2 dead batteries and an alternator without an alibi

[5.0loco]

So I changed out the stock alternator on my 1991 5.0l, but I didn't disconnect the battery because they were only two connectors (lazy I know). When Autozone checked my old alternator it was working but the bearing was probably going because you could hear/feel the difference versus the new one. When I reinstalled the new alternator, the car started fine and I let it sit for 5 days. On Monday morning, I tried to start it and I had no juice. I hooked up the trickle charger and it initially showed as "charging" but then after a while the little bulb that has a red battery symbol turned on (bad battery I think). I then tried to jump start it and I finally got it going, so I left it idling for over 1/2 hour to get charged. Tuesday (yesterday) morning, I tried to start and again no juice. So yesterday afternoon I took the Duralast battery in to autozone figuring that the 8 month old battery had gone bad somehow. They checked it and it wouldn't take a charge so I got a new one under warranty. So I installed it last night and it started right up. BUT, this morning tried to start it up and very little juice. I popped the hood and I touched the connections on the alternator and they were hot. So now this battery is maybe shorted out too...


So either:
I had bad luck with the new alternator and it's shorting out the batteries; OR
Did I f^ck something up when I didn't disconnect the battery and shorted something out that is causing this problem?
In any case I'm taking both the alternator and battery in the the store and try to get a new one of each under warranty, but I'd like to know if I should be looking at something else...
Thanks guys

 

[jrichker]

Typically it is something draining the battery. Small things like glove box or courtesy lights are often the culprits. If you have an aftermarket stereo or alarm system, it is also suspect.

The ideal method is to disconnect the negative terminal, and connect a Digital Multimeter (DVM) between the negative terminal on the battery and the negative cable. Set the DVM on a low current scale of 2-5 amps if it doesn't auto range. Watch the current draw, and then start pulling out fuses. When you see a sudden drop in the current, that circuit is the likely culprit. Note that the computer, radio & clock will draw less than 1/10 amp to keep the settings alive.

Using the negative side prevents nasty accidents like shorting the wrench to ground while disconnecting the battery cable. Electrically, the test will work with either positive or negative battery cable.

 

[5.0loco]

Typically it is something draining the battery. Small things like glove box or courtesy lights are often the culprits. If you have an aftermarket stereo or alarm system, it is also suspect.

The ideal method is to disconnect the negative terminal, and connect a Digital Multimeter (DVM) between the negative terminal on the battery and the negative cable. Set the DVM on a low current scale of 2-5 amps if it doesn't auto range. Watch the current draw, and then start pulling out fuses. When you see a sudden drop in the current, that circuit is the likely culprit. Note that the computer, radio & clock will draw less than 1/10 amp to keep the settings alive.

Using the negative side prevents nasty accidents like shorting the wrench to ground while disconnecting the battery cable. Electrically, the test will work with either positive or negative battery cable.

Thanks jrichker, I won't be able to try that until a charged battery goes in. I'm not sure it will help in my situation because judging from the fact that it took only 11 hours to drain the new and fully charged battery, I would think that only something more substantial could do it, such as parking lights. I don't have any aftermarket electronics nor have I messed with any wiring recently.

I'm leaning more toward the alternator, assuming there is a short or electrical problem in the alternator - would it draw current like this?

Another possibility is that a wire from the alternator is grounding, I did move these around when I swapped the alternator....

 

[jrichker]

If a diode inside the alternator shorts to ground, you can get a rapid drain.

 

[5.0loco]

Well just got back from the store and the "new" alternator was BAD so they gave me another one and the battery is on a charger.

I hope I'm not dealing with a short somewhere else, once the charged battery goes in I'm going to check the parasitic drain, what would be the maximum "normal" amount, say nothing higher than 50mA?

 

[jrichker]

Well just got back from the store and the "new" alternator was BAD so they gave me another one and the battery is on a charger.

I hope I'm not dealing with a short somewhere else, once the charged battery goes in I'm going to check the parasitic drain, what would be the maximum "normal" amount, say nothing higher than 50mA?

That sounds about right...

 

[David Pepiton]

I'm not sure who make duralast alternators but I've been seeing a bunch of issues with them. I know oreilly's uses delco-reme. if you continue to have isses and can not find the issue try taking your car to a automotive electrician here in AZ the one i trust is lefty's.

http://www.yelp.com/biz/leftys-auto-electric-company-phoenix

 

[5.0loco]

I'm not sure who make duralast alternators but I've been seeing a bunch of issues with them. I know oreilly's uses delco-reme. if you continue to have isses and can not find the issue try taking your car to a automotive electrician here in AZ the one i trust is lefty's.

http://www.yelp.com/biz/leftys-auto-electric-company-phoenix

Thanks for the recommendation David, can't beat going to a shop that has a good rep. Fortunately it was only the alternator. I put a new one in last week and it worked just fine. After the charged battery went in I tested at a low 25 mA of parasitic draw, so no short except the one that was likely in the first "new" duralast. Must have been a dud , if this one goes I'm going to 3g swap!

 

[jrichker]

Thanks for the recommendation David, can't beat going to a shop that has a good rep. Fortunately it was only the alternator. I put a new one in last week and it worked just fine. After the charged battery went in I tested at a low 25 mA of parasitic draw, so no short except the one that was likely in the first "new" duralast. Must have been a dud , if this one goes I'm going to 3g swap!

Thanks for the feedback, it helps improve the first time fix results for all of us.