Short clarification: most Hino 700 Series trucks are diesel engines (compression-ignition) and do not have spark ignition coils. They use glow plugs and an electronic glow system instead. If your truck is diesel, read the glow‑plug section below (this is the relevant repair). If you truly have a gasoline/auxiliary engine or a coil‑equipped variant, read the generic ignition‑coil section after that.

PART A — Diesel (Hino 700): Glow‑plug replacement (what you most likely need)

Why this repair is needed — theory in plain terms
- Diesel engines ignite fuel by compressing air until it gets hot enough to ignite diesel. Cold starts need extra heat in the combustion chamber; that’s the glow plug’s job. Think of glow plugs as tiny electric heaters stuck into each cylinder to warm the air/fuel for starting — like putting a small space heater inside each room before you light the stove.
- Symptoms of bad glow plugs: hard cold starts, excessive white/gray smoke on startup, long cranking, rough idle immediately after start, glow‑plug warning lamp on dash, loss of power at low temperatures.
- What can go wrong: a glow plug can burn out (open circuit), short internally, or corrode/seize in the cylinder. The glow relay/timer or wiring can fail and mimic bad plugs.

Components you need to know (detailed)
- Glow plug: a threaded heating element that screws into the cylinder head. It has a hex head/terminal for electrical connection and a heated tip inside the combustion chamber.
- Glow plug harness/terminals: connects plugs to relay/module; may have individual connectors or a busbar.
- Glow relay/module (glow controller): receives signals from ECM and battery, powers glow plugs. Some systems have a pre‑heat timer or ECM integrated control.
- ECM (engine control module): tells the glow controller when and how long to heat based on temperature, ambient temp, engine temp, etc.
- Battery/main power supply and fuses: supply high current to glow plugs.
- Cylinder head and sealing washer: plug seals combustion chamber; some plugs have crush washers or built‑in sealing.
- Tools: ratchet, deep sockets (usually 8/10/12/14mm depending on plug), torque wrench, insulated pliers, multimeter, penetrating oil, anti‑seize (if recommended), replacement glow plugs (exact part numbers), gloves, eye protection.

Safety first
- Work on a cold engine when possible. Glow plugs and head are hot after running.
- Disconnect negative battery terminal to avoid shorts when removing wiring.
- Wear gloves and eye protection. If fuel lines are nearby, avoid sparks.
- Use penetrating oil on seized plugs carefully; allow soak time.
- Note and mark any sensors/wiring harnesses before removal.

Step‑by‑step: replacing glow plugs (beginner‑friendly)
1. Preparation
- Park on level ground, set parking brake, chock wheels.
- Let engine cool if hot.
- Disconnect negative battery terminal.
- Remove engine covers/air intake components to access valve cover/plug area as needed.
2. Locate glow plugs
- Remove any engine covers, intake ducting, or harness clips obstructing access to the glow plug harness on the cylinder head.
- Identify each glow plug connector and label them if harness is not fixed (so you reattach correctly).
3. Disconnect wiring
- Gently unplug the electrical connectors from each glow plug/busbar. Some have locking tabs — release them first.
4. Remove glow plugs
- Spray penetrating oil around the base if plugs look corroded; wait 10–20 minutes.
- Use the correct deep socket and ratchet. Turn counterclockwise to remove. Keep the socket straight to avoid breaking the plug.
- If a plug is tight, back it a little then move in small increments; avoid sudden force. If it’s seized, heat the area (carefully) or seek professional help — head damage risk.
5. Inspect plugs and cylinders
- Look at plug tip: carbon build‑up, broken element, open circuit.
- Check threads in cylinder head for damage.
6. Test plugs (optional but recommended)
- Use a multimeter: measure resistance between terminal and body/ground. Typical readings vary by design (very low ohms for metal sheathed, or a few ohms). Consult parts spec. An open circuit indicates failed plug.
7. Install new glow plugs
- Apply any manufacturer‑recommended anti‑seize or sealing washer (only if specified). DO NOT overtighten.
- Hand‑start threads to avoid cross‑threading, then tighten to manufacturer torque (if you don’t have the spec, tighten snugly but carefully; best practice: look up exact torque in the workshop manual).
8. Reconnect harness and battery
- Reattach wiring to each plug in the correct order. Ensure connectors are clean and secure.
- Reinstall covers and intake components.
- Reconnect negative battery.
9. Test
- Turn ignition on and observe glow lamp sequence or use diagnostic tool to command glow cycle.
- Start engine; it should crank faster/easier and run smoother on cold start.
10. Final checks
- Clear any diagnostic trouble codes (DTCs) with a scanner.
- Monitor for smoke, rough idle, or warning lights.

Common problems and tips
- Plug breaks off in head: very difficult; may need cylinder head removal or special extraction tools — service shop required.
- Seized threads: do not force; you can re‑tread with helicoil if threads are damaged, but best done with proper tools/skills.
- Wiring harness corrosion: often causes failures; inspect and replace harness if terminals are corroded.
- Relay/module failure: if all plugs test good but no heat — check relay, fuses, and control signals from ECM.

