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This solenoid is responsible for switching the left side battery to 24V for cranking the starter motor.
The starter motor is 24V in JDM sourced HDJ81s.
The 24V switch-over process is controlled by a Timer ECU that coordinates the switching process from 12V to 24V for starting (cranking) and isolating the left side battery from the rest of the vehicle and charging system. The timer itself is not prone to failure, and is located inside the left front kick panel (on RHD models).
When this solenoid is wearing out, it can lead to "no crank" situations where only a relay (solenoid) is heard to be clicking. Other symptoms of a wearing solenoid are poor/weak or absent battery charging of one battery (the left one) or sometimes both batteries.
If you are testing this solenoid, as with any other solenoid, you must look for both current and voltage. Voltage alone does not necessarily provide clues to failure. It is possible to have good voltage readings when the circuit is not loaded, and yet poor current pass through and low voltate when it is loaded. ie: test this under a moderate load to emulate the starter motor's load when evaluating its performance.
Note 1: The starter contacts on Toyota (Denso) starters are replaceable and are a service item. Check your starter contacts, found inside the starter solenoid assembly, prior to replacing this solenoid if you are experiencing a no-crank situation and your batteries are fully charged and are tested to be in good condition.
Note 2: Check all battery connections, grounding points, starter solenoid, and series-parallel switching solenoid connections, along with fuses (fuse box) and fusible links (near the batter terminals) for having good, clean, corrosion-free contact points before digging in too far.
Toyota - Japan
2022 03 29