Almost sure I have a bad alternator but ...

[Saint Cad]

Car running fine and then won't turn over. Wacky electric stuff like out of Close Encounters. I've seen that before and it means my car is getting some juice but not enough. I open up my hood and clean the corrosion off of both of the teminals
and posts but the negative is really bad.

Maybe important: I couldn't get all of the negative corrosion off in the field. Also the terminal was so corroded that the bolt spun while trying to loosen the nut. I managed to pry the terminal of but after taking the 1/32 - 1/16" of corrosion off I could clamp it down. It seems tight but I'm not 100% sure that it has a good connection.

Try to jump it and it comes close but doesn't quite turn over. The battery is a cheap one and pretty old so I figure I'll get home and probably just replace it.

AAA comes out and hooks it up to their power and it starts. I drive home the 3 blocks but the funny thing is the odometer is flickering. As soon as I pull in my driveway (thank god) the car just dies. Obviously it won't restart and everything points to a bad alternator. Obviously I will put in a new battery and test the alternator by pulling the positive terminal but is it possible that it is something else?

[beowulff]

Bad battery, bad cables, (maybe) bad alternator.
Fix the cables and get a new battery, and then see if the battery won’t charge.

[greenslime1951]

One good test would be to measure the input voltage at the battery while the car is still running. If you're not getting 14 volts, then the alternator may be bad.

The trouble is that corroded cables can also cause a voltage drop, and you can't necessarily see the corrosion as it's often inside the strands. The best way is to measure the output at the alternator, but that's a bit harder to do.

If the cables and terminals are as bad as you say, though, I would recommed replacing the battery, the terminals, and the cables. Then measure the alternator's output again. If you now have 14 volts, you should be OK.

Don't forget to check the alternator drive belt for wear and proper tension.

Edit: When checking the voltage, put the negative probe on chassis ground (NOT the negative battery terminal) and the positive probe on the positive battery post. Make the terminal-to-post connection as tight as possible.

[tapu]

Just as it's never sarcoidosis, it's never the alternator. That's just a plot device.

[beowulff]

Originally Posted by greenslime1951

Edit: When checking the voltage, put the negative probe on chassis ground (NOT the negative battery terminal) and the positive probe on the positive battery post. Make the terminal-to-post connection as tight as possible.

I would measure the voltage from post-to-post, since that's what counts.
Measuring the voltage from chassis ground includes the cable voltage drop + any corrosion resistance, so it may give a misleading reading.

[Saint Cad]

I know there is corrosion at least in the visible part of both cable so they probably need to be replaced as well so battry and cables are going to be changed out.. Oh and the two terminals are different metals - neg is copper and pos looks like zinc. I can't remember ever seeing that before.

[Fubaya]

Originally Posted by Saint Cad

Obviously I will put in a new battery and test the alternator by pulling the positive terminal

Disconnecting the positive terminal can cause bad things to happen to your electronics because the battery provides voltage regulation. I once saw a car blow the airbags when the battery was disconnected with it running, but frying the computer is usually the biggest worry.

[engineer_comp_geek]

Originally Posted by greenslime1951

One good test would be to measure the input voltage at the battery while the car is still running. If you're not getting 14 volts, then the alternator may be bad.

If you have a good battery and alternator, you should measure something close to 12.5 volts with the car off and 14 volts or so with the engine running. If the battery has dead cells or is severely discharged though it may read a lot less than 12.5 volts, and even with a good alternator it may not reach 14 volts while the car is running. What you really want to look for is whether or not the voltage bumps up by a couple of volts or so when the car is running compared to when it is off.

Also, Fubaya covered it pretty well, but it bears repeating. DO NOT DISCONNECT THE BATTERY ON A MODERN CAR TO TEST THE ALTERNATOR! It's a great way to do some serious damage to a modern car's electrical system.

I strongly suspect the OP needs a new battery, terminals, and an alternator, but if I were fixing it myself I would test the battery before replacing it and maybe just charge it first (using an external slow charger), and also hold off on replacing the alternator until after the battery and cables have been taken care of, and then test the alternator to see if it is charging and needs replaced.

[Iggy]

Originally Posted by greenslime1951

...

Don't forget to check the alternator drive belt for wear and proper tension....

I had lots of electrical weirdness. Turned out to be the belt. Unfortunately I was so consumed with checking and replacing everything else that I never noticed. Then the belt broke while I was driving.

After the tow truck and a new belt all of the electrical weirdness went away. Guess alternators don't work so well if they aren't spinning right?

[Gary T]

Originally Posted by Saint Cad

Obviously I will put in a new battery and test ruin the alternator by pulling the positive terminal but is it possible that it is something else?

There, fixed it for you.

Originally Posted by beowulff

Bad battery, bad cables, (maybe) bad alternator.
Fix the cables and get a new battery, and then see if the battery won’t charge.

I agree.

[Saint Cad]

Update:
New battery and I threw my voltmeter on it and started the car. Jumped up to 14.5V but backed down to 13.9V over a few seconds. Thoughts?

[Gbro]

Originally Posted by Saint Cad

Update:
New battery and I threw my voltmeter on it and started the car. Jumped up to 14.5V but backed down to 13.9V over a few seconds. Thoughts?

Sounds like you are good to go.
If you change out any B.O. cables.

[Saint Cad]

Originally Posted by Gbro

Sounds like you are good to go.
If you change out any B.O. cables.

Didn't change the cables. It seems to be a pro job on the 2005 Jeep Libertys

[aceplace57]

Auto Zone is where I go.

Don't buy a part you don't need. AutoZone will test your car's parts for free. We can test your car's battery, alternator, starter and voltage regulator while they're still on your car. We can also give your car a complete starting and charging systems test.

http://www.autozone.com/autozone/pro...essories=false