ECM Power Source Circuit [04/2013 - 08/2015]: Description
WARNING: This page is about a different car, the 2015 Lexus IS 250 and 2014 Lexus IS 250. However, it is still accessible from the selected car via links, so may be relevant.
When the engine switch is turned on (IG), battery voltage is applied to the IGSW terminal of the ECM. The output signal from the MREL terminal of the ECM causes current to flow to the coil of the semiconductor pwr integration ECU (EFI MAIN1 relay), closing the contact and supplying power to terminals +B and +B2 of the ECM.