What you’re dealing with
- “Thermal switch” here = the coolant-temperature-activated fan switch (a simple on/off temperature switch) used on many Hino FD/FE/FF/SG/FA/FB trucks to start the electric cooling fan (or to signal a fan relay). It is NOT the engine coolant temperature sensor that feeds the ECU — it’s a mechanical/electrical switch that closes at a set temperature to operate the fan circuit.

Simple analogy
- Think of the thermal switch like a thermostat-operated light switch: when the room (engine coolant) gets hot enough, the switch flips and powers the fan (light). When it cools, the switch opens and the fan turns off.

Components you need to know (every component described)
1. Thermal switch (fan switch)
- Small threaded sensor that screws into the coolant passage (radiator neck, thermostat housing, or cylinder head area depending on model).
- Internal mechanism: usually a bimetal strip or wax pellet actuator that closes a contact at its designed temperature (e.g., 88–98°C). Has one electrical terminal (single-spade) or two terminals (switch-to-ground or switch-to-relay).
- Has a sealing washer or O-ring to prevent coolant leaks.

2. Wiring connector / harness
- Female spade or plug which clips onto the switch terminal. May include a rubber boot for weatherproofing.
- Carries the control signal (often grounds the relay coil through the switch, or supplies +12V to relay coil).

3. Fan relay (if present)
- A separate relay activated by the switch to supply high current to the fan motor. Commonly a 4- or 5-pin relay with coil and high-current contacts.

4. Fuse / fusible link
- Protects fan circuit from short. If blown, fan won’t run even if switch works.

5. Cooling fan motor and fan assembly
- The load that runs when the thermal switch closes. Can be axial electric fan(s) with motor or multiple motors.

6. Radiator / coolant passages / thermostat
- Where the switch senses temperature; the thermostat regulates engine temp and affects when the switch sees hot coolant.

7. Battery / ground
- Provides power; good ground path is essential for switch and relay operation.

Theory — why repair is needed and how system works
- Purpose: prevent engine overheating by switching the cooling fan ON when coolant reaches a preset temperature; stop fan when cooled to save power and reduce wear.
- How it works:
- At low temp: thermal switch is OPEN → circuit to fan relay coil is open → fan OFF.
- At set temp: switch CLOSES → completes relay coil circuit (often to ground) → relay energizes → sends battery power to fan motor → fan runs and cools coolant.
- When coolant drops below the switch’s opening temperature, switch opens and fan stops.
- Why repair: if switch fails, either the fan will not run (risk of overheating, especially at idle/congestion) or the fan will run constantly (wasted power, possible battery drain, increased fan wear). Intermittent failures cause unreliable cooling and unpredictable engine temps.

Tools & consumables
- Tools: multimeter, 10–19 mm wrench or socket set for sensor removal (size depends on switch), pliers, flat screwdriver to release connector, hose clamp pliers (if draining), drain pan, thermometer (optional), heat source (hot water kettle) for bench testing, torque wrench if torque spec known.
- Consumables: replacement thermal switch (OEM or proper temp spec), new crush washer/O-ring, thread sealant if specified (use coolant-safe), coolant to top up, gloves, rags.

Safety first
- Work on a cool engine. Hot coolant can scald. If you must open the cooling system, wear gloves and eye protection and relieve pressure slowly.
- Disconnect battery negative before doing electrical work to avoid shorting.
- Properly capture and dispose of drained coolant.

Diagnosis — how to tell the switch is the problem
1. Symptom checklist
- Engine overheats at idle/traffic but cools at speed → fan not working at low speed (likely switch, relay, fuse, or fan motor).
- Fan runs constantly, even cold start → switch may be stuck closed or wiring shorted to ground, or relay stuck welded.
- Intermittent fan operation → bad connector, corrosion, weak switch, or relay.

2. Simple checks
- Fuse: inspect fan fuse/fusible link.
- Relay: listen for relay click when engine reaches temp; swap relay with known-good same-type relay to test.
- Fan motor: apply 12V directly to the fan motor (bypass relay) to confirm motor works.

