Melted EEC diode/ high voltage

i just got a 1999 ford explorer with a 4.0 SOHC it runs fine except it came with a bad alternator and a melted EEC diode. So I put in a new alternator and diode, but it was putting out 15.5V and melted the new diode in two days . So I took it back and they said it was bad and gave me a new one. After i put this one in it gave me 16.5v and melted the new new diode in one day. I took this one back and they said it was fine and only putting out 15V. Is there anything else that can give me high voltage and melt a diode beside the alternator?

Who is “they?” McParts stores return policies are quite generous but their pay scale doesn’t allow them to employ a knowledgeable person and the second warranty replacement brings on some serious scrutiny for the store and the counterman who approves the second return. Your problem may be in the vehicles wiring and not faulty alternators but then it is possible that the parts stores supplier has a shelf full of faulty voltage regulators and you are at the leading edge of the problem. To protect your vehicle and your pocket book, buying a junk yard part and installing it along with another diode would be a reasonable move. If doing so results in the charging system functioning as designed you can take that second alternator back to the store and present them with your full story. I would be surprised if they didn’t give you a full refund including the core value if the junk part takes care of the problem.

Do you own a simple voltmeter and know how to use one? If so, put the pinpoint leads directly on the battery terminals (not the cable ends, but directly in the middle of the battery posts) and measure the voltage. Now start the car and measure again. Now leave the negative lead of the meter on the battery post and put the positive lead on the BAT terminal at the alternator. Tell us what the readings are.

I am sorry I do not know your vehicle But it sure sounds like a voltage regulator to me.

@Barkydog I’m almost 100% certain that the voltage regulator is built into the alternator on OP’s car.

So every time he got a new alternator, he also got a new voltage regulator.

FWIW, one of my colleagues had some problems with Ford remanned alternators (delivered by the Ford dealer, no less!). The first 2 were duds. The 3rd one worked correctly.

I suspect the rebuilder was having a bad day. We all know that none of the manufacturers actually rebuild their own alternators, starters, etc. That work gets farmed out. No secret there.

On most vehicles the voltage regulation is done by the ECM these days. I don’t know about a 1999 Ford, but my 1994 Mopar handles it this way. Possibly the fault lies in the ECM or wiring to it? Maybe something isn’t getting a good reference signal due to a poor ground?

“They” refers to Autozone. and I do own a volt meter and know how to use it ( worked as an electrician for a few years, know a lot about wiring in buildings but not a lot about cars) and they way i got my number is measure voltage on the terminals and then move the positive lead to the alternator. i get 12.5v before starting the truck i see it climb to 16V. and have good ground at the battery

OK, you have a VOM and know how to use it so look at the plastic harness clip that plugs into the back of the alternator from the side. The wires in that clip power the voltage regulator, trigger the dash warning light and give the regulator a return link to battery voltage to determine when to reduce the current. You need to search on line for a wiring diagram for your alternator to find the schematic and back probe that harness clip for proper voltage. If the voltage regulator is turned on and functioning but gets no return voltage signal it will push the current to the max.

OK, so I didn’t have much time to do anything today but I do think I solved the diode thing. After tracking down some wires i looked under the power distribution box under the hood and I saw someone messed with the wires. Someone spliced the wire coming out of the EEC diode and ran it to the fuse that feeds the alternator. after correcting that I now get a voltage reading of 15V. I will try to continue my hunt tomorrow after work.

Great start, the question is why?

everything works fine now. just had to put the right size fuses in

Thanks for the followup!