How to tell if it's the battery or the alternator that's dead?
My girlfriend's car wouldn't start up last night, just kept making a clicking sound when we turned the key in the ignition. Got our jumper cables out and hooked our cars up together but her car still would not start. Does this mean the alternator is bad? Or could the battery just be totally dead? The keyless entry on her car stopped working as well, does that mean it's the battery? Any help would be greatly appreciated. Thanks!
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My buddies blazer just went through the same thing, and he asked me to come fix it, I swore it was the alternator, but I took the battery out of my car just to make sure (we tried jumping it for a good hour to no avail) and sure enough, his old battery was just DEAD!
So, how can you tell if the alternator is failing without taking it apart and doing some measuring inside the alternator? It's really pretty simple. You will need a simple voltmeter. You can get one at Radio Shack for under ten dollars. Here's what you do - start the car, make sure all the accessories are off and rev up the motor to a fast idle. Set the Voltmeter to the DC scale (not AC or Ohms). Measure the voltage across the battery terminals - red lead of the voltmeter on the positive terminal, black on the negative (ground in most cars). The voltage should, and probably will, read around 14 volts. If it reads less than 12 volts you may indeed have a failed alternator and you can skip the next step. Next, turn on the heater, the rear window de-fogger, the radio, the headlights and anything else that draws power. Now rev up the motor and watch the voltmeter. It should still be reading around 14 volts. If it reads lower than 13 volts the chances are that your alternator is not up to snuff.
if the keyless entry doesn't work the battery is probably dead. if the solenoid on the starter clicks when you turn the key then the starter is probably ok, but not getting enough juice. measure the battery thats in. you should see 11 or 12 volts when it's not doing anything.
if you leave the jumpers hooked up for 5ish minutes and the car still doesn't start then its *most likely* the starter motor.
if you get the car started and it dies again shortly later then your alternator isn't working. when the car is running you can pretty much just take the battery out and the car will run off the alternator.
i've had issues where my battery cables were so corroded inside that the car just wouldn't start even though the battery and alternator were fine, but that's rare.
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