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Self-Diagnostic System: Pre-Check

  1. Diagnosis System - 
    1. As shown in the following diagram, ECUs of this vehicle are connected to each other by communication buses through which various signals are transmitted. See Fig 1. These communication buses are diagnosed by the gateway ECU. When the gateway ECU detects a communication error between ECUs, or B+ short or GND short of a communication bus, a DTC is output and memorized. The gateway ECU cannot diagnose accurately unless it can function normally. Therefore, first confirm the normal condition by performing step  2 and then trouble shoot against each DTC. This multiplex communication (gateway) system consists of 3 BEANs (body electrical area networks): the door system communication bus; the instrument panel system communication bus and the light system communication bus and AVC-LAN (audio visual communication-local area network) communication bus related to the audio and navigation system. (Dealer option, cellular mobile phone, etc., communication network can be added to the above). The gateway ECU forward data between BEANs and between a BEAN and AVC-LAN and transmits data between ECUs.
    2. If DTC of ECU communication stop is output, connectors may be disconnected, or communication buses may be open at 2 points. See Fig 2. It will not become abnormal with only 1 communication bus open.
    3. If 2 communication buses are open at the position as shown in illustration, DTC of ECU communication stop between those 2 buses is output. See Fig 3.
  2. Basic Inspection - 
    1. Check that the DTC can be read from the gateway ECU using the LEXUS hand-held tester. If the results are OK, go to USING HAND-HELD TESTER  under RETRIEVING DIAGNOSTIC TROUBLE CODES. If the results are not good, go to the next step.
    2. Check the gateway ECU power source circuit. See Fig 4. Disconnect the gateway ECU 22-pin connector located behind the upper left side of the instrument panel. Check that voltage of terminals 12 (BATT) and 22 (GND) is between 10-14 volts. See CONNECTOR IDENTIFICATION . Check that there is continuity of terminals 22 (GND), 11 (CG) and body ground. Turn the ignition switch ON. Check that voltage of terminals 2 (IG), 13 (ACC) and 22 (GND) is between 10-14 volts. If the results are OK, go to the next step. If the results are not good, replace or repair the faulty wire harness or connector. See COMPUTER DATA LINES in SYSTEM WIRING DIAGRAMS article.
    3. Check the wire harness between the gateway ECU and DLC3. See Fig 5. If there is a failure on SIL line, LEXUS hand-held tester cannot communicate with ECM or air bag sensor assembly. If the results are OK, replace the gateway ECU. See INSTRUMENT PANEL under REMOVAL & INSTALLATION in appropriate INSTRUMENT PANELS article. If the results are not good, replace or repair the faulty wire harness or connector. See COMPUTER DATA LINES in SYSTEM WIRING DIAGRAMS article.
Fig 1: Multiplex Communication System Schematic
G00221230Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
Fig 2: Identifying Conditions For ECU Output Of DTC
G00184920Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
Fig 3: Identifying 2 Open Communication Buses (Preventing Communication With 3 ECUs)
G00221231Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
Fig 4: Identifying Gateway ECU Power Source Circuit
G00221232Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
Fig 5: Identifying Gateway ECU & DLC3 Circuit
G00221233Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
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