What the distributor does (simple, mechanical theory)
- Purpose: the distributor times and routes high-voltage spark from the ignition coil to each spark plug in the correct order and at the correct moment. Think of it as a traffic controller that sends the spark to one cylinder at a time, timed to the engine’s rotation.
- Why it matters: if the distributor is worn, contaminated, or misinstalled, spark timing or distribution will be wrong → misfire, poor idle, low power, hard starting, stalling, higher fuel use, or engine damage from sustained knock.

Main components (what each one is and does)
- Distributor housing / body: holds everything, installs into the cylinder head/cam/drive and seals with an O-ring. Houses the rotating shaft and pickup system.
- Distributor shaft & drive gear: the shaft is driven by a gear (on the cam or oil pump) and turns the rotor. Gear condition and mesh determine whether the rotor and pickup have correct phase with the crank.
- Distributor gear sleeve/washer/O-ring: seals oil and centers the housing; worn O-ring leaks oil into the distributor and contaminates electrical parts.
- Rotor: a plastic arm on top of the shaft with a conductive contact that rotates under the cap. The rotor carries high voltage from the coil to each cap terminal as it spins.
- Distributor cap: insulated cover with terminals for each plug wire and a center terminal from the coil. Keeps the rotor isolated and directs the high-voltage arcs to the correct wire terminal.
- Cap terminals (and cap contacts): metal inserts in the cap conduct the spark from the rotor tip to the spark plug wire. Cracks or carbon tracking cause misfires.
- Spark plug wire towers / wires: carry high-voltage from the cap to each spark plug. Wires often labeled per cylinder.
- Pickup coil (ignition pickup or magnetic sensor): senses rotor/reluctor position and sends a timing signal to the ignition module/PCM or directly triggers the coil. On electronic systems this replaces points.
- Vacuum advance (if equipped): a vacuum diaphragm on the distributor that advances timing under light-load/high-vacuum conditions. Improves cruise performance and economy.
- Mechanical (centrifugal) advance (older designs): weights/plates in the distributor that advance timing with RPM.
- Ignition module (if internal): some systems have an electronic module inside the distributor controlling the coil firing. If it fails, no spark or intermittent spark.
- Hold-down clamp/bolt: secures distributor in the block and is used to tweak timing after installation.
- Cap clips / screws: hold cap on.

How the system works (step-by-step)
1. Engine rotation gives crank and cam position. The camshaft (or oil pump) drives the distributor shaft.
2. The pickup (or cam position sensor) senses the rotor/reluctor position and tells the ignition module/ECU when to fire the coil.
3. The coil charges and discharges at the commanded moment, sending high voltage to the distributor cap center.
4. Rotor distributes that high voltage sequentially to the cap terminals; the spark jumps from cap terminal to rotor tip across a tiny air gap into the rotor, then through the cap terminal into the spark plug wire to the plug.
5. Advance systems (vacuum/centrifugal/electronic) change the timing based on RPM and load so combustion occurs at an optimal point as engine speed/load changes.

Common failure modes (what goes wrong and symptoms)
- Cracked cap or carbon tracking: causes misfires, rough idle, cross-firing, or poor starting. Visible cracks or black lines inside the cap.
- Worn rotor contact: weak or intermittent spark.
- Oil fouled distributor (bad O-ring): oil inside cap contaminates contacts, causes misfire or complete failure.
- Worn distributor gear or excessive play: timing erratic, noisy, possible mechanical damage.
- Failed pickup coil or ignition module: no spark or intermittent no-spark.
- Bad vacuum advance diaphragm: poor throttle response and low fuel economy.
- Bad or corroded cap terminals or wires: weak spark to specific cylinders → misfire.
- Incorrect installation/incorrect timing: poor running, pinging, bad performance.

Tools & supplies you’ll need (basic, TJ-friendly)
- Socket set, ratchet, extensions, swivel or distributor wrench
- Screwdrivers (flat and Phillips)
- Torque wrench (recommended)
- Timing light (for final timing check)
- Marker/paint pen / scribe and masking tape for labeling wires
- Spark plug wire puller or pliers (careful with boots)
- Clean rag, parts cleaner or brake cleaner
- Replacement parts: cap, rotor, O-ring, gasket (and possibly cap clips, pickup if faulty)
- Dielectric grease (small amount on plug boots)
- Owner repair manual or factory service manual for torque and timing specs (I’ll say approximate steps below; confirm specs for your exact TJ year/engine)

Safety first
- Work on a cool engine. Disconnect negative battery terminal when doing electrical work. Keep loose clothing/tools clear of belts/fans. Label everything before you remove it.

Step-by-step: remove, inspect, replace, and reinstall distributor (beginner-friendly)
Notes before starting: TJs came with different engines (2.5L I4 and 4.0L I6 most common). The general procedure below works for the 4.0L I6 and similar distributor systems — check your manual for exact torque and timing specs.

1) Preliminary labeling and prep
- Label each spark plug wire and cap tower in firing order (marker + masking tape). Do NOT rely on memory. Label both ends of each wire.
- Remove plastic engine covers that get in the way.
- Disconnect negative battery cable.

