Details on Truck:
1992 Ranger, 4-cylinder, 2-wheel drive, 250,000 miles, oil leak likely

I got in my truck on Christmas eve after work and it wouldn't start on it's own. Had to get it jumped. co-worker said to check out the alternator, so I took it out and had it tested at an auto parts store. It failed the test and I purchased a refurbished replacement. Truck ran fine for a week.
New Year's eve, I picked up my kid from daycare and I had to get a jump again. When I was idling the engine, RPM's showed 4,500 and when I revved the engine, it would go down to 1500, like it was reversed somehow. ??? Got the truck on the road and drove about a mile and the engine died going 45 mph (after I flashed my brights at someone who pulled out in front of me). At this point, the headlights and hazards did not work, but the dome light was working somehow.
Tow truck driver said the alternator probably failed again, so I took it out and had it tested again.... failed.

I tried to fire it up last night with the replacement, replacement alternator and it wouldn't start without a jump. Once I jumped it, it ran a little rough and when I flashed the brights, the engine died.

I'm running out of ideas at this point. The battery shows 12.5 volts at rest and when the engine was running, it showed 13.5 volts. I suppose I could have really bad luck and this is the third bad alternator, but I doubt it. The guy at autozone said that the oil leak could be causing the alternator to fail, but I think he's full of it. What else could I check?

 

Have you had the battery tested? Are all your connections at the battery clean and free of corrosion? Its possible to get a dud alternator. Before getting the new one, get it tested in the store before leaving.

 

This. I used to work at Advance, and in the time I worked there, I probably had 3 failed alternators right off of the shelf, and numerous others where parts would actually fall out of the alternator.

 

Yeah, but two in a row and just for me? Is there anything in my truck that could be making it fail right off the bat? The autozone tech said that my battery or starter could cause the alternator to fail. I doubt his claims..... any insight into that?

 

Could be bad alternators but you FOR SURE have a bad connection either on the Battery Ground or the "+" wires, that is the only explanation for the engine dieing when extra power is needed, i.e. the high beams.
Disconnect the ground cable then follow the "+" cable and check it's connection, usually on the starter relay, loosen and clean, this is also where alternator power connects to system, if there is a problem here you will lose ALL power in the vehicle, because battery and alternator are the only power sources.

Battery cables can "look" OK, but they will corrode under the insulation, have some tape handy, use a razor knife to cut insulation back and off about 2" from the battery end of the cable, if you see whitish discoloration replace cable, if it looks OK then tape it.

 

Fixed it for you.

 

I don't listen to the guys that work at the parts stores.

 

I currently work for AutoZone. I'm the only one who's been in my store over a year other than my store manager. Im also about the only one who has a clue as to what I'm doing and can actually give advice. Its sad that a 20 year old has more know how than the other people I work with.