Abstract: The description discloses a glow plug system for a Diesel engine of a motor vehicle having a glow plug (RG1, RG2) which comprises a positive terminal for connecting a supply voltage (U1) and a ground terminal for connecting to a ground potential (GND), a controlling device (1) for controlling the electric power supplied to the glow plug (RG1, RG2) in operation, whereby the controlling device (1) comprises a measurement input (ADC1) and a ground input (ADC2) in order to determine, in operation, a measured value of the supply voltage (U1) in relation to a reference potential (GND?) applied to the ground input (ADC2).