1) Purpose and symptoms (theory)
- The blow‑by tube/PCV path vents combustion gases that pass the piston rings from the crankcase back to the intake (or air cleaner). Proper ventilation keeps crankcase pressure near atmospheric and removes moisture/oil vapor.
- Symptoms of a problem: crankcase pressure, oil leaking from seals, oil spraying from breather, blue smoke, excessive oil consumption, poor idle/stumble. These are either caused by excessive blow‑by (worn rings/valves) or by a restriction/failure in the venting path (clogged tube, faulty PCV/oil separator).

2) Locate components (theory + quick practical)
- Identify the crankcase breather/valve cover outlet, the blow‑by tube routing, any oil separator/PCV valve, and where it connects back to the intake or air cleaner. On Hino FD/FE/FF/SG/FA/FB series the breather usually runs from the rocker/cam cover to an oil separator or to the air intake/air cleaner housing.

3) Initial test for positive crankcase pressure (diagnostic step, why)
- With engine warm and idling, disconnect the breather hose at the valve cover and briefly feel for suction or pressure. Proper system usually shows slight vacuum (suction) or near‑atmospheric flow toward the intake; strong positive pressure (blow) indicates excessive blow‑by or blocked venting.
- Why: this distinguishes between a venting restriction (tube/PCV clogged) and excessive combustion gas leakage (internal engine wear). A restriction can produce pressure even if blow‑by is normal.

4) Inspect and remove the blow‑by tube and PCV/oil separator (ordered action + theory)
- Stop engine, allow to cool, then loosen clamps/remove fasteners and take the tube and any oil separator/PCV out. Inspect visually and by smelling: heavy sludge, varnish, sticky oil and carbon indicate a flow restriction. Check for collapsed/melted hoses and broken check‑valves.
- Why: debris or oil sludge in the tube or separator blocks flow, creating crankcase pressure and forcing oil past seals.

5) Clean or replace parts (ordered action + how it fixes)
- Clean metal tubes with solvent and brush; blow out with low‑pressure compressed air from the crankcase side toward the intake side. Replace soft hoses and any PCV valve or separator elements that are clogged, cracked, or the check valve is stuck. Replace clamps and gaskets as needed.
- How this fixes the fault: restoring an open, one‑way vent path lets blow‑by gases be drawn back into the intake for combustion or be vented correctly, removing positive pressure and stopping oil ejection/leaks and many breather‑related smoke issues.

6) Reassembly and leak check (ordered action)
- Reinstall tube/PCV/separator with new seals/clamps, ensuring correct orientation of any check valve. Start engine, check for correct flow direction at the valve cover port (suction or neutral), re‑check for oil leaks and return to normal idle/smoke levels.
- Why: correct reassembly restores the designed pressure balance and sealing; a misassembled or leaking connection recreates the fault.

7) Verify and quantify remaining blow‑by (if problem persists)
- If positive crankcase pressure and heavy blow‑by continue after cleaning/replacing the vent components, perform compression tests and/or leak‑down tests on cylinders. Excessive blow‑by typically means worn piston rings, ring land damage, or worn valve guides/seals.
- Why: cleaning the tube only fixes ventilation problems and blockages. It does not repair internal sealing failures; continued positive pressure indicates combustion gas is bypassing rings/valves.

8) Repair options for excessive internal blow‑by (ordered choices + why)
- If compression/leak‑down confirms ring/cylinder wear: options are piston ring replacement with cylinder honing or reboring and new pistons, or partial engine overhaul depending on wear. If valve guide/seal wear is the cause, service the valve train.
- How these repairs fix the fault: they restore the combustion seal so combustion gases no longer enter the crankcase, eliminating the source of excessive blow‑by rather than only treating the symptom.

Summary (short)
- Sequence: diagnose pressure → remove/inspect blow‑by tube and PCV → clean or replace clogged components → reassemble and retest → if pressure persists, do compression/leak‑down → repair internal engine wear as required.
- Cleaning/replacing the tube/PCV fixes pressure and oil‑breather symptoms caused by vent restrictions. If the root cause is worn rings/valves, ventilating the system only masks the symptom and a mechanical engine repair is required to stop the blow‑by.
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Goal summary (one line)
- The coolant thermal switch on Hino FD/FE/FF/SG/FA/FB (fan/engine‑cooling switch) is an on/off temperature sensor that tells the fan relay when to run; repair is replacing/testing that switch and any faulty wiring/relay so fan control returns to normal.

