Unplug the coolant temp sensor. Touch one multimeter probe to the sensor connector terminal and the other probe to an ECU pin. When the buzzer sounds, you have found THW. Mark it.
Do you have a specific 3S-FE pinout question? Leave the year and model in the comments below (or on your favorite forum), but always verify community advice against a factory PDF. 3s-fe ecu pinout pdf
If you are reading this, you are likely wrestling with wiring harness gremlins, planning a standalone engine management upgrade, or trying to diagnose a stubborn check engine light on your Toyota. The engine in question is the legendary Toyota 3S-FE —the workhorse 2.0-liter inline-four found in the Camry, Celica, Carina E, MR2 (rarely), and several Coronas from the late 1980s through the 1990s. Unplug the coolant temp sensor