Method for operating a passive access system for motor vehicles, with a plurality of vehicle-side transmit antennas for transmitting low-frequency signals arranged at different spatial positions on a vehicle, which are coupled to at least one control device, the control device being coupled to vehicle-side transceiver means for wireless communication, wherein an ID-transmitter which is mobile relative to the vehicle is designed with a 3D-receiving antenna arrangement for receiving low-frequency signals, wherein the ID-transmitter has key-side transceiver means for wireless communication, wherein a first transmission antenna on the vehicle side and a second transmission antenna on the vehicle side are controlled with the aid of the control device for the time-shifted transmission of first time-shifted low-frequency signals with respectively assigned transmission signal strengths, wherein the first and the second transmission antenna on the vehicle side being controlled with the aid of the control device for the simultaneous transmission of first simultaneous low-frequency signals with respectively assigned transmission signal strengths, wherein at least one of the time-shifted or simultaneous low-frequency signals is received in the mobile ID-transmitter with the aid of the 3D-antenna arrangement and a signal strength evaluation is carried out for each of the spatial directions, so that signal strength information dependent on the spatial direction is determined for each spatial direction of the 3D receiving antenna arrangement, wherein information relating to the signal strengths dependent on the spatial direction is transmitted from the ID-transmitter wirelessly from the key-side transceiver means to vehicle-side transceiver means of the control device, wherein the control device receives the information relating to the signal strengths dependent on the direction of space and, depending on the received information, controls one or more transmission antennas on the vehicle side for the time-shifted or simultaneous transmission of further low-frequency signals, the respective associated transmission signal strengths being selected depending on the received information relating to the signal strengths dependent on the direction in space, whereby in the mobile ID transmitter the further low-frequency signals are received with the aid of the 3D-antenna array and a signal strength evaluation is carried out for each of the spatial directions, so that further spatial direction-dependent signal strength information is determined for each spatial direction of the 3D-Antenna array, whereby a plausibility check is carried out depending on the further space-direction dependent signal strength information, checking whether the space-direction dependent signal strength information is consistent in order to detect an relay of the signal between the vehicle and the ID transponder via intermediate stations, characterized in that in the mobile ID transponder at least one of the first time-shifted low-frequency signals with the help of the 3D-Antenna arrangement is received and wherein, if the signal strength evaluation indicates a signal strength below a predetermined threshold in at least one of the spatial directions of the 3D-Antenna arrangement, the information relating to the spatial direction-dependent signal strengths in the control device causes another transmission antenna on the vehicle side to be driven instead of the transmission antenna on the vehicle side associated with the signal in order to transmit a further signal.