Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Check Power Supply And Ground Line Of Engine Control Module (ECM)
- After servicing or replacing faulty parts, perform Clear Memory Ref. to ENGINE (DIAGNOSTICS) (H4DOTC)>Clear Memory Mode . and Inspection Mode Ref. to ENGINE (DIAGNOSTICS) (H4DOTC)>Inspection Mode .
- Use the check board when measuring the ECM terminal voltage and resistance.
Wiring diagram:
Engine Electrical System Ref. to WIRING SYSTEM>Engine Electrical System>WIRING DIAGRAM .
- CHECK MAIN RELAY
.
- Turn the ignition switch to OFF.
- Remove the main relay.
- Connect the battery to main relay terminals No. 12 and No. 13.
- Measure the resistance between main relay terminals.
Terminals
No. 10 - No. 11:
Is the resistance less than 1 Ω?
- Yes: Go to 2 .
- No: Replace the main relay. Ref. to FUEL INJECTION (FUEL SYSTEMS) (H4DOTC)>Main Relay .
- CHECK GROUND CIRCUIT FOR ECM
.
- Disconnect the connector from ECM.
- Measure the resistance of harness between ECM and chassis ground.
Connector & terminal
(B134) No. 35 - Chassis ground:
(B134) No. 47 - Chassis ground:
(E159) No. 10 - Chassis ground:
(E159) No. 17 - Chassis ground:
(E159) No. 18 - Chassis ground:
(E159) No. 25 - Chassis ground:
(E159) No. 26 - Chassis ground:
Is the resistance less than 5 Ω?
- Yes: Go to 3 .
- No: repair the harness and connector.NOTE: In this case, repair the following item:
- Open circuit in harness between ECM and engine ground terminal
- Poor contact of coupling connector
- CHECK INPUT VOLTAGE OF ECM
.
- Turn the ignition switch to ON.
- Measure the voltage between ECM and chassis ground.
Connector & terminal
(B134) No. 23 (+) - Chassis ground (-):
(B134) No. 32 (+) - Chassis ground (-):
Is the voltage 10 V or more?
- Yes: Go to 4 .
- No: Repair the open or ground short in the harness of power supply circuit.
- CHECK INPUT VOLTAGE OF MAIN RELAY
.
Measure the voltage between main relay connector and chassis ground.
Connector & terminal
(B220) No. 11 (+) - Chassis ground (-):
(B220) No. 12 (+) - Chassis ground (-):
Is the voltage 10 V or more?
- Yes: Go to 5 .
- No: Repair the open or ground short in the harness of power supply circuit.
- CHECK INPUT VOLTAGE OF ECM
.
- Turn the ignition switch to OFF.
- Install the main relay.
- Turn the ignition switch to ON.
- Measure the voltage between ECM and chassis ground.
Connector & terminal
(B134) No. 38 (+) - Chassis ground (-):
Is the voltage 10 V or more?
- Yes: Go to 6 .
- No: Repair the open circuit in harness between ECM and main relay connector.
- CHECK INPUT VOLTAGE OF ECM
.
- Turn the ignition switch to OFF.
- Connect the connector to ECM.
- Turn the ignition switch to ON.
- Measure the voltage between ECM and chassis ground.
Connector & terminal
(B134) No. 12 (+) - Chassis ground (-):
(B134) No. 24 (+) - Chassis ground (-):
Is the voltage 10 V or more?
- Yes: Check ignition control system. Ref. to ENGINE (DIAGNOSTICS) (H4DOTC)>Diagnostics for Engine Starting Failure>IGNITION CONTROL SYSTEM .
- No: repair the harness and connector.NOTE: In this case, repair the following item:
- Open circuit in harness between ECM and main relay connector
- Poor contact of main relay connector
- Poor contact of ECM connector
Use the Manual With the Right Hardware
Pair factory procedures with proven DIY tools so the instructions are easier to execute.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.