Every editorial product is independently selected, though we may be compensated or receive an affiliate commission if you buy something through our links. Ratings and prices are accurate and items are in stock as of time of publication. Don't test an alternator by disconnecting the negative battery cable. Use a
voltmeter instead to conduct a simple, safe test. If you have a newer battery but your car won’t start, you may have a bad alternator. 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 without the negative cable, but this 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. Step 1Learn how to how to test an alternator without disconnecting your vehicle's battery.
Introduction
Tools Required
Here’s a video on how to properly and safely test your alternator.
Project step-by-step (2)
Conduct a Voltmeter Test
Step 2
If Your Alternator Passes
- If you connect the meter leads to the battery terminals and the meter shows that it's in the 13.8 to 15.3 volt ranges (engine running, lights and accessories off), that means the alternator is working as it should be.
- If the alternator is functioning correctly, it's likely that your dead battery was caused by a computer module that isn't shutting down when you turn off the car. If your alternator passes the voltmeter test, get your vehicle into a shop and pay a professional mechanic to find and correct the misbehaving module.
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Do It Right, Do It Yourself!
Originally Published: January 04, 2019
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