Video: How to Test an Alternator
If you have a newer battery, but your car won't start, you may have a bad alternator. Rick Muscoplat, an automotive expert at The Family Handyman, will show you how to test an alternator.
Test the alternator with a voltmeter
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Alternator test with a voltmeter
If it's in the range, it's good.
Connect the meter leads to the
battery terminals and look for 13.8
to 15.3 volts (engine running, lights
and accessories off). That means the
alternator is pumping out the juice.
If you're tempted to test an alternator by disconnecting the negative battery cable, don't do it. A good alternator may indeed keep the engine running, but it was never a good test. In
the pre-computer days, you could
pull it off without damaging anything.
Today, you risk frying every electrical
device in your vehicle. The second
you disconnect the battery, the voltage
regulator pegs the alternator to put
out maximum power. With no battery
in the circuit to act as a buffer, the
alternator can put out up to 150 volts,
depending on engine rpm. When the
smoke clears, that “simple test” could
end up costing you several thousand
dollars for new electronics.
Instead, get a cheap voltmeter (about
$15 at any home center or auto parts
store). With the engine off, battery voltage
should be between 12.5 and 12.8
volts. If it's below that, charge the battery
with a battery charger before you
conduct the test. Then start the engine
and check for increased voltage readings
as shown above. If you see higher
readings, chances are the alternator
is good (more sophisticated testing
equipment is needed to detect an open
or shorted alternator diode).
By the way, a dead battery in the
morning is usually caused by a computer
module that isn't shutting down
when you turn off the car. If your
alternator tests good, get your vehicle
into a shop and pay a pro to find the