I was already running late, leaving my house for work one-day last week, only to hop into the driver’s seat of my truck and hear the starter click. I tried turning the key again, but the truck refused to turn over. My guess, it was one of two electrical components in charge (pun intended) of electrical power. I suspected either the battery, it supplies electrical current to the starter to start the engine or the alternator that maintains the battery’s charge. If the battery is bad it is unable to store the chemical energy provided by the alternator. If the alternator is bad, the battery will run out of charge from running other electrical components.
Luckily, I was at home and had access to the tools I needed. I went in search of a voltmeter. If you want to perform this test, a cheap version can be bought from Harbor Freight Tools for under $10. The tool comes in handy for several different DIY troubleshooting methods and will be needed for part two of this article. After finding the voltmeter I ran the red test lead to the positive terminal of the battery marked with a + symbol. I then place the black test lead on the negative terminal marked with a – symbol.
Make sure the meter is set to read volts for direct current volts (DCV) on the switch, and set, to a voltage reader higher than 15. The first thing I checked was the alternator. This may seem a little backward, however, my truck required jumping off and the battery was already dead and it would produce a false test result. Once I had my truck jumped off, I watched the voltmeter. The alternator is supposed to charge somewhere between 14.2 or 14.7 volts across the battery.
My meter read 14.1 volts. This is a little low but fine. Then I had to test a load under these conditions to make sure the alternator could supply enough electricity for the trucks other electrical systems. A load is anything electrical that uses power from a source, like the battery. I turned my lights on and my air conditioner on high. These are significant electrical loads. I again checked the voltmeter to see if the alternator was able to stay consistent at around 14 volts. It read around 14.2 volts, fluctuating to 14.1 volts. If the meter would have read lower than 12.7 volts or gone higher than 14.7 volts it would indicate a bad alternator. At this point, I needed to check the battery.
By now the truck had run for about 10 minutes, giving the battery some time to recharge. To test the battery, the truck must be shut off. The battery should read just over or under 12.6 volts. My meter told me this battery had 12.3 volts. This was slightly lower but understandable considering it had only charged for a short time.
The truck had some kind of electrical problem that was drawing the battery down, but according to both of these tests, the battery and alternator were okay. The next thing to test for is an electrical short. Hopefully, after reading this you will have an idea on how to diagnose a bad battery or alternator. For the next car talk, I will diagnose an electrical short.