MustangMan00

Hey I got my alternator tested today(86k miles) because my radio lights were randomly dimming, also when I brought in my alternator to work(advance auto) my coworker said it was fried and I should def. replace it, but it had been working fine prior.

It came up with DIODE OPEN, does anyone know what causes this? I guess I need a new alternator huh?....$184 is a pretty penny.

Battery and starter checked out fine, I have an Optima redtop

-ToXiC-

replace it... all there is to it.

wingless

There are four diodes.

They can either open or short, both failures are bad.

Both failures will cause the alternator to stop charging.

They are easy to test w/ a DVM.

devongarver

Actually, most automobile alternators use 3 phase AC to DC. So it is not a normal 4 diode full wave bridge rectifier circuit which is what single phase AC uses to convert to DC. Also, Diodes don't short. They can burn open, but never have I seen or heard of one shorting. Also, if one opens, you will simply lose 1/3 of your wave, which will cause a fluctuation in the DC that is output to the electrical system. This is still not good, and can cause issues, but will not prevent the alternator from charging. They are also not easy to test unless you are familiar with the layout of your vehicles alternator, and you can see which diodes are which-this usually isn't possible as they are contained within a plastic housing, mounted inside the alternator. SO, basically, buy a new alternator.

MustangMan00

well a re-manufactured one from my work is $230, after my discount it was $184.

My coworker told me to go to this electric supply/repair place he knew very well. (He tested it again on a fancy machine and said my diagnosis was right(Diode failure). I'm getting it rebuilt for $120 and will be ready tomorrow.

SnakeConvert98

That's what i was going to suggest,,good for you!! You know the old saying "a penny saved is a penny earned" and we all know how pennys are tight nowadays!!

Cusp

A single open diode will take out 1/2 of one of the 3 phases. This will decrease the charging capacity of the Alternator. But it will also add whine to your sound system.

First choice would be to replace the alternator. But, it may be possible to find a repair shop to fix it for less than a rebuild unit. Maybe 1/2 as much.

If it were mine, I think I would crack it open to determine if I could replace the open component. If it were possible, the repair most likely would not cost more than about 10 or 20 bucks.

Solid state devices fail when their design limitations are exceeded. I speculate that the stock diodes are not designed with enough of a margin for over load.

MustangMan00

well I figured I would just get the whole thing rebuilt while they were in there it has 86k on it

devongarver

lol exactly, it saves Ford about 42 cents each to put a smaller diode in haha. J/K I talk out my A$$ on that one. But I bet it's about right lol.

MustangMan00

Well here it is $120 out the door, even nicely polished up...

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