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.
2004 Toyota Tacoma PreRunner V6-3.4L (5VZ-FE)FACTORY OEM SERVICE PROCEDURE
Component Tests and General Diagnostics
ECM Power Source Circuit
CIRCUIT DESCRIPTION
When the ignition switch is turned to ON, positive battery voltage is applied to terminal IGSW of the ECM and the EFI main relay (Marked: EFI) control circuit in the ECM sends a signal to terminal MREL of the ECM switching on the EFI main relay.
This signal causes current to flow to the coil, closing the contacts of the EFI main relay and supplying power to terminal +B of the ECM.
The electronic throttle control system where not equipped:
If the ignition switch is turned to OFF, the ECM continues to switch on the EFI main relay for a maximum of 2 seconds for the initial setting of the IAC valve.
INSPECTION PROCEDURE
CIRCUIT DESCRIPTION
When the ignition switch is turned to ON, positive battery voltage is applied to terminal IGSW of the ECM and the EFI main relay (Marked: EFI) control circuit in the ECM sends a signal to terminal MREL of the ECM switching on the EFI main relay.
This signal causes current to flow to the coil, closing the contacts of the EFI main relay and supplying power to terminal +B of the ECM.
The electronic throttle control system where not equipped:
If the ignition switch is turned to OFF, the ECM continues to switch on the EFI main relay for a maximum of 2 seconds for the initial setting of the IAC valve.
Wiring Diagram:
Step 1 - 3:
Step 4 - 6:
Step 6 (Continued) - 8:
Step 9:
INSPECTION PROCEDURE
Factory Service Tools
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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.
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