Why do I keep burning out diodes in my alternator?
 

I have a 91' Probe that has been a really reliable car. In fact, I replaced the original alternator at 240k because it would only charge when you were above 2000rpms. However, each of the next 3 brand new alternators only lasted about 2000 miles each. Same thing happens, a diode burns out. I can't find anything else out of the ordinary with the electrical system either. I have had this car since 1992 so I know everything that it has been through. I'm really puzzled and am also tired of replacing the alternator over and over :-) I checked the wiring, and I had the battery checked as well and it tested good.

I have posted on the Probe "forum" but since most of the kids are 18-21 they really don't know much about cars, and folks here tend to be more knowledgeable overall when it comes to routine maintenance and troubleshooting.

Thanks,

Bryan

Many years ago my Dad had this problem on a Grand Prix. The problem went away when he went with a different brand of rebuilt alternator.

well maybe the voltage regualtor?

Isn't all that integral in the alternator?

You said "brand new", what doess that mean? Ford or pepboys "brand new"?

As Cowboy mentioned above see if you can get a different brand/get your money back, some aftermarket rebuilds are just junk. Do you have heavy loads on it or is everything pretty well stock?

:-X01 It sounds like it is a manufacture defect of some sort. The diodes are used to convert AC current to DC current on a altinator. I wonder if who ever rebuilt the unit received a shipment of bad diodes.Oo.

I had a problem like this on my Bronco. Don't get mad at the limited details, but here goes. The mechanic I had working on it took a look at it and said that there was something that needed to be removed from the alternator prior to installation, otherwise it would keep burning out. I thought he was full of it, but he showed me a small tag that detailed the process. This was a long time ago, but that was the last alternator I needed. Look into that I guess, but yeah...not a mechanical whiz here. Why do you think I joined FTE!?!!

Some alternators for ford products come with a new plug pigtail. This does matter in the replacement and life of yoru part. Just because there is one on the car that fits your new/rebuilt part doesnt mean you dont need to replace it. If you got one with your part it needs to be installed.

It has also been my experience that some parts stores electrical parts are crap.

These alternators are *brand new* Bosch units from Napa and have an internal voltage regulator. "New" was one of my stipulations when I bought a replacement....I got 240k out of the first one and I wasn't going to save a few bucks to get stranded with a rebuilt unit.

My electrical system is almost stock. I have a small amplifier and am also using H4 headlamps(80W vs. 55W), but the amp was not even connected when the first two went out. No power windows, and I've not been using the A/C since September. These two items have been installed on the car for about 10 years anyway.

I thouroughly inspected the pigtail when replacing it the first time, it looked good and made a nice firm connection. A wiggle test didn't show anything strange going on either.

Maybe I need to think backwards and ask "what burns out a diode?"

I'd buy a Ford alternator... also, have you tried replacing the battery?

> "what burns out a diode?"

Bad ground or bad positive cable that for a moment disconnects the alternator (or a load) from the battery system when you hit a bump. When the load or battery reconnects to the alternator it blows the diode.

I would suspect a bad connector or the loss of the 12+ while charging. Look for a wire being pushed out the back when you plug in the connector.

On many foreign vehicles you can not use the battery to charge (jump) another car nor use a battery booster to charge the battery without disconnecting both battery cables. In many of them, if you drop below 10 volts you stand a good chance of killing the alternator.

I can only speak of German cars, but, there might be a field plug on your alternator that has to be either disconnected or connected. It is usually marked (+) with a square connector and looks like it can receive a 1/4" female connector. Depending on apps. it receives 12+ KEO.

IF YOU PUT A TEST LIGHT TO THIS CONNECTOR YOU BLOW THE ALTERNATOR.
Just so you do not get the idea to test the alternator though it.

Excessive current draw. Poor quality component.

That's the ticket. That's what I was trying to relate, but couldn't. This is exactly what my mechanic told me when I noticed this problem, and I never saw the problem again when he did it right. Like I said, I'm tlaking about a 1988 Bronco, but what balddave is saying is 100% what I was thinking of. Keep an eye out for that if you try for another alternator.

If you are asking, why do I need a new connector? Most likely, I can only go by what I have seen done on other connectors, not so much a Probe, is they run a zener diode (black plastic with copper markings with two metal legs) between the two positive wires inside the connector itself.