Summary first (one line): radiator electric fans are switched on by the vehicle’s control circuit (thermoswitch/ECT→ECU/relay), powered through fuse/relay to the fan motor (or motors); a fault is a broken input, control, power or motor ground — repair restores the correct sensor/control/power/ground path so the motor runs when required.

Ordered theory + diagnostic/repair actions and why each repair fixes the fault:

1) Safety and prep
- Theory: fans can start anytime; protect yourself and the electrical system.
- Action: engine cold, key off, disconnect negative battery when doing wiring/relay swaps; keep hands/loose clothing away from blades when testing live.
- How repair fixes fault: prevents accidental shorts/damage while you repair components.

2) Understand the control logic
- Theory: on Toyota vehicles like a 2Z the fan(s) are controlled either directly by a coolant-temperature switch or indirectly by the ECU which energizes a relay (and sometimes a second relay for high speed). A/C request often forces fan on via A/C pressure switch or ECU.
- Action: consult wiring diagram for this car: locate fuse(s), relay(s), fan motor connector(s), coolant temp sensor (ECT) and A/C pressure switch.
- How repair fixes fault: knowing the correct control path focuses repairs on the actual failed element rather than chasing symptoms.

3) Visual inspection
- Theory: connectors, wiring and fuses are common failure points (corrosion, broken wires, melted connectors).
- Action: inspect fuses, relay sockets, fan motor connector(s), wiring harness for heat damage, corrosion, crushed wires; check fan blades for jamming/damage.
- How repair fixes fault: repairing corroded connector or replacing a burned fuse/relay restores the physical continuity needed for current to flow to the motor.

4) Check fuses and relays (quick functional test)
- Theory: a blown fuse or open relay coil/contacts prevents power to fan(s).
- Action: with key on (but engine off) check fan fuse(s) for continuity; swap known-good identical relay if available or bench-test relay.
- How repair fixes fault: replacing a blown fuse or bad relay re-establishes the switched power path to the fan motor.

5) Verify fan motor power/ground when fan should be ON
- Theory: fan needs a battery positive feed and a return/ground (either switched positive or switched ground depending on circuit).
- Action: warm engine to fan-on temp or simulate fan-on by turning A/C on; backprobe fan connector: one pin should be ~12V (or be switched to ground) when fan commanded on. If no command, manually energize relay to check.
- How repair fixes fault: if you find the correct signal missing, repairing the upstream component (sensor/ECU/wiring) restores the control signal so the relay engages and the fan receives power.

6) Bench-test the fan motor directly
- Theory: the motor can fail (open winding, worn brushes, seized bearings).
- Action: disconnect fan connector, apply 12V directly from battery to motor terminals briefly (observe polarity if needed). Motor should spin smoothly and draw a reasonable current (listen for noise, seize).
- How repair fixes fault: replacing the fan motor assembly replaces defective windings/brushes bearings; a direct test proves if the motor is the failure.

7) Measure motor current draw
- Theory: excessive current suggests internal short/binding; zero/very high resistance suggests short/open motor.
- Action: use clamp meter or inline ammeter while motor runs; compare to spec (typically <25–40 A for large fans, varies by vehicle).
- How repair fixes fault: replacing an overcurrent motor prevents relay/fuse burnout and restores normal operation.

8) Test the coolant temperature sensor / thermoswitch and ECU fan output
- Theory: the ECU or separate thermoswitch determines when to energize the fan relay based on coolant temp; a failed sensor gives incorrect temperature reading so ECU never commands fans.
- Action: measure ECT sensor resistance vs temp or backprobe ECU output to relay; warm engine with a heat gun on sensor or use scan tool to read temperature value. If ECU shows normal temp but relay not energizing, the ECU output stage or wiring may be bad.
- How repair fixes fault: replacing a failed ECT sensor or repairing ECU output/wiring restores accurate temperature detection and the control command that turns the fans on.

9) Inspect and test A/C control inputs and pressure switch
- Theory: A/C request should force fan on; if A/C side works but normal temp control doesn’t, the problem is likely sensor/ECU; if nothing forces fan, wiring/relay or ground issue.
- Action: with A/C on, check if fans run; test A/C pressure switch continuity and wiring to relay/ECU.
- How repair fixes fault: repairing A/C input/pressure switch wiring restores the alternate fan activation path.

10) Check grounds and chassis connections
- Theory: a poor ground raises circuit resistance and prevents motor current flow even if voltage is present.
- Action: measure voltage drop between battery negative and fan motor ground while fan commanded on. Clean/repair ground straps and bolt connections.
- How repair fixes fault: restoring low-resistance ground allows full current flow so the motor runs at proper speed.

11) Wiring repair and connector restoration
- Theory: hidden breaks, high-resistance joints or corroded pins interrupt control/power paths.
- Action: repair damaged wires (cut out and solder or crimp with heatshrink), replace corroded connectors, use dielectric grease after repair. Replace harness sections if chafed.
- How repair fixes fault: re-establishes reliable electrical continuity and prevents intermittent failures.

12) Replace relay/fan assembly/ECT/ECU as needed
- Theory: replace the failed component identified by tests. If ECU output is confirmed faulty and wiring good, ECU repair/replace required.
- Action: replace only the failed items: fan motor or fan assembly if bench-test fails; relay/fuse if they failed; ECT sensor if reading wrong; ECU only after ruling out wiring and other parts.
- How repair fixes fault: replacing the defective component restores the intended control/power function so fans operate when commanded.

13) Final checks and validation
- Theory: verify system now behaves across conditions.
- Action: run engine to normal operating temp, ensure fans kick in at correct temperature, verify fans operate with A/C on, check for overheating and re-check trouble codes with a scanner.
- How repair fixes fault: validation confirms the repaired path controls the fan correctly and that cooling is restored.

Quick troubleshooting priority list (order to do):
1) Visual/fuse/relay swap
2) Force-run fans by applying 12V to test motor
3) Check for voltage/ground at connector when fan should be on
4) Test ECT reading with scan tool or resistance vs temp
5) Repair wiring/ground/connectors or replace failed motor/relay/sensor
6) Re-test and validate

Common failure modes and why replacement fixes them:
- Blown fuse/failed relay: electrically open — replacing restores the switched circuit.
- Bad fan motor: open winding/brushes/seized — replacement puts a working motor back in circuit.
- Corroded connector/wire break: high resistance/no continuity — repair restores low-resistance path.
- Faulty ECT sensor/thermoswitch: wrong/no command to relay — replacement restores correct temperature sensing and commands the relay.
- Bad ground: insufficient return path — cleaning/repairing ground allows full current to flow.

No extra commentary.
rteeqp73

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