Glow plug system, controlling device and method for controlling the power of a glow plug

The description discloses a glow plug system for a Diesel engine of a motor vehicle having a glow plug (RG, RG) which comprises a positive terminal for connecting a supply voltage (U) and a ground terminal for connecting to a ground potential (GND), a controlling device for controlling the electric power supplied to the glow plug (RG, RG) in operation, whereby the controlling device comprises a measurement input (ADC) and a ground input (ADC) in order to determine, in operation, a measured value of the supply voltage (U) in relation to a reference potential (GND′) applied to the ground input (ADC). The controlling device has a test input (ADC) which, in operation, is connected to a test voltage source via a first resistor (R) and to the positive terminal of the glow plug (RG, RG) via a second resistor (R), whereby the controlling device, in operation, determines a difference of the electric potential of the test input (ADC) and the electric potential (GND′) of the ground input (ADC), determines a deviation of the difference of these potentials from a reference value and, if this deviation is unequal to zero, uses the deviation to correct the measured value of the supply voltage (U) and uses the corrected value of the supply voltage (U) as for controlling the power. Furthermore, the description discloses a controlling device for a glow plug system of this type and a method for controlling the power of a glow plug.