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Engine Controls - System & Component Testing: Diagnosis Lexus RX I

Testing & Diagnostics 2 illustrations ~517 words

Circuit Description

When ignition switch is turned on, 9-14 volts is applied to the terminal IGSW at ECM main relay control circuit, sending a signal to terminal M-REL at ECM switching EFI main relay on. When ignition is turned off, ECM will keep EFI main relay on for 2 seconds or less in order to reset Idle Air Control (IAC) valve.

When engine is being cranked, intake air flow is slow, so fuel vaporization is poor. A rich mixture is necessary to achieve good startability. While engine is being cranked, 9-14 volts is supplied to ECM terminal STA. Starter signal is mainly used to increase fuel injection pulse for starting injection control and after-start injection control.

Note. The following test assumes starter cranks normally with ignition in START position. If starter does not crank normally, check and repair starter circuit before continuing with this test.

Diagnosis & Repair (Using Lexus Scan Tool)

  1. Connect Lexus scan tool to Data Link Connector (DLC) No. 3. (Scheme 45)or (Scheme 46). Turn scan tool on. Turn ignition on. Using scan tool, read STA signal with ignition on and then while cranking starter.
  2. With ignition on, STA signal should be off. With ignition switch in START position, STA signal should be on. If STA signal is not as specified, go to next step. If STA signal is as specified, no problem is indicated at this time. Diagnose by symptom. See TROUBLE SHOOTING - NO CODES article.
  3. Check for open in wiring harness between ECM and starter relay. See appropriate wiring diagram in WIRING DIAGRAMS article. Repair as necessary. If wiring is okay, replace ECM and retest.

Scheme 45

Scheme 45
Scheme 45

Scheme 46

Scheme 46
Scheme 46

Diagnosis & Repair (Using OBD-II Scan Tool) - ES300

  1. Access ECM harness connectors behind glove box. Backprobing ECM harness connector, measure voltage between ground and terminal No. 7 (Gray wire) STA at ECM E7 connector. see scheme 6
  2. Turn ignition switch to START position. With starter being cranked, voltage should be 6 volts or more. If voltage is not as specified, go to next step. If voltage is as specified, no problem is indicated at this time. Diagnose by symptom. See TROUBLE SHOOTING - NO CODES article.
  3. Check for open in wiring harness between ECM and starter relay. See appropriate wiring diagram in WIRING DIAGRAMS article. Repair as necessary. If wiring is okay, replace ECM and retest.

Diagnosis & Repair (Using OBD-II Scan Tool) - RX300

  1. Access ECM harness connectors behind glove box. Backprobing ECM harness connector, measure voltage between ground and terminal No. 7 (Black wire) STA at ECM E8 connector. see scheme 7.
  2. Turn ignition switch to START position. With starter being cranked, voltage should be 6 volts or more. If voltage is not as specified, go to next step. If voltage is as specified, no problem is indicated at this time. Diagnose by symptom. See TROUBLE SHOOTING - NO CODES article.
  3. Check for open in wiring harness between ECM and starter relay. See appropriate wiring diagram in WIRING DIAGRAMS article. Repair as necessary. If wiring is okay, replace ECM and retest.

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