PART B — If you truly have an ignition coil (gasoline/ignition‑coil equipped engine) — generic coil‑on‑plug replacement

Why this repair is needed — theory
- Ignition coil transforms low battery voltage into high voltage needed to jump the spark plug gap and ignite air/fuel in gasoline engines. Analogy: the coil is a step‑up transformer — like a small power station that takes a normal household voltage and multiplies it to a lightning bolt for the plug.
- Symptoms of bad coil: misfire on one or more cylinders, rough idle, poor acceleration, increased fuel consumption, check engine light with misfire codes (P030x), visible arcing or burned coil housing.
- What can go wrong: coil internal windings fail (open/short), insulation breaks down causing arcing to ground, connectors/wiring fail, heat‑related failure, oil/contamination causing shorts.

Components you need to know (detailed)
- Ignition coil (coil‑on‑plug or coil pack): contains primary and secondary windings, iron core, housing, high‑voltage terminal or boot, and electrical connector for the primary.
- Spark plug: mates with coil (on coil‑on‑plug designs), creates the spark using high voltage.
- Boot (insulating connector): on some coils provides sealing and connection to spark plug.
- Primary connector/pin: low voltage supply from ignition switch/ECU (12V switched) and trigger signal (from ECU or ignition module).
- Ground path: coil primary circuit completes to ground through ECU/ignition driver.
- Battery, main fuses, ignition switch, engine control module (ECM), crank/cam sensors — these provide timing and power.

Tools and supplies
- Socket set, deep spark‑plug socket, ratchet, extension
- Multimeter
- Insulated pliers
- Replacement coil(s) and spark plug(s) if needed
- Dielectric grease (small amount on boot)
- Torque wrench for spark plug torque
- Screwdrivers, small pick (for connector tabs)

Safety
- Disconnect negative battery terminal when unplugging coils to avoid accidental sparks.
- Avoid working on a hot engine when possible; coils can be hot after run.
- Keep hands and tools clear of moving belts if engine starts.

Step‑by‑step: coil‑on‑plug replacement (generic)
1. Preparation
- Park, set brake, disconnect negative battery.
- Remove engine covers to access coils.
2. Identify faulty coil
- Use diagnostic trouble codes or swap‑test coils: if you have a misfire on cylinder 3, swap coil 3 with coil 1; if misfire moves to cylinder 1, the coil is faulty.
3. Unplug electrical connector
- Release locking tab and pull connector gently. Don’t yank wires.
4. Remove coil
- Remove any retaining bolts. Pull coil straight up with a slight twist; avoid bending.
5. Inspect spark plug and boot
- Check spark plug for wear; some coils fail because of bad plugs. Replace plugs if needed.
- Clean the boot and plug well before installing new coil.
6. Install new coil
- Apply a small dab of dielectric grease inside the boot to help seal and ease future removal (do not overdo).
- Push coil onto the plug until seated, bolt down to specified torque (light torque; check manual).
- Reconnect electrical connector.
7. Reconnect battery, test
- Reconnect negative battery. Start engine; verify smooth idle and that misfire code is cleared.
- Use scan tool to clear codes and monitor misfire counters.

Testing coils (basic)
- Resistance test: measure primary coil resistance across low‑voltage terminals (usually fractions to a few ohms); measure secondary (between HV terminal and primary terminal or ground) — usually thousands of ohms. Consult spec. An open circuit or short to ground is bad.
- Swap test: swap suspect coil with a known good cylinder and see if misfire moves.
- Use an ignition coil tester or oscilloscope for definitive testing.

Common failure modes
- Heat cycling leads to internal winding failure.
- Oil or coolant contamination causes shorts or arcing.
- Broken connector pins or corroded terminals restrict primary current.
- Poor spark plug gap or worn plugs increase load on coil and shorten life.

General troubleshooting flow (both systems)
- Check codes first with scanner.
- Verify battery voltage and ground connections.
- Inspect wiring/harness and fuses/relays.
- Test individual component (plug, coil or glow plug) with multimeter and/or bench tester.
- Swap components when possible to isolate faulty part.

Final notes and best practices
- Always replace glow plugs/coils with the correct OEM or equivalent part numbers.
- Replace spark plugs when replacing coils if they’re worn — a weak plug stresses a new coil.
- Keep connectors clean and dry; use dielectric grease on spark plug boots (gasoline) and on outside of coil connectors if recommended.
- Always torque to manufacturer specs; under‑ or over‑torque can cause leaks or broken threads.
- If a component breaks in the head/block (seized plug/coil mount), stop and seek professional shop help to avoid costly head damage.

If your Hino 700 is diesel, do the glow‑plug procedure above. If you have a gasoline or auxiliary engine that actually uses ignition coils, follow the coil‑on‑plug procedure. Always verify torque and resistance specs in the Hino workshop manual for your exact engine model before you start.
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