A vehicle's electrical system is charged by the power generated by the motor. If components in this system malfunction, then the volt meter on the vehicle's instrument panel will jump. The jump will most likely be caused when the accelerator is engaged; the volt meter will rise when it is pressed and fall when it is released.
The alternator is what converts the mechanical power created by the engine into electricity that can be stored in the car's battery and used by the vehicle's electrical system. The alternator helps regulate the variable input from the engine. For example, if the engine is revving high, it is creating more power. If the engine is revving low, it is creating less power. The alternator ensures that no mater what level of power is being created, the vehicle receives a steady stream. If the alternator is malfunctioning, then the volt meter may measure the changes in engine power.
A malfunctioning battery can also cause jumps in the voltage. If the battery is not holding a charge well, then it is dependent on the power supplied by the alternator. If the battery power is low and not receiving much of a charge, then it will register as low. However, as soon as more power is put into the battery from the engine, the voltage will spike.
The volt meter in the vehicle is tuned for a specific sensitivity. For example, the meter should record changes only in a certain range. However, if the volt meter malfunctions, then it can result in the apparatus becoming sensitive and recording natural fluctuations, thereby exaggerating them. This can be caused by a loose connection or short.
If the wiring leading to the volt meter or from any part of the vehicle's electrical system becomes frayed or damaged, the interference can cause jumps in the volt meter. For example, an exposed wire will likely cause no issues unless it touches another live wire or part of the vehicle; this can cause the voltage to behave erratically.