3. Test the switch in-situ or on bench
- Back-probe the connector with multimeter set to continuity or ohms. With engine cold the switch should show OPEN (no continuity). Run engine to temperature or warm the coolant; at operating temp the switch should CLOSE (continuity).
- Bench test method: remove switch, attach meter across switch terminals; heat the switch in hot water while monitoring temperature with thermometer; switch should close near its rated temp. Do not use an open flame; keep electrical terminals dry.
- Voltage check: with switch connected, check for voltage on relay coil side when switch is closed (or check for ground if switch grounds the coil). If switch closes but relay gets no power or ground, trace wiring.

Removal and replacement procedure (workshop-style, beginner-friendly)
Preparation
- Park on level ground, engine cold. Put chocks under wheels. Disconnect negative battery terminal.
- Identify switch location (often on radiator neck, thermostat housing, or cylinder head). Consult vehicle layout pictures if needed.

Draining enough coolant
- You don’t always need to fully drain the system. If switch is in a radiator neck or housing, place a drain pan under the fitting and loosen switch slowly to avoid splashing. If the switch location will spill coolant, open the radiator drain petcock partially to reduce pressure/coolant level below the switch.

Disconnecting electrical connector
- Remove rubber boot (if present). Depress tab and pull connector off. If corroded, use penetrating spray and work gently with pliers or small screwdriver to release clip — don’t pull on wires.

Unscrew and remove switch
- Use the correct size wrench or socket. Turn counterclockwise. Expect coolant to leak; have rags and catch pan ready.
- Inspect sealing washer/O-ring. Replace with new.

Install new switch
- If the new switch uses a crush washer, place it on the thread. Some require a small smear of thread sealant; use only specified coolant-safe thread sealant or Teflon tape sparingly — many switches rely on the washer for sealing.
- Hand-start the thread to avoid cross-threading. Tighten to seat. If you have torque spec from manual, use it (typical small sensor torque is low, often 10–20 Nm; if unknown, snug plus 1/8–1/4 turn). Don’t overtighten — you’ll strip housing or break the sensor.
- Reconnect electrical connector and rubber boot.

Refill and bleed coolant
- Top up coolant to correct level. Start engine with heater on high to allow air to purge. Squeeze upper radiator hoses carefully (cold only) to help dislodge air. Some systems have bleed screws — open as per manual. Keep watching coolant level and top up as needed. Check for leaks around the new switch.

Reconnect battery and test
- Reconnect negative battery. Run engine to operating temperature. Observe fan operation: switch should close and fan should start at specified temp. Check for leaks. Verify no warning lights or abnormal behavior.

Testing after install
- Use multimeter to confirm switch continuity at hot temp and open at cold.
- Confirm relay operation and fan current draw is within spec. Check for correct fan on/off behavior; at idle you should see fan cycle when warm; when driving at speed the fan may not run if ram air is sufficient.

What can go wrong — detailed failure modes and causes
1. Stuck open (fan never turns on)
- Failed open thermal switch (internal contact welded open or actuator failure).
- Broken/disconnected wiring to switch or relay.
- Blown fuse or failed relay.
- Faulty fan motor or wiring from relay to fan (fan tests good when directly powered).
- Switch mounted where it doesn’t see hot coolant (air pocket or wrong location) — symptom occurs even though engine overheating.

2. Stuck closed (fan runs constantly)
- Switch contacts welded closed or internal element jammed.
- Short to ground in the wiring harness downstream of the switch.
- Relay welded closed or stuck.
- ECU control in some systems might command fan on — but for simple thermal switch systems, a closed switch means switch failure or short.

3. Intermittent operation
- Corroded connector or water intrusion causing intermittent contact.
- Weak or aging switch with variable actuation temperature.
- Poor ground at engine/chassis affecting relay coil return.
- Air pockets in coolant preventing accurate sensing (switch sees cold air or fluctuating temps).

4. Coolant leak from switch
- Old/damaged O-ring or washer, incorrect installation, or over-tightening that cracks housing.
- Thread sealant left inside switch port, causing damage.

5. Incorrect replacement part
- Using a switch with wrong actuation temperature will cause premature or late fan activation. Always match OEM part number or temperature spec.

