Short answer up front: Toyota 3B / B / 11B / 13B are diesel B‑series engines and do NOT have spark plugs or spark plug wires. They use glow plugs and a glow‑plug wiring/relay/timer system to aid cold starting. Below is a complete beginner‑level, step‑by‑step guide that explains the theory, every component, how the system works, what goes wrong, and exactly how to replace/test glow plugs and their wiring (the equivalent “spark plug wires” repair for these engines).

Why this matters (theory, simple analogy)
- Diesel vs petrol: Petrol engines use spark plugs (an electrical spark) to ignite the air/fuel. Diesel engines rely on compression heating; glow plugs are small electrical heaters that help raise the temperature in the combustion chamber for cold starts.
- Analogy: Think of each glow plug like a tiny electric space heater threaded into each cylinder. The glow system (switch/relay/timer + wiring + battery) is the lamp and circuit breaker that turns those heaters on before you crank the engine.
- Why repair is needed: If a glow plug or its wiring/relay fails, starting will be hard or impossible when cold, you’ll get long cranking times and heavy white smoke on cold starts, rough idle after starting, or the glow light on the dash may not behave correctly.

Main components (detailed)
1. Glow plugs
- What: Threaded heaters that screw into the cylinder head; tip sits in combustion chamber. They contain a heating element and a metal body.
- Appearance: A hex portion for a socket, a threaded shank, and a terminal on top for the wire (often a spade or female push terminal). Some have an insulating washer or O‑ring.
- Function: Heat the air in the combustion chamber before and during starting.
2. Glow plug wiring harness
- What: Heavy gauge wires and connector terminals from the glow plugs to the relay/timer and battery.
- Function: Carry high current (amps) from battery/relay to glow plugs. Terminals are often push‑on or ring types.
3. Glow relay / timer (control box)
- What: Electromechanical relay or electronic timer that switches battery power to the glow plugs when the key is turned to preheat position (older trucks) or when ECU commands it.
- Function: Supplies high current to plugs for a timed period and often keeps plugs powered during cranking or warm‑up as required.
4. Dash indicator lamp
- What: The glow/plugs warning lamp that tells you when to wait or preheat is active.
- Function: Gives driver feedback; often tied through the relay/timer circuit.
5. Battery & grounds
- What: 12 V battery and engine/chassis grounds.
- Function: Supply the current needed and provide return path. Weak battery or poor ground reduces glow plug current and effectiveness.
6. Fuses/links and connections
- What: Inline fuse or fusible link protecting the circuit.
- Function: Prevents wiring damage on short circuits.

How the system works (step sequence)
1. Key turned to preheat (or ECU triggers): Glow relay/timer closes, sending battery voltage to glow plugs.
2. Glow plugs draw high current and heat quickly (seconds to tens of seconds depending on plug type).
3. Dash lamp indicates preheat; when the timer finishes the lamp may go out (depending on setup) and you crank engine.
4. During cranking the relay may continue to supply plugs for a short time; once running, plugs may be switched off or kept on briefly.
5. Good battery and wiring keep voltage high enough so plugs heat to the right temperature quickly.

Common failure modes (what can go wrong)
- Individual glow plug open circuit (internal filament burned): That cylinder will be cold — hard starting and white smoke.
- Carbon fouling or shorted element: Reduced heating performance or short draws too much current.
- Broken or corroded connector/terminal: Intermittent or no power to plug.
- Seized or broken glow plug in head: Can snap off during removal — requires extracting broken stud from head (difficult/expensive).
- Damaged wiring harness (melted, frayed) or bad crimp joints: High resistance → low current → ineffective heating.
- Failed relay/timer or dash lamp circuit: No current to plugs.
- Weak battery or poor ground: Voltage drop reduces plug current.
- Stripped threads from aggressive removal: Poor sealing, leaks, or need for helicoil/repair.

