Brief safety note: work on electrical systems with the ignition off and battery negative disconnected when you are disconnecting connectors or replacing parts. Use insulated tools when probing live circuits.

1) System theory — what the cooling fans are and how they’re commanded
- Most Hiace vans use one or two electric radiator fans. They remove heat from the radiator by forced airflow when airspeed or engine load is insufficient.
- Control elements: coolant-temperature sensor (or thermo-switch) and/or the engine ECU, an AC pressure/AC switch input, a fan relay (or relays), fuses, wiring and the fan motor(s). Some models use a fan control module that provides PWM (variable-speed) control.
- Command logic: the temperature sensor reports coolant temp to the ECU (or closes a thermo-switch). When temperature or AC demand is high the ECU (or thermo-switch) energizes the fan relay coil (or the control module) which closes the high‑current circuit and supplies 12 V to the fan motor. The motor returns to battery negative through a ground. For PWM systems the ECU modulates the relay/module to vary fan speed.
- Failure modes and symptoms: fan not running, runs intermittently, runs at wrong speed, or runs constantly. Causes are broadly: power/fuse/relay/wiring open or high resistance, failed fan motor (worn brushes, seized bearings, burned windings), failed temp sensor/thermo-switch, failed ECU/fan control module, or a bad AC switch.

2) Ordered diagnostic sequence (theory + what each test isolates)
- Visual & basic checks: inspect fuses, relays, connectors and wiring for corrosion, melted insulation, burnt terminals. Why: fuses/relays/wiring are the simplest path for power loss.
- Verify symptom behavior: does the fan ever run (on key-on, with AC on, when warmed up)? Why: determines if control signal (AC or temp) is available.
- Check fuse continuity and relay operation: a blown fuse or stuck relay is a hard open/short. A good relay clicks when energized — confirms control circuit is sending activation to relay coil.
- Power/ground at fan connector: with ignition on and AC request or engine at operating temp, measure voltage at the fan motor connector. If you see battery voltage on the supply pin and a good ground, the motor should run. Why: isolates motor vs control. If voltage is present and motor doesn’t run the motor is bad.
- Direct-power test on motor: apply 12 V directly to the fan motor (bypass relay) to confirm motor spins freely at full voltage. Why: proves whether motor is mechanically/electrically healthy.
- Check control signal to relay coil: back-probe relay coil terminal while replicating fan demand (AC on, warm engine, or jumper temp-switch). If the ECU grounds or supplies the relay coil, the control circuit is good. Why: isolates sensor/ECU path.
- Test temp sensor/thermo-switch: measure sensor resistance vs temperature spec or force the thermo-switch closure (where applicable). If the sensor never indicates high temp the ECU won’t command the fan. Why: isolates sensing element.
- Check PWM/control module output (if present): use an oscilloscope or duty-cycle meter to check for PWM at the fan supply when the ECU commands fan speed. No PWM while ECU requests speed indicates module/ECU fault; PWM present but motor doesn’t vary indicates motor or wiring problem.

3) Ordered repairs and why each fixes the fault
- Replace blown fuse or faulty relay
- What you do: swap with known-good relay or replace fuse/relay with correct rating.
- Why it fixes: restores the high-current switching device that connects battery power to the fan. A relay coil or contacts can fail open or stick; replacement restores the circuit.
- Repair/replace damaged wiring or connectors
- What you do: clean corroded terminals, repair broken wires, crimp or solder and heat-shrink splices, replace melted connector housings.
- Why it fixes: open circuits, high resistance or poor grounds prevent full voltage/current reaching the motor or prevent relay coil activation; restoring low-resistance connections returns proper voltage and control signals.
- Replace fan motor (or fan assembly)
- What you do: bench-test and, if motor fails, remove fan shroud and replace motor or entire fan assembly.
- Why it fixes: worn brushes, seized bearings or burned windings stop the motor from converting electrical energy into rotation. A new motor restores mechanical drive to move air.
- Replace fan control module or fix PWM circuit
- What you do: verify PWM absence then replace module/replace ECU-controlled relay assembly if serviceable. Follow manufacturer procedure for replacement and any relearn.
- Why it fixes: if the control module or ECU output is faulty the motor won’t receive correct voltage or PWM. Replacing the failed control restores the command and variable speed control.
- Replace coolant-temp sensor or thermo-switch
- What you do: swap the sensor/switch with new part and confirm ECU sees correct temp or that the switch closes at spec temp.
- Why it fixes: a failed sensor gives the ECU false temperature information, so the ECU doesn’t command the fan. New sensor restores correct input so the ECU will energize the relay when needed.
- Replace AC pressure or AC switch
- What you do: test AC request input; replace switch if it doesn’t signal when AC is on.
- Why it fixes: the AC system requests fan operation; a failed switch prevents that request and stops fan activation during AC operation.
- Final verification
- What you do: with repairs made, run the vehicle to operating temperature and with AC on; confirm fan engages at proper temps/speeds and that there are no error codes.
- Why: validates the repair under real conditions and ensures the system is now completing the sensing → control → power flow.

4) How each repair maps to the electrical/control path (concise)
- Fuse/relay/wiring repair → restores broken/high-resistance segments in the power/control path so the fan motor gets full current.
- Fan motor replacement → restores the electromechanical conversion (current → torque → airflow).
- Sensor/module replacement → restores correct inputs or control signals so the relay/module is commanded at the right times.
- PWM/control fixes → restores proper modulation so fan runs at correct speeds instead of not running or running at wrong speed.

Done.
rteeqp73

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