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With the engine running, check the output at the back of the alternator and see if it’s the same as what the voltmeter in the car is reading. If it’s higher, the problem is excessive load on the fuse center, wiring, or ignition switch (most ignition switches have a maximum capacity of 60 amps). If the output is the same, the belt driving the alternator may be slipping or the alternator may have an internal problem. Once again we are looking for voltage between 13.6 and 14.8.