Tools & materials you’ll need
- Hand tools: ratchet, deep socket set (glow plug socket typically 8–12 mm hex; use deep socket), extensions, wobble if needed
- Torque wrench (recommended)
- Multimeter (DC volts and ohms)
- Small clamp‑on ammeter (optional) for current draw testing
- Penetrating oil (e.g., PB Blaster) if plugs are stuck
- Compressed air or brush to clean around plugs before opening
- Replacement glow plugs (OEM spec for each engine) and replacement connectors/wiring if necessary
- Electrical crimper, heat shrink tubing, dielectric grease
- Safety: gloves, eye protection, battery terminal wrenches
- Service manual (strongly recommended for exact torque specs and wire routing)

Safety and preparatory steps
- Work on a cool engine. Glow plugs sit in the head; hot heads risk burns.
- Disconnect the negative battery terminal before unplugging electrical connectors (prevents shorting across battery).
- Clean around each glow plug base with compressed air or brush to keep dirt out of combustion chamber when plugs are removed.
- Label wires or photograph routing if connectors look similar.

Diagnostic tests (how to tell what’s bad)
1. Visual inspection: Corrosion, broken terminals, melted insulation, oil/fuel around plug.
2. Simple continuity/resistance test (multimeter)
- Remove connector from glow plug.
- Measure resistance between top terminal and glow plug body (ground). Good plugs usually read low resistance (single‑digit ohms). Exact value varies; compare to service spec or to another plug on the engine.
- An open/infinite reading = bad (open element). A much higher‑than‑others reading = weak.
3. Voltage under load test
- With relay engaged, measure voltage at the top terminal of each glow plug while engine is in preheat. Voltage should be close to battery voltage minus small drop (~11–12 V). If voltage is much lower, check wiring, relay, or battery.
4. Current draw (recommended)
- Clamp an ammeter on each feed wire to measure current. Typical glow plug currents vary; each plug often draws several amps. Compare to spec or between cylinders; a significantly lower current means weak plug/high resistance.
5. Relay test
- Check relay for switching to battery voltage when key is in preheat. Also bench test the relay by energizing coil and checking contact continuity.

Step‑by‑step: removing and replacing glow plugs
1. Prepare
- Disconnect negative battery terminal.
- Clean area around glow plug with compressed air to avoid debris falling into cylinder.
- Note and mark wiring positions if necessary. On most B‑series the wires are one per plug and identical; still keep track.
2. Remove wiring
- Carefully unplug the wire terminal at the top of each glow plug. Wiggle the connector, don’t pull the wire itself.
- If connectors are corroded, use a small screwdriver to pry gently while protecting plastic.
3. Remove the glow plug
- Use a deep socket that fits the hex on the glow plug. Turn counterclockwise.
- If it’s seized: apply penetrating oil and let soak several hours. Reapply heat cautiously (not recommended if there are plastic parts or fuel nearby). Be patient — forcing may break the plug.
- If a plug snaps: stop. Removing a broken stub requires extraction tools or head removal — advanced repair.
4. Inspect
- Look at the tip and threads. Some carbon deposit is normal; heavy deposits or melted tips indicate problems.
- Inspect head threads for damage.
5. Install the new glow plug
- Clean threads in head lightly. Some people use a very light smear of anti‑seize; others advise against it because it changes torque readings or can cause seizing down the line. Best practice: follow factory manual. If you use anti‑seize, use sparingly and reduce torque per manufacturer's guidance.
- Hand‑thread the plug to avoid cross‑threading.
- Torque to specification. If you do not have the manual: many glow plugs are tightened to roughly 8–14 ft‑lb (10–20 Nm) on many small diesels, but that’s a general range — get the exact spec if possible.
6. Reconnect wiring
- Reattach the connector onto the plug terminal. Ensure a clean, tight fit. Use dielectric grease on terminals to resist corrosion.
- If replacing wire ends: use proper heavy‑duty connectors, crimp firmly, and cover with heat shrink.
7. Reconnect battery and test
- Reconnect the negative battery terminal.
- Turn key to preheat and check dash lamp and voltage at plugs.
- Crank engine and observe starting behavior. Check for error symptoms.

Replacing glow plug wiring/terminals (step sequence)
- If the wire insulation or connectors are damaged, replace with correct gauge wire (heavy gauge to carry high current) or replace the entire harness.
- Cut out bad section, strip, crimp on appropriate terminal (ring or spade as original), then heat‑shrink the joint.
- Route wires away from exhaust, sharp edges, moving parts; secure with clips/zip ties and protect with loom.
- Ensure good engine/chassis ground continuity.

