The easiest way to test an alternator is to check the output voltage, and a lot of instrument clusters even include built-in voltmeters. Other vehicles sometimes have handy warning lights on the dash that turn on whenever the output voltage of the alternator drops below a certain level. These are both convenient visual indicators, and it’s almost as easy to check the voltage output of an alternator with a handheld voltmeter, but voltage doesn’t tell the whole story.
Any thorough alternator testing procedure should also include checking the amperage output, which can be accomplished with either an automatic tester or a clip-on ammeter in conjunction with a carbon pile.
Testing Alternator Output Voltage
Although dashboard warning lights and voltmeters are convenient, it’s never a good idea to put too much stock in them. Instrument cluster voltmeters can provide faulty readings due to bad wiring, and a failed warning light can actually cause an alternator to malfunction in some cases. Instead of relying on built-in monitors, it’s important to test an alternator by checking the voltage output right at the unit.
Alternator output voltage should be checked with the engine running and all of the accessories off. It’s also important to check the voltage at the alternator since checking at the battery may provide an unreliable result. To that end, the positive lead of the voltmeter should be connected to the positive output terminal on the alternator, and the negative lead should be connected to ground. The negative lead can be attached to virtually any metal component that is properly grounded. The one major exception is the alternator housing, which shouldn’t be used as a ground for testing purposes.
The output voltage of a healthy alternator should be somewhere in the vicinity of 14 volts, and that value should increase slightly if the engine speed is increased. If the voltage is especially high, the regulator may be bad. If the voltage is especially low, the alternator may be bad. Low voltage may also be caused by poor electrical connections, a loose drive belt, low engine idle speed, and a number of other issues.
Testing Alternator Amperage Output
In addition to testing the output voltage of an alternator, checking the amperage output is also vital. This can be accomplished with specialized electronic testers, but it’s also possible to check with an appropriate ammeter and a carbon pile, which is a device that is capable of placing a load on a vehicle’s electrical system. In order to perform this test, the ammeter should be clamped around the alternator’s positive output wire, and the carbon pile should be connected to the battery.
Before testing the output of an alternator with an ammeter and a carbon pile, it’s important to zero out the meter. It’s also a good idea to perform the test with all of the accessories in the vehicle turned off. The engine should then be started and held at about 1,500 RPM. While the engine is held at that speed, the carbon pile can be used to create a simulated load. That will cause the alternator output to increase. If the alternator is in proper working order, the ammeter reading should be roughly equal to the rating of the alternator.
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