Skip the auto shop - Our mechanics make house calls

Q: How Does a Mechanic Check a Car Alternator's Voltage Output?

asked by on

How does a mechanic check a car alternator's voltage output?

A: The first thing a mechanic will do is to ch...

The first thing a mechanic will do is to check the battery cables to make sure they are clean. Then the battery is checked to make sure it is at the proper voltage of close to 12.6 volts, using a battery voltmeter. Once a good battery voltage is verified, the battery will be load tested to see if it can sustain a fifty percent rated capacity load for fifty seconds without dropping the voltage below 9.6 volts.

Now that the battery is tested well, the alternator charging system can be tested properly. The mechanic may use a volt meter to check the voltage output and an amp meter to measure the amperage output of the alternator during testing. Once the testers are hooked as required, the mechanic will start the alternator test. The engine is started and the voltage and amperage are recorded at idle. Then the engine rpm is raised to between 1500 rpm and 2000 rpm and another voltage and amperage reading is taken. Now all the lights and accessories are turned on and the same readings are recorded. The engine accessories are turned off and the engine is turned off.

The readings taken are analyzed to see if the voltage and amperage are within the vehicle specifications. The readings under load at high idle speeds should be about 13 volts and 85% of amperage capacity. This is generally the reading for most vehicles and vehicle specifications may be different. Some alternators are regulated internally and others are controlled by a computer module. Always follow the vehicle manufacturer recommendations for testing your charging system. If the readings are below the specified voltage, then the alternator is undercharging. If the alternator voltage is over the maximum voltage, the system is overcharging and battery or component damage may occur.

Was this answer helpful?
Recommended services
Get a Quote

Need advice from certified mechanic? Get help now!

Over 1000 mechanics are ready to answer your question.
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Save up to 30%

Get a quote

What others are asking

Q: The headlights wont shut off and the horn keep honking when I turn them on.

It sounds like you have a rubbed through wiring harness or two. All the components you mentioned are switched on the ground side. That means if there is a short to ground on the switch side before the relay, then...

Q: Car dies quickly or won't turn on

You likely have a fuel pressure issue. Diagnostics will be required for this vehicle. The Aurora has a Cadillac engine with standard GM electronics. With the proper tools, a technician can pinpoint the cause of the stalling issue and get...

Q: vehicle will turn over but not start

Vehicle electronics are very sensitive to voltage fluctuation. Crossing battery terminals/jumper cables can do a lot of harm when mistakes like this are made. Your symptoms sound like a shorted PCM (Power Control Module). Since you can't get the vehicle...

Related articles

How Long Does a Heater Control Valve Last?
Keeping the right amount of coolant in a car is essential in keeping the engine at the right temperature. Failing to have the right amount of coolant or even bad elements...
How to Renew Your Car Registration in Oklahoma(DELETED)
Having your car registered with the Oklahoma Tax Commission is an important part of driving legally in this state. You will need to do this within 30 days of moving to Oklahoma...
Rules of the Road For Iowa Drivers
Driving on the roads requires knowledge of the rules, many of which are based on common sense and courtesy. However, even though you know the rules in...