Testing the relay/timer
- With battery connected, probe relay control terminal while turning key to preheat — it should energize (you’ll see battery voltage at the plug feed side).
- If relay fails to supply voltage, swap with known good relay or bench test (apply 12 V to coil and check contact).
- Some systems incorporate the relay into a timer box — a failed module needs replacement.

Special cautions and gotchas
- Broken glow plug inside head is costly: avoid excessive force. If stuck, use penetrating oil and controlled, steady force.
- Don’t crank the engine excessively when plugs are bad — fuel accumulates and causes heavy white smoke or flood.
- Always clean the area before removal — a little dirt can cause big problems when it falls in the cylinder.
- Use proper terminals and heavy gauge wire. Thin wire causes big voltage drop and poor heating.
- Replace plugs in sets if several are old or the vehicle sees cold weather (matched performance is better).
- If you see fuel or oil fouling a plug repeatedly, investigate injectors or valve seals — glow plug failures may be a symptom.

Quick troubleshooting checklist (practical)
- Symptoms: hard cold start, long cranking, white smoke → suspect glow system.
- Step 1: Check battery voltage (12.4 V+), terminal tightness, and engine ground.
- Step 2: Check dash glow lamp behavior when key turned to preheat.
- Step 3: Measure voltage at glow plug tops during preheat.
- Step 4: Measure each plug’s resistance; replace opens/weak ones.
- Step 5: Check relay and harness for continuity and corrosion.

Final notes
- Get OEM or correct‑spec glow plugs for your exact engine model (3B, 11B, 13B have specific part numbers).
- If you’re unsure about torque or exact part numbers, consult the factory service manual for your engine model before final torque or purchase.
- Replacing glow plugs and wiring is a straightforward DIY if plugs come out cleanly and wiring is accessible; stop and seek help if a plug refuses to budge or breaks — extraction is tricky.

That covers the entire system, components, theory, failure modes, and a detailed how‑to for replacing/testing the glow plugs and their wiring on Toyota B‑series diesel engines (3B, B, 11B, 13B).
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- Safety first
- Wear safety glasses and gloves.
- Work on a flat level surface, use wheel chocks to stop the vehicle rolling.
- Never rely on a jack alone — always support the vehicle with quality jack stands rated above the vehicle weight.
- Keep a fire extinguisher nearby when working with fluids.

- What “differential service” means for Toyota B‑series (3B, B, 11B, 13B) vehicles
- Routine service = drain and replace differential (rear axle) gear oil, inspect for metal particles, check seals and fasteners, reseal the cover.
- More advanced service = replace pinion seal, axle seals, bearings, or ring & pinion; measure backlash and preload. Advanced work needs extra tools and skill.

- Basic tools you need (descriptions and how to use each)
- Floor jack
- What it is: hydraulic jack to lift axle or vehicle.
- How to use: position on correct lifting point, pump handle to lift slowly. Raise enough to place jack stands. Do not crawl under vehicle supported only by the jack.
- Jack stands
- What it is: adjustable mechanical stands to support vehicle.
- How to use: position under axle or frame, set to appropriate height, lower vehicle onto stands; shake lightly to confirm stability.
- Wheel chocks
- What it is: blocks placed behind wheels.
- How to use: wedge behind wheels to prevent movement before lifting.
- Drain pan (shallow oil pan)
- What it is: container to catch old gear oil.
- How to use: place under differential drain or lower cover to collect fluid. Have a second container or rags for spills.
- Basic socket set and ratchet (metric set up to 19 mm typically)
- What it is: sockets and a ratchet handle to remove bolts and nuts.
- How to use: use correct socket size, pull handle perpendicular to ratchet to break fasteners loose. Use breaker bar for stubborn bolts.
- Breaker bar
- What it is: long non-ratcheting bar for extra leverage.
- How to use: fit socket, apply controlled force to break tight bolts. Avoid sudden jerks.
- Torque wrench (click‑type)
- What it is: tool to tighten bolts to a specified torque.
- How to use: set the desired torque, tighten slowly until it clicks; bolts must be torqued to spec to avoid leaks or stripped threads.
- Why required: cover bolts and axle nut/pinion nut require specific torque. Essential for safe reassembly.
- Screwdrivers (flat and Phillips)
- What they are: hand tools for prying or removing small fasteners.
- How to use: use correct tip to avoid damage; a flat screwdriver can help pry the cover but be careful to avoid gouging.
- Gasket scraper or putty knife
- What it is: metal or plastic blade for cleaning mating surfaces.
- How to use: scrape old gasket material/RVT from surfaces until clean and smooth.
- Wire brush & rags
- What they are: cleaning tools to remove rust and oil.
- How to use: brush mating surface lighty; wipe dry before resealing.
- Rubber mallet or dead‑blow hammer
- What it is: non‑marring hammer.
- How to use: tap the differential cover or axle parts gently to loosen; avoid steel hammer on gear surfaces.
- Pry bar (small)
- What it is: lever tool for separating cover from housing.
- How to use: insert gently and pry evenly around cover; don’t bend cover or housing.
- Seal puller (basic)
- What it is: small hooked tool to remove axle/pinion seals.
- How to use: hook behind the seal lip and pull out; avoid scratching bore.
- Funnel or gear oil pump
- What it is: pump or funnel with hose needed to refill differential through fill hole.
- How to use: attach pump hose to fill port and pump oil until level reaches fill hole; or use funnel and squeeze bottle for slow fill.
- Why required: fill hole is high; pouring without a pump is messy and slow.
- Gloves and absorbent mats
- What they are: personal protection and housekeeping items.
- How to use: wear to keep hands clean and reduce slipping on oil.

