Intelligent and connected vehicles (ICVs) have become the mainstream in the development of automobile industry. Many emerging technologies have been proposed to provide users with comfortable and convenient driving experience. However, even though these technologies significantly improve the quality of service, some of the communication interfaces they used are vulnerable and easily attacked. Note that although many malicious attacks can be carried out in various ways, their final step must be in the in-vehicle network, i.e., the controller area network (CAN) bus. In order to protect the security of the CAN bus, it is of great importance to design an intrusion detection system (IDS), which can monitor the message transmission in real time. In this article, we design a novel external IDS based on vehicle voltage signals, named VehicleEIDS. It does not occupy the bandwidth or computing resources of the CAN bus and maintains the original CAN bus protocol as well. The system can be directly installed in the automobile gateway to monitor the external intrusion, and can be connected to the CAN bus as an independent external device to protect the automobile security. In addition, VehicleEIDS is robust against the factors of vehicle status, the number of attacking electronic control units (ECUs), and the sending frequency of attack data. It is only related to the voltage signals of external intrusion device. Once external intrusion devices send attack data to the CAN bus, VehicleEIDS can quickly identify its abnormal voltage signals, with the accuracy of more than 97%.