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How to test car alternator / dynamo?

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Every car uses electricity to power up lot of devices and sensors like engine management chips, air conditioning, music systems, headlights etc. Surprisingly enough not all the electricity comes from battery as it comes from the alternator also called dynamo. Battery supplies only the first cranking power and when engine starts then the alternator takes over the electric demand. When you start your car, if you experience a delay in cranking of the engine or the headlights are dim, then there is a problem either with the battery or the alternator. The best way to find out the issue is to use a voltmeter or a multi-meter. 

When checking the alternator, you measure only the voltage, so a voltmeter is ideal; the multi-meter measures not just the voltage but other electrical properties too. 

Before checking the alternator, check the battery. It is required to start the vehicle, and spinning the alternator to keep the battery charged. So, if the battery is low, the vehicle may not start and you will not be able to check the alternator. Unlike European countries, where the weather tends to get cold and the battery is usually the culprit, in GCC the battery could be in a better state. 

To check the battery, turn off the engine before attaching the voltmeter. Connect it to the battery by placing the red end (positive) to the red battery terminal and the black one (negative) to the black terminal. If the meter shows a reading above 12.2 volts, the battery is healthy. If it is less, then you will have to recharge the battery rendering the alternator to be healthy. 

Also remember to check the connecting cables for abnormal wear - if crackled or frayed, replace them. A poor connection can cause a voltage drop reducing the flow of current through the charging circuit.

Look at the alternator gauge and an volt/amp gauge can read the alternator output. Turn on all accessories like headlights, heater or blower of the AC and see if the the gauge decreases voltage. If the voltmeter is higher when the engine is running, it means that the alternator is charging.

You can also use your ear. Try to hear the alternator when the engine is running. In case of any issue with the alternator, you can hear a weird screeching sound emanating from the front of the car.

Another way of the checking any problem with the alternator is to run the engine for a few minutes. Then turn it off and touch the alternator. If very hot, it is an indication that something is wrong. 

If the alternator is working well, your multi-meter should read somewhere in the range of 12.6 volts with the engine off and 14-15 volts when the engine is running. If voltage is below 13.5, then it is a sign that the alternator is not able to meet the battery’s charging requirements.

Try accelerating the up the engine to 2000 RPM or higher. If the voltage is too low and you are sure that all connections are well placed, then it might just be time to change the alternator.

If you need more help, or need specific answer for any question then try the ‘Carnity Forum’ (http://carnity.com/) section or find car businesses near you with the Carnity ‘Business listing’ (http://carnity.com/business_listing) section.


Edited by Carnity

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