How to Know My Alternator Is Broken
- 1). Run the engine for 30 minutes, so the alternator will fully charge the battery (if both are working right). You can drive the whole time if that will make the time pass faster. As with the headlights, radio and AC, the alternator doesn't lose power just because the engine is running faster.
- 2). Change the voltmeter to its direct-current setting; batteries produce DC power, not AC.
- 3). If you don't have a voltmeter, change your multimeter to volts, not amps. This converts the multimeter to a voltmeter. This is especially important because leaving it in the ammeter mode keeps resistance too low and you could short your battery. A voltmeter, by contrast, has high resistance, so it won't damage the battery.
- 4). Remove the key from the ignition. Turn off all electrical devices, such as headlights and air conditioning.
- 5). Set the meter readout to display between 9 and 15 volts.
- 6). Touch the voltmeter's two metal probe tips to the two battery terminals. Ensure a positive reading by touching black to negative and red to positive. A voltage of less than 12.5 indicates something is definitely wrong, but doesn't distinguish whether it's the battery or the alternator. A voltage of 12.8 or more probably means your battery and alternator are both fine, but you'll still need to do a load test to be sure.
- 7). Test the battery under a modest load. First, insert the ignition key and turn on the hazard lights. Don't start the engine, since a few lights are enough for this test. The battery should still manage around 12 volts. If your reading is lower, then you'll have to end the alternator test right there until you fix the battery or get a new one. According to BatteryStuff.com, 80 percent of battery failures come from the reversible process "sulfation." This is a coating of the internal metal plates with electrolyte. Use the links in Resources to learn how to reverse the process with an Epsom salt solution and proper overnight charging with a three-step charger.
- 8). Continue the alternator test if you have a good battery. Start the engine. Wait 10 minutes for the battery to recover any charge lost starting the car. Take another volt reading of the battery. Less than 13 volts means the alternator is bad. More than 15.5 volts means the battery isn't properly taking up a charge from the alternator. Instead, the charge is flowing from the alternator right into the voltmeter. In that case, your alternator is fine but your battery isn't.
- 9). Check for voltage reduction anywhere between the alternator and battery. A reading of more than 0.4 volts between two points along their joining wiring needs resolving--for example, a cleanup of corrosion or dirt, or fixing loose wiring.
- 10
Test for voltage loss between each battery terminal and its respective cable clamp. White matter builds up there and reduces conductivity. If the voltage drop is greater than 0.1 volts, this is too much for such a short distance. Turn off the ignition, put on some insulating gloves, remove the clamp and chip away at the residue with the end of a screwdriver or a similar implement. Use common sense. Fan the area to clear the air of any explosive battery gas. This could otherwise ignite if the clamp and terminal spark. Such explosions often lead to blindness.
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