Owner's Problem Description
The technician is facing an issue with a Toyota 4Runner SR5 that does not reach a negative signal on the injectors, preventing the vehicle from starting. The engine has a diagnostic trouble code (DTC) of 11, which indicates a potential power supply issue to the ECU. Previous checks on connectors, relays, and fuses show no faults. The technician is seeking further diagnostics before ordering a new ECU.
Professional Technical Analysis
The Toyota 4Runner SR5 in question is experiencing a non-start condition due to a diagnostic trouble code (DTC) 11, which typically indicates an issue with the ECU's power supply. The vehicle's electrical system should be thoroughly analyzed, focusing on the ECU power supply circuit, which includes checking the ignition switch, main relay, and any potential corrosion at the connector terminals. A comprehensive check of the injector voltage and ground signals is essential since the absence of a negative signal on the injectors means the PCM is not controlling the fuel injectors properly. This could be a result of a faulty ECU or an open circuit in the signal pathway. Considering the vehicle's age and typical electrical issues, it is crucial to verify the integrity of the wiring harness and connectors that feed power to the ECU. If the tests confirm that the power supply is intact and no shorts or opens are present, replacing the ECU may be the next step. Past experiences with similar models indicate that the ECU can fail due to internal component breakdown, often linked to issues like capacitor failure. As a preventative measure, ensure that all electrical connections are clean and secure to avoid future failures.
Possible Causes
Most common causes (ordered by frequency):
- ECU Power Supply Failure: Investigate the ignition switch and main relay for faults that could interrupt power to the ECU. Voltage should be checked at the ECU connector, where a stable 12V+ supply is required for proper operation. If the voltage is absent, the circuit leading to the ECU must be traced for opens or shorts. - No voltage present at the ECU connector, ignition and main relay functioning, but ECU shows no activity.
- Faulty ECU: Internal failure of the ECU can prevent it from sending the correct signals to the injectors. The DTC code 11 points towards a malfunction that could be due to component degradation, often seen in older models. Testing with a known good ECU can help confirm this. - DTC 11 persists even after checking power supply, testing with a substitute ECU yields normal operation.
- Faulty Wiring or Connectors: Corrosion or damage in the wiring harness or connectors leading to the ECU can result in intermittent connections or loss of signal, affecting injector operation. Visual inspection and continuity testing should be performed to identify any issues. - Visual inspection reveals corrosion at connectors, intermittent voltage readings when wiggling wires.
Diagnostic Steps
Professional Diagnosis Process
Follow these systematic steps to accurately diagnose the issue. Each step builds on the previous one to ensure accurate diagnosis.
- STEP 1 - Initial Voltage Check: Use a multimeter to measure voltage at the ECU connector. Ensure that there is a stable 12V+ supply when the ignition is on. If voltage is absent, check the ignition switch and main relay for proper operation.
- STEP 2 - Ground Integrity Test: Check the ground connection for the ECU to ensure it is secure and free of corrosion. A poor ground can lead to erratic ECU behavior. Measure resistance between the ECU ground pin and battery negative; it should be less than 5 ohms.
- STEP 3 - Injector Signal Testing: With the ignition on, use a scope to check for the negative signal at the injector connector. Absence of signal indicates an issue with the ECU or wiring. If present, measure the voltage and pulse width to ensure proper operation.
- STEP 4 - ECU Replacement Verification: If all previous tests confirm the power supply and ground are intact but the issue persists, replace the ECU with a known good unit. After installation, recheck injector signals and clear any DTCs.
