The proposal is for a motor vehicle security arrangement with a theft-protection control device which effects an exchange of data with at least one functional control device, especially an engine control device in order to control its operational readiness, in which the data exchange takes place via a data line interconnecting at least the theft protection control device and the functional control device. To the data line (1) may be optionally connected an external device (4) designed for the exchange of data with at least one of the control devices (2, 3, 3', 3''). The data line is a line already present in the motor vehicle, especially a K and/or L diagnosis line to ISO standard 9141. For data exchange purposes, one of the control devices (2, 3) connected via the data line (1) perceives the functions of the control unit. If an external device (4) is connected to the data line (1, 1'), it assumes the function of control unit from the control device (2, 3) monitoring the data exchange in order to operate the data exchange with the control devices (2, 3, 3', 3'') connected via the data line (1).