Goal: understand what the TE‑20 “thermal switch” does, how to diagnose it, and how the usual repair restores correct operation. Steps are in order; theory is given with each action. Safety: disconnect battery before doing electrical work; avoid burns when the engine is hot.

1) What the thermal switch is and why it exists (theory)
- Function: a temperature‑sensitive electrical switch that closes or opens a circuit depending on engine/air temperature to control a cold‑start heater (Thermostart/intake heater or glow device) or an associated solenoid. Its purpose is to give extra heat/air/vaporisation only when the engine is cold so the tractor starts reliably, and to turn that aid off when the engine is warm.
- How it works internally: typically a bimetal strip or wax‑pellet thermostat drives a contact; below a set temperature the contact closes to feed current to the heater; above that temperature it opens. Some designs are normally closed when cold, others normally open when warm — the important part is a defined trip temperature.
- Symptoms of failure: hard cold starting (switch stuck open), constant running of the heater (stuck closed) causing poor running/smoke/fouled plugs, intermittent operation (corroded contacts or broken wiring).

2) Locate and visually inspect (theory + action)
- Action: find the thermal switch on the inlet manifold or near the carburettor or the engine block where the factory placed it for a good temperature pick‑up. Inspect wiring, connectors and the heater element itself.
- Theory: visual inspection often reveals the most common faults — corroded spade connectors, chafed insulation, water ingress — which break the circuit or create high resistance. High resistance means insufficient current to the heater, so it can appear like a switch failure.

3) Electrical verification (theory + ordered tests)
- Action A — continuity test (cold): with battery disconnected, remove connector and use a multimeter on continuity or ohms. At ambient/cold engine temperature the switch should show continuity (or open) depending on its design; check service info or assume that a cold‑start switch closes when cold. Note the reading.
- Action B — heat test (hot): warm the switch gently (hairdryer or by running engine briefly) while monitoring continuity; it should change state at the designed temperature.
- Action C — voltage under load: reconnect battery, back‑probe the connector and crank the engine cold; the heater circuit should have battery voltage when the switch is in the closed (cold) state.
- Theory: the multimeter checks contact continuity and the heat test checks the temperature‑sensitive mechanism. Voltage under load confirms that full supply reaches the heater — a switch with continuity but no voltage when installed indicates wiring/fuse/starter circuit fault.

4) Mechanical removal (ordered)
- Action: disconnect battery negative. Disconnect the electrical connector on the switch. Unscrew or unclip the switch (often a threaded body with a sealing washer). Note sealing method (gasket, washer).
- Theory: removing the switch lets you inspect for physical damage, corrosion on terminals, or seized body. If threads/seal are damaged, coolant/air leaks or poor temperature sensing will occur.

5) Decide repair or replacement (theory)
- If failures are: corroded terminals, bent contacts, or stuck mechanism due to surface corrosion, cleaning/rehabbing can work.
- Action: clean contacts with contact cleaner and a fine file if light corrosion, replace terminal boot or connector.
- Theory: removing oxidation restores low resistance contact so the heater receives full current.
- If failures are: intermittent thermal action, no change on heat test, broken internal spring or stuck mechanism, replace the switch.
- Action: fit a new switch with the same thread, temperature rating, and current capacity. Use correct sealing washer or thread seal as specified.
- Theory: a new switch restores the temperature setpoint and reliable contact action — the heater will be powered only when required.

6) Reassembly and functional test (ordered)
- Action: install new/cleaned switch, tighten to correct torque, reinstall connector, secure wiring, reconnect battery. Cold test: with a cold engine, back‑probe for battery voltage at the heater circuit while cranking; heater should have voltage. Warm the engine (or run briefly) and retest — voltage should be removed when warm.
- Theory: correct operation means the switch is now controlling the heater as designed. If voltage appears at the heater only when cold and disappears when warm, the circuit and thermal behaviour are correct.

7) How the repair fixes the fault (explicit)
- If symptom was hard cold starting and diagnosis showed open or high‑resistance contacts: cleaning or replacing the switch restores a low‑resistance closed circuit when cold, supplying full current to the heater. The heater vaporises fuel / preheats intake so mixtures ignite easily; problem solved.
- If symptom was heater always on and diagnosis showed the switch stuck closed: replacing the switch restores the open state when warm, preventing continuous heating that fouls plugs or causes rich running.
- If symptom was intermittent operation and diagnosis showed wiring/connector damage: repairing wiring/terminals restores reliable current flow and sensor signalling, eliminating intermittent heating and the associated starting and running problems.

8) Quick troubleshooting checklist (ordered)
- Visually inspect connectors and wiring.
- Check fuse and battery supply to heater circuit.
- Multimeter continuity of switch cold and hot.
- Check voltage at heater with engine cold.
- Replace switch if it fails to change state on heating or shows internal damage.
- Confirm correct operation after installation.

Safety/notes (brief)
- Use the correct replacement switch rated for the heater current and correct temperature trip.
- Avoid overtightening threaded sensors — use correct torque and sealing washer to avoid air/coolant leaks or damaged threads.
- Corrosion is common on older tractors; prefer replacing small connectors with quality ring/spade terminals and heat‑shrink boots.

That is the ordered procedure with theory and how each repair action corrects the underlying fault.
rteeqp73

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1949 Ferguson TE20 Won't Run AGAIN!!!! My friend Gord is having problems with his Ferguson TE20. It runs terribly if it runs at all. It was here a year ago with similar ...

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