Quick troubleshooting flow (short)
- No fan: check fuse → check relay (swap) → test fan motor directly → test switch for continuity when hot → inspect wiring/ground.
- Fan always on: unhook switch; if fan stops, switch is shorted/closed; if fan stays on with switch removed, check relay/wiring/ECU.

Maintenance tips
- Replace rubber boot or connector if brittle/corroded.
- Clean connector pins and apply dielectric grease to prevent corrosion.
- When replacing, always use a new sealing washer/O-ring.
- If coolant is contaminated or system has air pockets, bleed fully to ensure accurate sensing.

Final checks after repair
- Run vehicle to operating temp in stationary and low-speed conditions to ensure fan cycles correctly.
- Road test in traffic to confirm no overheating.
- Recheck connector and switch area for leaks after cooldown.

Common specs and notes
- Actuation temp varies by model; typical fan switches operate ~88–98°C but confirm for your truck.
- If truck uses ECU-controlled fans, the “thermal switch” may be a different sensor; verify part numbers in parts manual for the exact Hino chassis code.

Conclusion (practical summary)
- The thermal switch is a simple temperature-controlled on/off switch for the cooling fan. Diagnose by checking fuses, relay, fan motor, and then the switch (continuity hot/cold). Replace the switch by draining/relieving coolant, removing connector, unscrewing sensor, fitting new sensor with new washer, tightening to seat, reconnecting, refilling/bleeding coolant, and testing operation. Watch for wiring corrosion, seal failure, and wrong temperature parts.
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1991 HINO FE For Sale https://www.truckpaper.com/listings/trucks/for-sale/211084531 1991 Hino FE17 COE Refrigerated Van Truck, S/N ...

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Overview
- Purpose: check, adjust (if applicable), or replace valve lifters (tappets/hydraulic lifters) on Hino FD/FE/FF/SG/FA/FB series engines. Procedure covers how to identify worn lifters, how to remove and replace them, what tools are required, and what parts may need replacing.
- Important: Hino engines have different variants—some use mechanical tappets (adjustable) and some use hydraulic lifters (self-adjusting). Always confirm engine type and specifications in the factory service manual for your exact model and year before starting.

Safety (must-read)
- Work in a well-ventilated, flat area with the parking brake on and wheels chocked.
- Disconnect negative battery terminal before working around valvetrain to prevent accidental cranking.
- Allow engine to cool fully before opening the valve cover—hot oil/metal will burn.
- Wear safety glasses, gloves, and use jack stands if raising the vehicle.
- Keep a fire extinguisher nearby when working with fuel/oil.

Tools — basic tools (you said you have basic tools) and how to use them
- Combination wrench set (open + box end)
- Use for loosening/tightening bolts in tight spots where a socket may not fit. Choose the correct size for the fastener and pull toward the box end for better grip.
- Metric socket set with ratchet and extensions
- Use to remove valve cover bolts, rocker shaft bolts, and other hex fasteners. Use extensions to reach recessed bolts. Match socket size to fastener and apply smooth, steady force.
- Torque wrench (click-type)
- Required to tighten critical fasteners (valve cover, rocker bolts, etc.) to factory spec. Set to required torque and tighten slowly until the click, then stop.
- Screwdrivers (flat and Phillips)
- For prying small clips, removing air hoses, or lever action on fasteners. Use correct tip to avoid stripping.
- Pliers (needle-nose and regular)
- For removing clips, cotter pins, or handling small parts. Use for gripping, not for heavy torque.
- Oil drain pan and rags
- Collect oil from spilled oil when removing or handling lifters; keep the work area clean.
- Clean containers or labeled trays
- Keep bolts and parts organized and labeled for reassembly.
- Feeler gauge set (if engine uses adjustable tappets)
- Measures valve clearance. Slide blades between rocker and valve stem to set clearance. Use calibration blades matching factory spec.
- Brake cleaner or parts cleaner and lint-free rags
- Clean components and mating surfaces before reassembly; avoid getting solvent into oil passages.
- Flashlight or inspection light
- Illuminates the valvetrain area for inspection.

