad diode? & What would happen if it was not fixed in a timely manner? The mechanic said it could cause "other electrical problems".

getting it fixed either way, but was just curious. :)

Thanks!
To answer:

He tested the battery (which is the original and 4 years old) said he wanted to change the battery because with the dead cell, it was showing that the alternator was bad. I advised him to change the battery, and he called back stating that the alternator had a bad diode which had caused the bad cell in the battery.
actually in thinking on it, and rereading the response below about lower rpms and idling in reference to the alternator, I have noticed that it was idling slightly harder (shakier, a little noisier) for the past few months or so, don't know if that is a possible symptom or not?
 
If any of the alternator's diodes have blown out, the alternator will not have sufficient output to maintain the battery after a given amount of time. What I am wondering is what exactly prompted the technician to test your charging system when all you came in for was fluid change? Especially being that you had no failed charging system symptoms. If you do have an original battery, this could be one reason that prompted him to test it, if presuming that your battery is 4 years old and perhaps may have shown signs of excess terminal corrosion, and the fact that batteries usually begin to show prominent wear at about that length of life. It is possible that he caught the bad alternator during the battery test. Be cautious of unneeded repairs though. Can he show you the report that the testing system gave indicating that the alternator is indeed not providing sufficient output? Most repair facilities will be happy to show you this so that you do not have to just trust whatever the tech is stating. The tech's determination must be upon a factual basis that the alternator IS indeed bad. You see, just because a cell is dead in a 4 year old battery does not indicated alternator replacement is warranted.

Diodes do fail over time by their constantly being loaded with current that they are forced to redirect. Explained, they stop and redirect the negative portion of current in the alternator (ie. as they originally put out alternating current, (AC) thus, labeled an alternator (verses a generator) Since autos use direct current (DC), the volatage of the negative portion is "flipped over" by what is known as a bridged diode. Car alternators actually use three diode bridgeings and are nicknamed a triode set. This smooths the current out and makes the alternator put out better current, especially at lower speed revolutions. This is why alternators are used instead of generators; their output at idle is much better than a generator.

Other electrical problems: addressed.
I question this tech's statement, but there are a few exceptions:
Deficiencies in voltage and/or current will only exclusively cause battery problems only unless you have a vehicle that uses multi voltage modules such as BMW's from the mid 80's or the brand new Volvo's that use this method in order to decrease the amount of wires on the automobile.
 
Bad diodes in the alternator definitely do not cause bad cells in the battery. Someone is pulling the wool over your eyes. a bad diode might weaken the alternator, but if it was too weak to charge the battery, your "Battery" light would have come on and stayed on on the dash. That's what it's for. And in that case the battery might be a little discharged, but that's what it's for! You recharge it and it is fine.

A bad cell in a battery is caused by the repeated charge/discharge cycle, that causes lead to flake from the battery plates and pile up inside on the bottom of the battery. It has nothing to do with the alternator.

A mechanic on here once wrote that he made a practice of taking his car to different garages and acting like he didn't know anything about cars. He said about half tried to get him to do work that he knew his car didn't need.

I would immediately thank them, ask them for a bill that tells what was done and take the car somewhere else, and take no excuses. Ask for your alternator back if it's not too late, and have a tow arranged for in case that becomes necessary. You can ask family, friends, neighbors and whoever if they have found a garage they have had good results with.

Wish you the best.

Best regards,
Mike
 
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