Summary (quick): The cooling fan(s) on a Toyota Hilux pull air through the radiator when the truck is not being driven fast enough for natural airflow. They stop the engine from overheating at idle/slow speed and help the air‑conditioning. Typical failures: fan motor burns out, relay/fuse/wiring fails, temperature sensor/ECU doesn’t command the fan, or a viscous clutch (older models) seizes. Below is a beginner‑level, thorough explanation of every component, why the repair is needed, how the system works, what can go wrong, and how to diagnose and replace parts safely.

1) Why this repair matters (theory, in plain terms)
- Engine heat: burning fuel produces heat. Coolant absorbs that heat and carries it to the radiator. The radiator dumps the heat to the air.
- Fan’s job: at low speed or idle there isn’t enough natural airflow through the radiator, so the fan forces air through it to remove heat. With A/C on the fan helps condense refrigerant.
- Analogy: radiator = radiator on a house heater; coolant = water circulating through pipes; water pump = heart; thermostat = a thermostat valve; fan = an electric room fan that turns on when the room gets too warm.
- If fans don’t work: engine temperature rises, thermostat and head gasket can fail, coolant can boil, and the engine can be damaged. So functional fans are essential.

2) Main components (and what each does)
Note: Hiluxes come in generations and markets. Some have electric radiator fans (most modern ones), others older ones use a mechanical viscous fan clutch. I list both sets of components.

Electric fan system components
- Fan blade(s) and fan assembly: plastic or composite blades located in a shroud that mounts to the radiator. The shroud directs airflow so the fan moves air through the radiator efficiently.
- Electric fan motor: spins the fan blades. Typically sealed, brush‑type or brushless depending on age.
- Fan relay(s): electrically‑operated switch(s) that allow the high current needed by the motor to be switched by a low‑current command from the ECU or temperature switch.
- Fuse(s): protect the circuit against short circuits or motor overcurrent.
- Coolant temperature sensor (engine coolant temperature sensor — ECT): measures coolant temp and sends a signal to the ECU. The ECU uses this to decide when to turn the fan on.
- Fan control module or ECU logic: in some models the ECU drives the relay(s) directly; in others a separate fan control module reads the temp sensor and controls the relay(s).
- A/C pressure/switch: when A/C is turned on, a pressure switch or A/C request signal forces the fan on (to help the condenser).
- Wiring harness and connectors: deliver power, ground, and control signals between components.
- Ground straps/mounts: solid engine/chassis ground is required for motor return.

Mechanical viscous (clutch) fan components (older Hilux)
- Fan blade bolted to viscous fan clutch.
- Viscous fan clutch: a fluid coupling that engages/disengages the fan depending on temperature (thermostatic clutch) or speed.
- Drive: clutch mounts to water pump flange and is driven by the engine via the accessory belt.
- Fan shroud: directs airflow.

3) How the system works (step‑by‑step)
Electric fans:
- Sensor reads coolant temp. If temp > threshold (varies by model, often 90–105°C), the ECU/fan control energizes the fan relay.
- Relay closes, sending battery voltage to the fan motor. Motor runs and pulls air through the radiator, cooling the coolant.
- When coolant drops below a lower threshold, the ECU/heater logic turns the relay off.
- If A/C is on, an A/C signal can force fan on regardless of coolant temp.

Viscous clutch fan:
- Fan constantly spins with the engine; the viscous clutch either engages strongly (full airflow) when hot, or slips when cool to save power.
- Thermostatic element in clutch senses air temp and varies coupling.

4) Common failure modes and what goes wrong
- Dead fan motor: brushes worn, bearings seized, or burned windings. Fan won’t spin. Often silent electrical fault or motor hum without rotation.
- Blown fuse: indicates overcurrent or short; fan won’t get power.
- Faulty relay: relay clicks but does not pass current; or no click at all.
- Bad coolant temp sensor or wiring: ECU never sees high temp so never commands fan.
- Damaged wiring/connectors/corrosion: intermittent or no power/ground to fan.
- Fan shroud or blade damage: reduced airflow, vibration, or noise.
- Viscous clutch stuck locked (fan always engaged) or stuck slipped (fan never provides airflow).
- AC pressure switch or control issue: A/C turns on but fan doesn’t, A/C performance poor.
- ECU/fan control module failure: rare but possible.
- Mechanical interference: debris jammed in shroud, radiator crushed preventing air flow.
- Overheating for unrelated reasons (thermostat stuck closed, water pump failure, clogged radiator) — fans alone will not solve those.

5) Diagnostic flow — how to check the system (beginner level)
Safety first: work on a cool engine. Disconnect negative battery when doing electrical work. Keep hands and tools away when fan could start. Wear eye protection and gloves.

Basic checks (no tools):
- Turn key to ON (engine off) and operate A/C. Does the fan come on? If yes, fan motor and basic power path likely OK.
- Start engine and let it idle to warm. Does the fan come on when engine reaches normal operating temp? (May take several minutes.)

Using simple tools (multimeter, test light, jumper):
- Check fuse(s): locate fuse box, find fan fuse(s), inspect. Replace if blown.
- Check relay: listen for click when fan should run (hot engine or A/C on). Swap relay with same type from another circuit (e.g., horn or other) if available.
- Check voltage at fan connector: with engine at temp and fan commanded on, backprobe connector with multimeter — one terminal should be battery voltage. If voltage present and fan not running, motor likely bad.
- Direct power test ("bench test"): disconnect fan connector and, with extreme caution, briefly apply battery positive to motor positive and battery negative to motor negative (use a fused jumper). If fan spins strongly, motor is OK; problem is wiring/relay/control. If it does not spin, replace motor/assembly.
- Motor current draw test: measure current draw with clamp meter; compare to spec if available. Excessive draw indicates binding; zero draw indicates open circuit.
- Test coolant temp sensor: check resistance/voltage outputs as per manual, or observe temp reading on dash. If sensor is dead, ECU may not command fan.
- Inspect harness/connectors: look for melted insulation, corrosion, loose pins.

