The main relay plays an important role in the efi toyota system, in allowing power to be retained to devices like the iscv long enough for them to return to a "start" position. This power is provided through the use of capacitors within the ecu itself.
Being a stepper motor, the ecu expects the motor (ISCV) to be in a particular position when it goes through its initialisation. If the motor is already in a different position, when the ecu controls the motor it will advance further than expected resulting in unexpected results.
Have you ever been in a 1jz powered cressida where it is a bitch to start / maintain idle or when it is running the idle is around 350-400rpm? That is because the ISCV isn't in the position the ecu is expecting.
How do i wire up the MREL ecu pin ( main relay aka m-rel)
The ecu expects power on the following inputs
* Batt - Constant Power
* IGSW - Ignition Power
When these conditions are met, the ecu should then switch the power internally and provide 12-14 volts output to the MREL pin, which is then used to trigger the main relay or commonly referred to as the efi relay.
The main relay has its fused high amperage input directly from the battery with constant power.
The earth is constantly connected.
The main relay output power conencts into the +B and +B2 pins on the ecu, which allows the ecu to reset the stepper motor in the iscv once the ignition is turned off.
When i first saw this, i was scratching my head thinking why would Toyota have the ignition power enter the ecu, via a pin, then come back via a relay into the B+ pins. But it wasn't until I discovered the power to the mrel pin is retained for a few seconds after the ignition is turned off and the poor idle experienced on subsequent restarts I saw the reasoning behind it.
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