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Old 03-13-2012, 07:10 AM   #1
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How to check to see if alternator is bad

I've been having issues starting the Jeep lately. I took the battery to a shop to see if it was good or bad, and it appears to be okay thus far but it is about 5 years old. It does support lights, amps, and a winch.

What would cause an alternator to go bad? Are there other things I should look at besides the belt to see why my battery either won't charge or hold a charge?

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Old 03-13-2012, 07:15 AM   #2
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I would check the cables and posts. Jeeps are really picky about the connections. Usually the brushes in the alt will cause it to go bad. Older altenators can be rebuilt, but the newer styles are much trickier.

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Old 03-13-2012, 07:33 AM   #3
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I had to replace mine a few months ago. It looked fine from the outside but everything inside Had rusted up and broke apart. When It died on me driving back to my house, I took it off, (really easy, just 2 bolts, IIRC) and took it to O'Reilly's and had it tested. When he hooked it up to the machine in the store you could hear all the broken stuff rattling around inside it. Taking it out and putting putting the new one in took less than 30 minutes. Try that before you start throwing parts and money at it.

Edit: Mine was the OEM alt.)
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Old 03-13-2012, 08:06 AM   #4
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Hook a volt meter to the battery. Engine off should be 12 to 12.8 volts. Engine on should be 13.7 to 14.7 volts if your alternater is charging.
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Old 03-13-2012, 08:22 AM   #5
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Quote:
Originally Posted by formula96 View Post
Hook a volt meter to the battery. Engine off should be 12 to 12.8 volts. Engine on should be 13.7 to 14.7 volts if your alternater is charging.
This

I'd say anything below 13.4V while running is not enough to properly charge the battery. Ideally should be around 14.4V.
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Old 03-13-2012, 08:52 AM   #6
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SOunds good. Once I get the battery back (assuming they say it is okay still), I'll try it on and off to see what kind of voltage readings I get.

IF it is the alternator, should I go OEM or go with an aftermarket unit? Recommendations?
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Old 03-13-2012, 08:55 AM   #7
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Quote:
Originally Posted by Rolf View Post
This

I'd say anything below 13.4V while running is not enough to properly charge the battery. Ideally should be around 14.4V.
If the voltage is on the low side when running, I'll usually turn the lights on and recheck it.
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Old 03-13-2012, 09:09 AM   #8
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When you took it to the battery shop did they check the cold crank amps. I had the same problem with mine not wanting to start and died wile idling. Took my alternator to to have it tested any it was fine. Took my battery to the shop and the cold cranking amps where way low. Changed the battery and have no problems since. A good way to check to see if it is the alternator start your jeep and disconnect the battery. If the alternator is good your jeep should keep running. Good luck I hope you figure it out.
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Old 03-13-2012, 09:14 AM   #9
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Yeah, they checked it. Initially it is supposed to be 800 CCA's. But when tested, it was 159. After charging they had it up to 550. So they are charging it overnight and I'll pick it up at lunch today.
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Old 03-13-2012, 10:34 AM   #10
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Charging systems check....

Quote:
Originally Posted by s14sh3r View Post
If the voltage is on the low side when running, I'll usually turn the lights on and recheck it.
This is a good idea, but I usually have EVERYTHING turned on......blower for heater, windshield wipers, flashers, and anything else I can think of.......

This checks your charging system under WORST CASE scenario....... anything less it should keep working fine......

If you do not see AT LEAST 13.5 VDC with all of this stuff turned on,you have a charging systems issue.

Most common, glazed/old/worn/improperly tensioned/seated belt. Make sure the belt is not bottoming out in the alternator groove. Belts work on the SIDES of pulleys. If you grab your alternator pulley (engine NOT running), see if you can spin it by hand. You should NOT be able to. If you can, one of the above issues is present. This allows slippage under load and decreases current/voltage output.
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Old 03-13-2012, 10:57 AM   #11
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If the battery is 5 years old, replace it....While you are at it, go thru all battery, and battery to fusebox connections and disassemble/clean with wire tooth brush and retighten.

A simple voltage test on runnin/not running alt is good for quick troubleshooting out on the road, but it needs to be tested under load on test stand.
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Old 03-13-2012, 03:04 PM   #12
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I'd take that battery, have it bronzed, and put it on the mantel! I can't get 3 years out of one!
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Old 03-13-2012, 04:40 PM   #13
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Quote:
Originally Posted by firemt04 View Post
When you took it to the battery shop did they check the cold crank amps. I had the same problem with mine not wanting to start and died wile idling. Took my alternator to to have it tested any it was fine. Took my battery to the shop and the cold cranking amps where way low. Changed the battery and have no problems since. A good way to check to see if it is the alternator start your jeep and disconnect the battery. If the alternator is good your jeep should keep running. Good luck
I hope you figure it out.
It doesn't take 13.5 volts to keep a vehicle running. Although, I don't recommend this test with today's vehicles, even if the vehicle continues to run, the alt can still be bad. Think about it. If the engine will run on a half dead 12 volt battery, it will run on a weak alternator. Simple test is to turn everything off except your low beams. Then turn the vehicle off and notice if the lights get dimmer. If they do, then the alt was putting out more voltage than the batt. Works best at night on a wall. Or take it to the nearest autoparts store to have it checked for free. Takes a couple minutes.
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Old 03-13-2012, 05:04 PM   #14
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i believe there is a way to test the alternator by disconnecting the wires from it and measuring the Ohms. i'm not sure how many Ohm's should be read and it may vary from alternator to alternator depending on the CCA output. anyone?
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Old 03-15-2012, 07:38 AM   #15
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Got the battery back yesterday.

550 CCA at the charge (battery when new was 700).

Voltage unhooked: 12.34V
Voltage hooked up, nothing running: 12.34V

Voltage with high beams, fogs, windshield, rear fogs, a/c, and stereo going: 13:8V

That voltage when running seems a bit on the end of the acceptable scale, so I'm going to look over the alternator to see if I can find signs of water or mud damage. But it looks like the battery is on the last legs at a minimum.

What kind of alternator are people going with?
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Old 03-15-2012, 04:33 PM   #16
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Quote:
Originally Posted by erickpl View Post
Got the battery back yesterday.

550 CCA at the charge (battery when new was 700).

Voltage unhooked: 12.34V
Voltage hooked up, nothing running: 12.34V

Voltage with high beams, fogs, windshield, rear fogs, a/c, and stereo going: 13:8V

That voltage when running seems a bit on the end of the acceptable scale, so I'm going to look over the alternator to see if I can find signs of water or mud damage. But it looks like the battery is on the last legs at a minimum.

What kind of alternator are people going with?

That voltage is fine for everything running. Sounds lik the battery is a gonner.

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