Tools — extra/special tools you will likely need and why (detailed descriptions and how to use)
- Factory service manual or repair guide specific to your Hino model
- Contains torque specs, valve-clearance specs, lifter type, camshaft timing procedure, and step-by-step diagrams. Essential. Use it as the authoritative reference.
- Valve cover gasket (new; replacement part)
- Valve cover gasket must be replaced when removing the valve cover to prevent leaks. Use a new gasket and clean mating surfaces.
- Torque angle gauge or calibrated torque wrench (for bolts requiring angle tightening)
- Some engines use torque+angle specs; this ensures correct preload on bolts.
- Camshaft timing/sprocket holding tool (camshaft locking tool)
- Holds camshaft(s) in position while removing rocker gear or replacing lifters/pushrods so timing isn’t lost. Prevents incorrect cam timing and engine damage.
- Valve spring compressor (if replacing lifters that require removing valves)
- Compresses valve springs safely to remove retainers and drop the spring and keepers so you can remove the lifter/rod or service the valve train. Use only if removing valve springs or valves.
- Lifter removal tool (hydraulic lifter/pushrod puller) or magnetic pick-up
- Some lifters are hard to extract; a special puller or long magnet helps remove lifters from bores without damage.
- Pushrod straight edge/inspection tool and tube for indexing pushrods (if applicable)
- Marks and stores pushrods so they can be reinstalled in the same location if you want to reuse them. Inspect for bending; replace if bent.
- Dial gauge with base (for measuring lifter collapse or cam lift)
- Measures lifter piston travel and cam lift for diagnosis. Required if you want to quantify lifter collapse.
- Impact driver (hand or air) and breaker bar
- For stubborn bolts; breaker bar for initial heavy loosening, impact for stuck fasteners. Use carefully to avoid damage.
- Engine hoist/support (only if cylinder head removal is required)
- If lifter bores require access that needs head removal, support is needed. This is major work and typically beyond beginner scope.
- New engine oil, oil filter, and oil draining tools
- When lifters are replaced, fresh oil and filter are usually recommended because debris and metal particles may be in the oil.

Parts — what may need replacing and why
- Valve cover gasket
- Replace every time valve cover is removed to prevent oil leaks.
- Lifters (hydraulic or solid tappets)
- Replace if noisy (ticking that doesn't adjust away), collapsed (hydraulic lifter piston doesn’t move), scored pitted, or if lifter bore is damaged. Symptoms: persistent tappet noise, misfire, low oil pressure localized, excessive valve lash variation.
- Pushrods
- Replace if bent, scored, or worn at contact points. Bent pushrods cause improper valve operation.
- Rocker arms/rocker shafts and bushings
- Replace if visibly worn, cracked, or if rocker movement is sloppy. Worn rocker surfaces cause repetitive wear and noise.
- Valve stem seals (if removing valve springs)
- Replace if oil consumption or blue smoke on startup present.
- Camshaft (rare)
- Replace if lobes are excessively worn or mushroomed; lifter replacement alone won’t fix a worn cam.
- Head gasket or cylinder head parts (only if head removal needed)
- Replace if cylinder head is removed, or if head gasket is damaged.
- Engine oil and filter
- Replace to remove contaminants and protect new lifters.
- Lifter bore liners/sleeves (rare, engine-specific)
- Required if lifter bores are damaged; often requires machine shop work.

Diagnosis — how to tell what you need to do
- Persistent ticking or tapping at all RPMs, unchanged after oil change or startup, suggests mechanical wear or collapsed hydraulic lifter.
- Tapping that quiets after a long drive or changes with oil temperature may indicate hydraulic lifter bleed-down or low oil pressure—check oil level and pressure first.
- Excessive valve lash on adjustable tappets measured with feeler gauge beyond spec means adjustment is required.
- Misfire or rough running combined with noisy lifters may indicate a bad lifter or valve train wear.

