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Remember the good old days of battery and alternator testing? With just a multimeter you can test if the alternator is charging and if the battery is in a proper state of charge. In most of the latest model vehicles, the battery and alternator are managed together to minimize load on the engine for increased fuel economy.
These days, it takes a lot more than a multimeter to diagnose a battery and alternator. You should use a battery test that can measure capacitance and scan to see if the alternator should charge. Here are six tips to use when diagnosing a vehicle with a dead battery.
1 One Test Is Not Enough
If there is only battery-to-battery voltage on a running engine, does this mean the alternator is “bad?” No. It just means the alternator isn’t charging, but doesn’t reveal why. Therefore, it does not indicate a faulty alternator. Too often alternators get cursed at by technicians because of this test alone. Since the battery and alternator work together as a system, you will need to test the battery with a tester and scan tool to check the commanded code and voltage.
2 Find It
For some people, it is normal to see voltages as high as 15.1 volts for extended periods of time. For others, it’s normal to see as low as 13 volts. For some people, a constant 13.6 volts is an indication of a problem. However, it is quite normal for others to see the alternator not charging at all to improve fuel economy. Take time to look at the service information and test with a scan tool to see if the ECM is controlling the alternator.
3 Extra Reasons, Extra Problems (Code)
It is bad practice to use the positive and negative battery terminals to power aftermarket accessories on some vehicles. This can change the internal resistance of the battery. Extra connections can cause the battery to incorrectly report the discharge status, even if the battery is in normal condition. This can cause the battery to overcharge and/or a code to be set which can cause the charge light on the instrument cluster to come on. You may have to remove additional items and test again.
4 Interrupted Alternator Communication
If communication is lost between the PCM and the alternator, the PCM will flash the battery light, but the regulator will continue to charge the alternator at about 13.6 volts. The PCM will assign a trouble code regarding the loss of communication. Imagine trying to diagnose a battery light that seems to have no reason to come on. That means it doesn’t seem to have a valid reason until the scanner is added to testing.
5 Milliamps, Not Amps
In many vehicles, the PCM controls the alternator duty cycle. This controls the amps and voltage in a much smoother and faster increment than can be measured with a meter so that current is supplied in a more efficient way. This change may be only 10 milliamps. You can’t buy a mediocre connection in the terminal.
6 Temperature Problems
Batteries are a chemical reaction. The reaction temperature between the plates and the electrolyte is important in the way a battery charges and discharges. Some vehicles will use a temperature sensor on the battery. Other vehicles will use the ambient temperature or under-hood temperature readings as measured by sensors inside the ECM. If you encounter a vehicle with low battery power, look up the temperature-related PID data using your scan tool. When these sensors fail, they usually produce data that is too hot or too cold for the condition.
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