Fault phenomenon
    A 2003 Passat 1.8GLS ABS fault light is on. With VAG1552 to detect, select the ABS control unit address code 03, press the Q key to confirm, the instrument displays: "Kwirenotswitchingtopositive", indicating VAG1552 and ABS control unit can not establish communication. Then the other system to detect and found that all systems have the same situation, can not be detected.
    Failure analysis
    "Kwirenotswitchingtopositive" means that the K-line can not be switched to the positive pole, the K-line is short-circuited or the K-line interface of a certain control unit is damaged, resulting in grounding and communication. Open the ignition switch, check the K-line voltage with a multimeter, the voltage is about 0.22V, K-line resistance to detect the resistance of about 0.5Ω. Can be judged initially, this is due to K-line short-circuit caused by the diagnosis of equipment and vehicle control unit can not communicate.
    View the 2003 Passat 1.8GLS circuit diagram (shown in Figure 1), analyze the car diagnostic interface line. This car K line and CAN-BUS line coexistence. ABS control unit, airbag control unit, automatic transmission control unit, engine control unit and instrument control unit at the same time using the K line and CAN-BUS (the control unit, the air control unit, the radio control system, the control unit, the engine control unit and the instrument control unit) Line for diagnosis and communication. Diagnostic seat 7 feet for the K line; 6, 14 feet for the CAN-BUS line, through the instrument gateway and ABS, automatic transmission and other control unit for communication and diagnosis.
    Volkswagen cars have a common feature: the majority of the control unit of the K line are connected together, the diagnostic instrument can be accessed through a different address code different control unit. The drawbacks of doing so are to avoid interference with multiple computer systems. Especially in the car computer more and more situation, this problem is more and more prominent. As a result, Volkswagen Audi has been able to diagnose the transition to can-bus, while also retaining the K-line as a diagnosis of some specific systems.
    Troubleshooting
    Understand how it works, proceed with troubleshooting. First, the table will be connected to the red table pen in the first 7 feet of the diagnosis, black table pen ground, from easy to difficult, one by one to disconnect the control unit. After some effort, disconnect the receiver, open the ignition switch, K line voltage rose to
    11.5V, with VAG1552 into the control unit, can be normal detection. Check the cable machine and found the cable after the plug has been cut off, there is no dressing, the wire exposed to the receiver and the receiver shell access to the ground, resulting in K-to-ground short circuit (this car has been installed DVD). Eliminate this failure, ABS failure will be solved.