- Optional/advanced tools (explain why you might need them)
- Torque angle gauge
- Why: some pinion nuts use angle torque. Useful for accurate preload procedures.
- Dial indicator with magnetic base
- Why: measures gear backlash. Required if you replace ring & pinion or need precise gear setup.
- Bearing puller and hydraulic press
- Why: to remove and install bearing races and carrier bearings. Required if replacing bearings or doing full overhaul.
- Impact wrench and impact sockets
- Why: speeds removal of stubborn bolts/axle nuts; not required but helpful. Don’t use for final torque—use torque wrench.
- Shop manual or factory service manual
- Why: provides exact torque specs, oil capacity, sealing methods, and special procedures for your specific axle model. Strongly recommended.

- Consumables & fluids
- Correct gear oil (API GL‑5) — common choices:
- 75W‑90 synthetic or 80W‑90 mineral gear oil for most Toyota rear diffs.
- Use 85W‑140 only if specified or for very hot/heavy duty applications.
- Limited‑slip differential needs the correct limited‑slip additive per manual.
- How to use: fill until oil reaches fill hole level; do not overfill.
- RTV silicone or differential cover gasket
- When to use: if original gasket is damaged or cover is gasketed rather than a metal gasket.
- New pan gasket or gasket maker, crush washer (if pinion drain plug uses one), axle seals, pinion seal
- Why: seals and crush washers often fail with age; replaced to stop leaks.
- Replacement bolts (if corroded) and thread locker
- Why: rusty/stripped bolts should be replaced; thread locker on specified fasteners prevents loosening.

- Step‑by‑step service procedure for a beginner (general — follow shop manual for torque and capacities)
- Prepare vehicle
- Park level, chock front wheels, loosen rear wheel lug nuts a little if removing wheels.
- Lift rear with floor jack and support securely on jack stands under axle tubes or recommended points.
- Access differential
- If required for access, remove wheels and/or inside differential dust shields.
- Position drain pan beneath differential.
- Drain gear oil
- If differential has a drain plug: remove drain plug with appropriate socket; allow oil to fully drain.
- If no drain plug: loosen and remove lower cover bolts gradually while keeping pan underneath; pry cover off gently to avoid spill.
- Inspect old oil: look for metallic flakes (indicates gear/bearing wear) and smell (burnt smell indicates overheating).
- Clean interior
- Use rags to wipe inside housing and magnet(s). Use a putty knife/gasket scraper to remove old gasket material from cover and housing.
- Clean magnet(s) on cover — metal filings will stick here.
- Inspect gears and components visually
- Look for pitting, scoring, broken teeth, heavy wear, or excessive play in bearings.
- If gears look normal (even wear pattern, no chunks or heavy scoring), proceed to reseal. If heavily worn/damaged, consider repairs/replacement (see replacement section).
- Replace seals & plug crush washer if needed (recommended if leaking or old)
- If pinion or axle seals look worn or leaking, use seal puller to remove old seal and install new seal carefully level with bore. Lightly coat new seal lip with gear oil before installation. Use a seal driver or socket to tap new seal flush.
- If drain/fill plugs use crush washer, replace washer.
- Reinstall cover or tighten drain plug
- If cover uses a gasket: fit new gasket or apply a thin, even bead of RTV silicone to mating surface (follow manual cure time).
- Reinstall cover bolts finger‑tight, then torque in a crossing pattern to specified torque.
- If you used RTV, allow recommended cure time before filling if specified.
- Refill with gear oil
- Remove the fill plug on the differential cover or housing.
- Using a gear oil pump or bottle with hose, pump gear oil until it starts to weep from the fill hole — that is the correct level.
- Reinstall fill plug and torque to spec.
- Clean up and lower vehicle
- Wipe any spilled oil, reinstall wheels if removed, lower jack and remove stands.
- Tighten wheel lug nuts to correct torque.
- Road test
- Drive gently and check for leaks after first short trip. Recheck fluid level when cool if necessary.

