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Discussion Starter · #1 ·
hey eveybody

im a mud guy at times, and it never fails me to kill alternators. i bought one over the summer with a lifetime warranty. well my battery died at the end of summer. i just got around to putting a new one in today, but the check engine light is still on. (which usually came on when my alternators were going out) ran great, but i want to check the alternator. it isn't really a big deal, since itll be free, but i wanted to check it without having to remove it. is there anyway to do this??

thanks:thumbsup:
 

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mk20rockeye said:
start your jeep and then take the negative terminal off the battery. that has never failed me. the engine runs off the alternator once started.
That "trick" won't work on lots of late model vehicles.

Just charge the battery and then drive to Autozone or Advance. They will come out and check your alternator in the parking lot.
 

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Discussion Starter · #6 ·
if the removing the nagative connection is true then it might be the alternator then. the battery i had before was pretty dead, and when i jumped it, it would run fine, but stumble off and die. now it starts and runs great. but it could be because theres enough charge in the battery..sound true. so if i go home and pull the negative connection and it dies, then its the alternator for sure??
 

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haha, those may work but the only sure fire way is to start the car, get a multimeter on the battery or alternator (either works) and the voltage at idle if working should be between 12.5 and 13.5volts. At 2000 rpm, should be 13-14 volts, if its ever below 12v the alternator is not charging. Thats how we check em at our NAPA store since we dont have a bench tester. Hope that helps :)
 

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Testing an Alternator Manually
Now that we’ve gone over some of the signs of a bad alternator, let’s talk about how to actually test one without a multimeter. While a multimeter is the most accurate way to test an alternator, there is a way to test it manually.
 

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start your jeep and then take the negative terminal off the battery. that has never failed me. the engine runs off the alternator once started.
NEVER do this to any OBD-II vehicle. You can destroy the PCM.

Test the alternator output with a $ 6-15 Volts/Ohms multi-meter from your local hardware/home improvement/Walmart type store.

Handheld auto parts store alternator testers are somewhat inaccurate, and tend to pass marginal alternators. The best test is to remove the alternator, and give it a Load Test inside a test machine.
 
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This is what I use. Mine is a very old Milton but they make inexpensive knock-offs. It is known as a carbon pile tester. It puts a load on the electrical system with high resistance wires very similar to a toaster oven. It shows the battery charge, and when the load is activated, shows the alternator output. A healthy alternator will respond by matching the load. Worth every penny.
Circuit component Electrical wiring Gas Electronic component Electrical supply
 
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