Ordered, concise — theory, diagnosis, and step-by-step repair for the radiator/cooling-fan thermal switch on a Jeep Wrangler TJ (1997–2006 family). Assumes you mean the coolant temperature / fan thermal switch that controls the electric radiator fan (not the ECU coolant sensor).

1) What the thermal switch is (theory)
- Function: a temperature-actuated electrical switch (wax-piston or bimetal type) that changes electrical state at a set coolant temperature and thereby energizes the fan relay (or tells the PCM to run the fan).
- Electrical role: it provides a ground/closure or open to the fan control circuit; the fan relay supplies battery power to the fan when the switch closes.
- Failure modes: open when it should close → fan doesn’t run → overheating; stuck closed → fan runs continuously → battery drain/noisy fan; intermittent → unpredictable cooling/overheating.

2) Symptoms that point to the thermal switch
- Radiator fan never runs at operating temp (engine overheats at idle/traffic).
- Fan runs all the time.
- Fan runs intermittently or only after several restarts.
- No fan relay activation when engine is hot (verify by listening/feeling relay).
- DTCs related to fan control (if equipped).

3) Ordered diagnostic checks (do these before replacing)
- Safety first: engine cold, park, e‑brake on, battery negative disconnected if you will mess with connectors.
- Visual: inspect connector and wiring for corrosion, broken wire, or melted insulation at the switch. Clean/repair wiring if needed.
- Check relay and fuse: swap fan relay with identical relay to rule out relay. Check fuse continuity.
- Direct fan test: with the engine cold and key on, jumper the fan relay coil or connect fan directly to 12V briefly (use proper jumper) to confirm fan motor works.
- Switch continuity test (multimeter): unplug connector, measure continuity/ohms across the switch terminals. At ambient (cold) it’s normally open (or per spec); heat the sensor to operating temp (see next) and it should change state. If no change, switch is bad.
- Hot test (safe method): remove switch and bench-test in hot water while monitoring temperature and continuity, or warm engine to operating temp and measure in-situ. The switch should change state near normal fan-on temperature (~thermostat range; TJ values ~190–210°F depending on sensor). If wiring is suspect, trace/back-probe harness to verify signal reaches relay/PCM.

4) How to replace the thermal switch — step-by-step (in order)
- Tools/parts: replacement thermal switch (correct part for TJ year/engine), small drain pan, pliers, open-end wrench or deep socket for sensor, thread sealant appropriate for coolant sensors (no silicone), shop rags, coolant for top-up.
- 1) Park on level ground, engine cold. Disconnect negative battery terminal.
- 2) Relieve small pressure (remove radiator cap only when cold). Place drain pan under area to catch coolant. You may need to drain a small amount of coolant so level falls below the sensor — otherwise expect a coolant drip when removing the switch. You do not normally need to fully drain the system.
- 3) Locate the thermal switch (radiator or engine coolant outlet/thermostat housing area — consult vehicle-specific diagram if needed).
- 4) Disconnect the electrical connector from the switch; inspect connector for corrosion. Repair/clean if needed.
- 5) Unscrew the switch with the correct wrench/socket. Catch any spilled coolant.
- 6) Prepare the new switch: apply a small amount of appropriate thread sealant if required by the part (do not overdo — some sensors use a paper or rubber gasket). Follow part instructions.
- 7) Install the new switch and tighten to snug / factory torque (snug plus a small fraction of a turn; avoid over-torquing plastic housings).
- 8) Reconnect electrical connector. Refill coolant to correct level if you drained any; bleed air from the cooling system per factory procedure (run engine with radiator cap off, heater on high, watch for air bubbles and top up).
- 9) Reconnect battery negative. Start engine, bring to operating temperature, observe fan operation and check for leaks. Confirm fan comes on at expected temperature and turns off as engine cools. Verify no leaks at the sensor.

5) How the repair fixes the fault (concise physics/electrical explanation)
- The bad switch either failed to close or failed to open at the designed temperature. Replacing it restores a temperature-sensitive mechanical/electrical element that changes state at the proper coolant temperature. That correct state change sends the correct electrical signal to the fan relay/PCM so the relay energizes the fan only when needed. Thus cooling is restored at high temperature (prevents overheating) and the fan no longer runs unnecessarily (prevents battery drain and noise).

6) Quick post-repair checks (ordered)
- Start cold, warm to normal operating temp; verify fan cycles on/off at reasonable temps.
- Check for coolant leaks around new switch.
- Confirm heater performance and that the temperature gauge is reasonable.
- Re-scan for stored DTCs and clear if present; ensure no new fan-control codes.

Safety notes (brief)
- Never open the cooling system when hot. Use eye protection and gloves.
- Dispose/catch coolant properly.

That’s the ordered diagnosis and repair theory plus how the fix restores proper operation.
rteeqp73

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