The ECU has two rows (A & B) with 40 pins each (A1–A40, B1–B40). You identify pins by looking at the harness side (the plug going into the ECU) with the retaining clip facing up.
| Component | ECU Pin | Action | | :--- | :--- | :--- | | Suction Control Valve (SCV) | A3 (+) / A4 (-) | Regulates fuel into the high-pressure pump | | Injector #1 | C1 (High) / C2 (Low) | High-side / Low-side driver | | Injector #2 | C3 / C4 | High-side / Low-side driver | | Injector #3 | C5 / C6 | High-side / Low-side driver | | Injector #4 | C7 / C8 | High-side / Low-side driver | | Main Relay Control | A2 | Sends ground to turn on the main relay | | Check Engine Light (MIL) | A9 | Switched ground (illuminates the light) | isuzu 4hl1-1 ecu pinout
| Pin | Component | Wire Color | Behavior | | :--- | :--- | :--- | :--- | | A-14 | Suction Control Valve (SCV) Negative | Blue/Red | PWM (Duty cycle 15% - 85%) | | A-15 | Suction Control Valve (SCV) Positive | Blue/White | 12V Battery feed | | A-30 | Cylinder #1 Injector (Return) | Brown | High voltage pulse (100V+ peak) | | A-31 | Cylinder #2 Injector (Return) | Green/Black | High voltage pulse | | A-32 | Cylinder #3 Injector (Return) | Yellow/Black | High voltage pulse | | A-33 | Cylinder #4 Injector (Return) | Blue/Black | High voltage pulse | | B-37 | EGR Vacuum Solenoid | Yellow/Red | Ground switched by ECU | | B-40 | Fuel Pump (Supply Pump) Relay | Blue/Black | Ground switched (ON during crank) | The ECU has two rows (A & B)
Danger Zone: Injector pins (A-30 to A-33) carry high voltage (up to 180V). Do not probe these with a standard multimeter on DC volts; use an oscilloscope or a high-impedance probe. Shorting these to ground will destroy the ECU driver ICs. Do not probe these with a standard multimeter
The ECU expects two pedal signals (A25 main, A26 sub) to be inversely proportional with exact ratio (typical 2:1). If not, the ECU triggers P2138 and limits engine to idle.