A weak alternator can have a number of negative effects on your vehicle. A weak alternator can limit the electricity supply to the fuel injectors of your vehicle. As the fuel injectors are an integral part of generating power in your engine, this can result in poor performance. A weak alternator can also lead your engine to misfire, idle roughly as well as accelerate poorly. On severe occasions a bad alternator can cause your engine to constantly stall, causing damage to its internals of it.