Car Repair Advice by Professional Automotive Experts
How Do I Repair My Car?
/ How Do I Check
My Car Alternator?
How Do I Check My Car Alternator?
Most batteries wear out every 3 to 5 years and need to be replaced. Always replace
your battery with an equal replacement battery to assure proper operation. Automotive
batteries have a + positive terminal (red), - negative terminal (black). The battery
in this illustration has a protective cover over the positive terminal to prevent
short circuit in case of an accident. Electricity is stored in the battery and then
supplied to the vehicle when the engine is not running. While the engine is running
the vehicles alternator charges the battery for future use. (Note: never
disconnect the battery while the engine is running. If the battery cable is
disconnected from the battery a spark can be generated which can cause the
battery to explode or a major electrical malfunction to occur.)
Step 1 - Checking Battery
To check a battery surface voltage remove the positive terminal protective cover.
Connect the +positive side meter lead (red) to the positive side battery terminal.
Connect the - negative (black) side meter lead to the negative battery terminal.
With the vehicle not running and the car sitting over night the battery voltage
should be between 12.5 and 12.8 volts.
Step 2 - Identify Alternator
The alternator is rotated by a drive belt driven by the vehicles engine while
it is running. Electrical voltage and amperage are generated to recharge the battery
and supply voltage to the electrical system of the car. The alternator is held in
place with mounting bolts. There is a main electrical wire on the rear of the alternator
that supplies voltage to a main voltage junction box. If your alternator is not
charging properly, your battery will slowly drain down from operating all the electrical
systems in your car and stop the car from running.
Step 3 - Checking Alternator Output
Connect the voltage meter lead the same way you would in a battery static voltage
check, Start engine (do not drive) at engine idle the voltage should be between
13.6 to 14.3 volts. If not the alternator may need replacing.
Step 4 - Checking Alternator Amperage Output
Testing the amperage output of the alternator is good for measuring the
amount (not the level) of voltage the alternator can produce. This test can be
tricky because if the alternator is weak it can still show it as producing
amperage. Which is good, but if the voltage is low, it will still allow the
battery to go dead. To check the amperage output of an alternator an amp meter is
needed. Once the meter is connected start the engine. Next turn on all
electrical accessories and raise the engine idle to about 1200 RPM. The
alternator should output the max amperage it was designed to produce. Example: a
90 amp alternator should output about 88 amps. Note: An alternator cannot
sustain maximum output for long periods of time. If the alternator is forced to
operate at maximum output it will overheat and fail. An alternator is designed
to operate at max amperage output only for a brief amount of time.
Common Problems:
WARNING! Always wear protective eyewear, gloves and necessary
clothing before inspection or work begins. Never crank an engine over when you are
near the battery or engine. A battery is filled with explosive gasses that can ignite
when extreme heat or a spark is present. A battery could have residual acid on the
outside casing so use caution when handling a battery and the surrounding area.
Use baking soda to neutralize any suspected acid residue before work begins.
Wear protective eye wear and clothing and remove all
jewelry when checking your battery and charging system.
How Do I Repair My Car?
/ How Do I Check
My Car Alternator?
Additional Resources
Additional "How to" Car Repair or Services
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