Theory (how it works)
- Function: the switch senses coolant temperature and changes state at a preset temperature. That state change either completes a ground or provides a signal to the fan relay coil so the relay connects battery power to the fan motor.
- Types: common designs are (a) mechanical on/off (bimetallic strip or wax‑actuated plunger) that opens/closes a contact, or (b) an electronic thermistor that changes resistance. In either case the relay interprets the switch change and powers the fan.
- Fault modes and effects:
- Stuck open / failed open contact or open circuit = fan never energises → overheating at idle/slow speed.
- Stuck closed / short = fan runs continuously → battery/alternator load, premature fan wear.
- Intermittent or corroded contact = intermittent cooling, fluctuating temps.
- Leakage at switch fitting = coolant loss → overheating and contamination.

Ordered workshop procedure (diagnose, test, replace, verify)
1) Safety and preparation
- Park on level ground, engine cold. Chock wheels, wear eye/hand protection.
- Disconnect negative battery terminal before removing electrical parts.
- Have a multimeter, appropriate sockets (usually small hex or deep socket for switch), thread sealant or correct seal/gasket, small container for coolant, funnel, and clean rags.

2) Confirm symptom and isolate the fan circuit
- Run engine to normal operating temp (careful of hot surfaces). Observe gauge and fan behaviour.
- If fan does not run when hot, or runs constantly, note symptom. Check fuses first.

3) Check fan motor and relay quickly (prevents unnecessary switch replacement)
- Locate fan relay in relay box. With engine cold and battery connected, energise the fan directly (jump 12V to fan motor connector or energise relay socket) to confirm fan motor spins freely. If motor dead, replace motor before or instead of switch.
- Check relay by swapping with identical relay or bench testing relay coil/resistance. If relay is bad, switch replacement won’t help.

4) Electrical test at the thermal switch connector
- With connector unplugged and engine cold, measure continuity/resistance across switch terminals (or to chassis ground if single‑wire). Note cold reading.
- Heat the engine to operating temperature (or remove switch and bench‑test in hot water) and watch for the change: an on/off switch should change from open to closed (or vice versa) at the specified temp; a thermistor will show a marked resistance change.
- Alternatively, check for relay control voltage: when engine hot, measure for 12V or ground at the switch connector as the fan should command the relay.

5) Bench test the removed switch (recommended)
- Remove switch only when engine/coolant cooled enough. Capture a small amount of coolant to avoid big drain.
- Unplug connector, use appropriate socket to remove switch.
- Place switch in a pot of water with a thermometer. Slowly heat water and monitor switch continuity with multimeter. Record the temperature at which it changes state. Compare to spec or expected operating range (typically near thermostat cut‑in).
- A failed switch will not change state, or will change erratically.

6) Replace the switch
- Drain coolant to below switch level (or be ready to catch a small amount) and remove switch.
- Clean threads in housing. Fit new switch with correct sealing method (new sealing washer or pipe sealant as specified). Do not over‑torque; tighten to manufacturer torque spec or snug plus a small quarter turn.
- Reconnect electrical connector.

7) Refill and bleed cooling system
- Refill coolant to correct level. Bleed trapped air per Hino procedure (run engine with heater on, top up coolant, open bleed screw if fitted) until thermostat opens and level stabilizes.

8) Functional verification
- Reconnect battery. Start engine and warm to operating temperature. Observe fan operation: it should switch on at the expected temperature and off after temperature drops. Check for no coolant leaks at switch.
- Confirm temperature gauge stable and no overheating. Confirm fan is not running continuously unless designed to.

Why the repair fixes the fault (concise)
- Replacing a defective thermal switch restores a reliable temperature‑sensing contact or resistance. If the internal contact was open/ corroded/shorted, the relay received the wrong command (no power or permanent power). A new switch provides the correct change of state at the intended temperature, allowing the relay to energise the fan only when required. Proper sealing of the new switch also eliminates coolant leaks that can cause low coolant and overheating. If the fan or relay were faulty, those must be repaired too — replacing only the switch won’t fix a bad relay or motor.

Quick trouble‑shoot checklist if replacement doesn’t cure
- Verify wiring continuity from switch to relay and relay to fan.
- Verify relay coil and contact operation.
- Verify fan motor draws expected current (no open/short).
- Verify thermostat and coolant flow; a stuck thermostat can mimic switch faults.

Safety note (final)
- Avoid working on a hot cooling system; pressurised coolant will scald. Use protective gear and follow manufacturer torque/sealant recommendations where available.
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