- When replacement parts are required, why, and what to get
- Seals (pinion seal, axle seals)
- Why replace: leaking oil causes low lubricant level, overheating, and bearing failure. Old seals get hard and crack.
- Replacement: OEM or quality aftermarket seals sized for your axle. Replace both axle seals if one is leaking or as preventive maintenance when you have the unit open.
- Gasket or RTV
- Why replace: old gaskets leak; RTV ages and can crack.
- Replacement: OEM gasket or high‑temp RTV recommended for differentials.
- Gear oil
- Why replace: oil ages and loses lubricating additives; metal contamination is possible.
- Replacement: choose the correct viscosity and API GL‑5 rating for your model. Add limited‑slip additive if your differential is LSD.
- Bearings (pinion bearing, carrier bearings)
- Why replace: noise, roughness, excessive end play or visible damage warrant replacement. Damaged bearings will destroy gears if not addressed.
- Replacement: matched bearing sets and races (often sold as kits).
- Note: installing bearings correctly typically requires a press and shims to set pinion preload and carrier bearing preload.
- Pinion seal + crush washer + pinion nut
- Why replace: if pinion was loosened or if seal leaks; crush washers are often single‑use.
- Ring & pinion gears
- Why replace: chipped teeth, heavy wear, or incorrect gear mesh. If gears are damaged, both ring and pinion must be matched and installed together.
- Replacement: buy a matched ring & pinion set and installation shims or a repair kit. Accurate setup (bearing preload and backlash) is critical and usually done with a dial indicator and torque‑angle procedures.
- Fasteners, bolts
- Why replace: rusted, stretched, or rounded bolts should be replaced to achieve proper torque and safety.

- When to stop and seek professional help (no further questions — follow these)
- If you find metal flakes, broken gear teeth, noisy bearings, or cannot reassemble to correct torque/preload/backlash — stop and consult a professional.
- If the job requires bearing removal/installation, shim measurement, or gear pattern setup (dial indicator and marking compound) — these are precision tasks that require shop tools and experience.

- Quick troubleshooting signs
- Whine at cruising speed → likely pinion bearing or gear mesh issue; inspect oil for metal.
- Clunk on acceleration/reverse → possible worn ring & pinion, loose carrier bearings or damaged spider gears.
- Visible oil leak from cover or axle flange → replace gasket/seal and refill.

- Quick parts checklist to have on hand before starting (beginners)
- Correct quantity and type of gear oil
- New cover gasket or RTV
- New fill/drain plug crush washer(s) if applicable
- New axle seals and pinion seal if leak suspected
- Rags, gloves, drain pan, cleaning solvent

- Final tips
- Get the vehicle’s factory service manual or a reliable repair guide for torque specs, fluid capacity, and any model‑specific notes before starting.
- Take photos during disassembly to ease reassembly.
- Work methodically, keep fasteners organized, and clean mating surfaces thoroughly.

No unnecessary talk — follow the above, and if you encounter damaged gears, bearings, or cannot set backlash/preload, have a shop do the rebuild.
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