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Tech Tip #5 – Charging System Check
The vehicle’s alternator has two jobs; the first is to supply all the electricity needed by the vehicle while it is running, and its second job is to recharge the vehicle’s battery. A battery with a low charge or internal problems can place too much of a load on the alternator and possibly overheat and damage the windings or diodes in the alternator. So it is important to perform the battery checks and charge the vehicle’s battery fully before starting any alternator testing.
Do not be tempted to test an alternator by disconnecting the negative battery cable with the engine running. The alternator may keep the engine going, but it is not a good test method. In the pre-computer days, you could pull it off without possibly damaging anything. With today’s computer controlled cars, you risk frying every electrical module in your car. As soon as you disconnect the battery, the voltage regulator tells the alternator to put out maximum power. With no battery in the circuit to act as a buffer, the alternator can easily put out voltage spikes over 100 volts, depending on engine rpm. That “simple test” could end up costing you several thousand dollars for new electronics.
So to begin testing the alternator, like we said before, first verify you have a good, charged battery with a simple voltage check. Your battery should have a minimum of 12.5 Volts before testing the alternator. Once we have verified the battery is charged sufficiently, we can test the alternator output by simply starting the vehicle and monitoring the voltage present at the battery. The battery will initially be a large draw on the alternator during the initial re-charge after startup, so let the engine run for a few minutes to allow the alternator output to stabilize before testing. Raise the engine speed to about 2000rpm and you should have a reading between 13.8V to 15.3V.
If your volt reading is at or near battery voltage, that means the alternator is not functioning at all. This could be an internal problem with the alternator (bad windings, diodes, etc.) or a problem in the wiring or control circuits to the alternator. Less than 13.8V indicates a weak alternator that is still putting out some electricity, but not enough to keep up with the demands of the vehicle and charge the battery sufficiently. Any voltage over 15.3V indicates an overcharging condition that can lead to damage to the vehicle’s electronics and be overcharging of the battery. See the Alternator Charging Voltage chart.
There are a couple other items to check in the charging system. Alternators work by generating an AC voltage and then converting the AC voltage to DC voltage through a series of diodes called a rectifier. An alternator with one or more bad diodes can cause AC voltage spikes which can cause lots of hard to diagnose electrical problems. To check the alternator diodes we can measure how much AC “ripple” is present in the system by measuring for AC voltage at the battery. This AC ripple voltage should be less than 0.5VAC.