Procedure — inspection, adjustment, and replacement (high-level, beginner-friendly steps)
- Prepare: gather tools, obtain service manual, drain a small amount of oil if necessary for a cleaner job area, disconnect negative battery, and remove air intake components obstructing valve cover.
- Remove valve cover: clean around cover to avoid dirt falling in, remove bolts with socket/ratchet, lift cover off (may need gentle pry), remove old gasket and clean mating surfaces with solvent and rag.
- Identify lifter type: inspect lifter bores and service manual—if there is a visible lifter plunger and no adjustment screw on rocker arms, it’s hydraulic. If rockers have adjustment screws or locknuts, tappets are adjustable.
- For adjustable tappets (mechanical lifters)
- Rotate engine by hand (socket on crank pulley) to place cylinder at TDC compression for the cylinder you're adjusting (use service manual firing order).
- Use feeler gauge to measure clearance between rocker and valve stem.
- If clearance out of spec, loosen locknut, turn adjuster to set specified clearance using feeler gauge, then tighten locknut while holding adjuster. Re-check clearance after tightening.
- Repeat for each cylinder following firing order and TDC compression steps in manual.
- Replace valve cover gasket and reassemble, torque bolts to spec.
- For hydraulic lifters (most common)
- Inspect for noise and pumping action: remove rocker assemblies and carefully lift out lifters or pushrods to inspect lifter surface and piston movement.
- If lifter piston is stuck/collapsed or shows heavy wear/scoring, replace the lifter assembly.
- If pushrods are bent or scored, replace pushrods.
- If cam lobes show wear, lifter replacement alone may not fix it; consider replacing camshaft and lifters together.
- When installing new hydraulic lifters, prime them if recommended (some lifters require pre-filling with clean oil). Follow manufacturer’s priming instructions.
- Reinstall pushrods and rockers, torque rocker shaft/bolts to spec and check for correct valve lift/lash per manual (some hydraulic systems have a preload procedure).
- Removing stuck lifters
- Use a lifter/puller tool or magnetic tool; apply steady pull while protecting bore surfaces. If lifter is seized in bore, do not hammer—seek professional help or consider cylinder head removal.
- Reassembly
- Use new valve cover gasket, apply gasket per manual (some need sealant at corners), torque bolts to spec.
- Reconnect battery, refill oil and replace filter if you've drained or plan fresh oil.
- Start engine, let idle and listen for noise. Some hydraulic lifter noise may disappear after a short break-in as oil fills them. Check for leaks.
- Break-in and checks
- After startup, run engine to operating temp, re-check valve cover for leaks.
- Re-torque valve cover bolts after first heat cycle if manual specifies.
- Monitor oil pressure and engine noise for several hundred kilometers.

When full part replacement or head removal is required (summary)
- Replace lifters if piston stuck, collapsed, noisy after oil change, or visibly damaged.
- Replace pushrods and rockers if worn or bent.
- Replace camshaft if lobes are damaged—cam damage typically means machine-shop level repair and is expensive.
- Remove cylinder head and/or replace head gasket only if lifter bore damage or head work is necessary—this is advanced work requiring engine re-timing and precision machining.

Common beginner pitfalls and how to avoid them
- Not using the service manual: leads to wrong spec and possible engine damage. Get the manual.
- Reusing a valve cover gasket: causes oil leaks—always replace gasket.
- Incorrect torque: over- or under-tightening rocker bolts causes broken bolts or loose parts. Use a torque wrench and correct specs.
- Mixing lifters/pushrods positions: index or mark pushrods if reusing; mismatched/or bent pushrods cause problems.
- Not priming hydraulic lifters: causes noise and poor performance—follow lifter priming procedure if required.
- Skipping oil/filter change after repairs: contaminants can damage new lifters—change oil/filter.

Quick checklist to have with you
- Factory service manual
- Full metric hand tool set + torque wrench
- Feeler gauge set (for adjustable tappets)
- Valve cover gasket
- Replacement lifters, pushrods, rocker arms (if needed)
- New engine oil and oil filter
- Parts cleaner, rags, containers for bolts
- Camshaft holding tool and lifter removal tool (if replacing lifters)

Final note (concise)
- Follow the factory service manual for exact specs, timing, and procedures for your specific Hino engine. If lifters are seized, cam lobes are damaged, or head removal is required, this can exceed beginner skills—consider a qualified shop for those tasks.
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