6) Replacing an electric fan assembly (general steps — adjust to your year/model)
Tools: metric sockets and ratchet, screwdrivers, pliers, trim tools, multimeter, replacement fan assembly or motor, dielectric grease, gloves.

Safety: engine cold, battery negative disconnected, support radiator if needed.

Steps:
1. Remove battery negative cable and, if needed, battery to gain access.
2. Remove any intake ducting, radiator upper cover, or trim blocking the shroud.
3. Unplug the fan electrical connector(s). Release any clips holding wiring.
4. Remove bolts securing the fan shroud/assembly to the radiator (usually top and bottom bolts). Some Hiluxes separate the fan from the shroud—note how assembly comes out.
5. Carefully lift the fan & shroud out, taking care not to bend radiator fins. If shroud is split into upper and lower halves, remove as appropriate.
6. If replacing just the motor: remove screws/bolts holding motor to the shroud and remove motor from blade assembly. Transfer blade to new motor if recommended (some motors are sold as part of complete assembly).
7. Install new motor/assembly. Make sure fan spins freely by hand and clears shroud by a few millimeters.
8. Reinstall shroud/assembly, tighten bolts snugly (do not overtighten plastic). Reconnect wiring with dielectric grease on terminals to prevent corrosion.
9. Reinstall removed ducts, battery, and start engine. Test fan operation at idle and with A/C on. Check for abnormal noise or vibration.

Replace entire fan if motor is non-serviceable or blades/shroud damaged.

7) Replacing a viscous (mechanical) fan clutch
Tools: long socket set, fan clutch removal tool or proper wrench, possibly remove accessory belt or use holding tool.

Simplified steps:
1. Cool engine, disconnect battery negative.
2. Remove fan shroud or loosen it to gain access.
3. Loosen and remove bolts holding the fan clutch or fan blade to the water pump flange (different models have different attachment methods).
4. Remove the assembly and replace with new clutch/blade, ensuring correct orientation and torque of bolts (follow torque specs in service manual if possible).
5. Reinstall shroud and belts, reconnect battery, and run engine to verify proper operation and no wobble.

8) Tests after repair and final checks
- With engine warm, verify fan engages automatically at operating temp.
- Verify fan runs when A/C is on.
- Check for leaks or rubbing, check radiator for bent fins or debris.
- Road test if needed; watch the temperature gauge for stability.
- Check connector security and apply dielectric grease to prevent future corrosion.

9) Tools and parts checklist
Basic tools:
- Metric socket set (8–19 mm), ratchet, extensions
- Screwdrivers, pliers, trim tools
- Multimeter and test light
- Jumper wire with in‑line fuse (for bench testing)
- Torque wrench (recommended)
- Fan clutch tool (if viscous type)
Parts to have:
- Replacement fan motor or full fan assembly
- Replacement relay and fuse (carry spares)
- Replacement coolant temp sensor if suspect
- New fan shroud or fan blades if damaged
- Dielectric grease, zip ties, electrical repair connectors if wiring damaged

10) Common beginner mistakes and how to avoid them
- Working on hot engine: always let it cool. Hot coolant and moving fans are dangerous.
- Forgetting to disconnect battery for electrical work: risk of shorting and injury.
- Over‑tightening plastic fasteners: strip threads or crack shroud.
- Not supporting the radiator/shroud when removing: can damage radiator mounts or bend fins.
- Ignoring root causes: replacing fan without checking thermostat, water pump, or blocked radiator will not fix overheating.
- Not testing the circuit: swapping in a new fan without verifying power/ground can mask electrical faults.

11) Troubleshooting quick reference
- Fan never runs:
- Check fuse → relay → power at connector → direct power to motor. If motor spins with direct power, problem is control wiring/relay/sensor.
- Fan runs constantly:
- Check coolant level, thermostat, coolant temp sensor (stuck reading high), or fan relay stuck closed.
- Fan runs weakly/noisy:
- Motor bearings failing, blades damaged, or shroud interference; replace motor/blade/shroud.
- Fan runs only with A/C but not when hot:
- Sensor/ECU/fan control or wiring to temp sensor likely at fault.
- Fan works intermitently:
- Look for loose connectors, corroded ground, or failing relay.

12) When to replace parts vs. repair
- Replace entire fan assembly if shroud/blades damaged or motor not serviceable.
- Replace motor only if it’s serviceable and bearings/brushes are bad.
- Replace relay/fuse immediately if faulty; cheap parts and easy to swap.
- Replace temp sensor if readings are off and other components are confirmed good.
- Consider a wiring repair if connectors are corroded or insulated wires are damaged.

13) Final safety notes
- Never place your hands near a running fan. Disconnect battery when possible.
- Use a fused jumper when bench testing motors to avoid damaging circuits.
- Dispose of old parts and fluids per local rules.
- If you feel unsure at any point, a professional shop can complete the repair and verify cooling system integrity.

That covers the components, theory, failure modes, diagnostics, and how to replace both electric and mechanical fans on a Hilux in beginner‑friendly terms. Follow the steps carefully and prioritize safety.
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