2) Set engine to #1 cylinder TDC compression stroke
- Remove the #1 spark plug (usually front-most on the I6). This lets you feel/compress-check to ensure you’re on compression stroke.
- Slowly turn the engine with a 3/8" wrench on the crank bolt until the timing mark on the harmonic balancer lines up with the TDC mark on the timing tab.
- Confirm compression stroke: with #1 plug out, place your thumb over the plug hole and feel for air pressure as you rotate to the mark (or observe valve/lifter movement). If you get no compression at the TDC mark, rotate 360° and you’ll get to TDC on compression stroke (cam-driven distributor rotates half crank speed).

3) Mark rotor and cap position
- With distributor cap on or off (remove cap if needed), mark the rotor position relative to the engine (make a reference line on rotor pointing to the block or to the nearest cap tower) so you know where rotor was pointing when the engine was at TDC.
- Mark the distributor housing relative to the block with scribes or paint so you can see how it sits before removal.

4) Remove cap & wires, then remove distributor
- Label and remove each spark plug wire from the cap.
- Remove cap screws/clips and lift the cap (set aside).
- Note rotor position again and mark rotor relative to housing.
- Disconnect any vacuum hose to vacuum advance and any electrical connector to pickup/ignition module (if external).
- Loosen but do not remove the hold-down clamp bolt — backing it up a few turns usually frees the distributor. For full removal, remove the bolt and clamp, then lift the distributor straight up and out. Note: the rotor will be engaged with the gear; you may need to wiggle and rotate slightly to clear.
- Pull the distributor out carefully so the drive gear disengages. Keep track of any shims or washers.

5) Inspect parts
- Cap: inspect for cracks, carbon tracks, corrosion on terminals.
- Rotor: inspect for worn or pitted contact.
- Pickup coil / module (if accessible): look for oil contamination or corrosion.
- Distributor gear: check teeth for wear, missing teeth, or metal shavings.
- O-ring: replace if hardened or damaged.
- Housing: clean inside with parts cleaner; avoid soaking electronics.

6) Transfer required parts to the new distributor (if replacing)
- If you bought a new distributor but need to transfer the pickup plate or other bits, do so carefully. In many replacements the rotor and cap are new.
- Replace the O-ring and any seals.

7) Reinstalling the distributor (key alignment method)
This is where beginners make mistakes. The rotor position when seated must correspond to #1 spark plug tower and the engine at #1 TDC compression stroke.
- Make sure engine still at #1 TDC compression.
- Rotate the rotor so it points to the #1 cap terminal position (or original mark you made).
- Insert the distributor straight down while rotating slightly if needed until the rotor lines up with the marks. The distributor gear will mesh and the rotor will end up pointing where you set it. Because the gear ratio and spline engagement can shift rotor slightly, you may have to carefully rotate the rotor prior to insertion so that when the gear meshes it is pointing to the right cap tower.
- If your new distributor has a guide notch, align that with the block index mark. If not, use the marks you made earlier.
- Push distributor down fully and re-install hold-down clamp and bolt but do not fully torque — leave slightly loose so you can adjust timing.

8) Reinstall cap and wires
- Install the cap with correct orientation, clip/screw it on.
- Reinstall spark plug wires to corresponding towers (use your labels). Apply a small amount of dielectric grease inside boots.

9) Final timing adjustment with timing light
- Reconnect battery negative.
- Start engine. Let it warm up to normal temp.
- Attach timing light to #1 wire, connect power/ground per light instructions.
- Aim timing light at timing marks. With the distributor hold-down slightly loose, rotate the distributor body slowly until the timing mark aligns to your factory spec (typical factory base timing is often around 6–10° BTDC on 4.0L; look up exact spec for your year/engine). On many injectors systems you’ll have to run at specified idle RPM and may need to disconnect vacuum advance to set initial timing — check factory procedure.
- When timing is correct, tighten the hold-down bolt to the specified torque (consult manual; a common range is 15–25 ft·lbs). Re-check timing after tightening (tightening can move the distributor slightly).

10) Re-check everything and test drive
- Make sure all wires are secure and vacuum hose connected.
- Verify idle, rev remotely to ensure advance works smoothly and no misfire.
- Test drive, monitor for stumbling, pinging, or unusual noises. Re-check timing if needed.

Testing/diagnostic tips
- Cap/rotor visibly cracked or carbon-tracked → replace cap and rotor.
- Intermittent no-spark: wiggle distributor plug/connector while cranking. If it changes, connector or pickup coil/module issue.
- No-spark: check for secondary continuity on wires/coil and pull codes (if you have scan tool). Check for spark by grounding a plug and cranking (safe method: use insulated tool and keep distance).
- Use a multimeter to check pickup coil continuity/Ohms (specs vary — open circuit = bad). If pickup/mode failure suspected, replace distributor or module.
- Oil in distributor usually means O-ring/gasket failed. Replace O-ring and clean/replace contaminated cap/rotor.
- Excessive gear wear usually requires replacing or repairing gear and possibly the shaft; ignore that and it will fail.

Maintenance & replacement guidance
- Replace cap and rotor as preventive maintenance every 30–60k miles or with ignition problems.
- Replace the O-ring if any sign of oil leaking into the distributor.
- Replace the distributor if pickup coil or internal ignition module fails (often not serviceable separately on some units).
- Keep spark plug wires and plugs in good condition; bad plugs/wires can masquerade as distributor failure.

Quick analogies to make it stick
- Distributor = railway switch tower: it sends the train (spark) down the right track (plug wire) at the right time. If the switch is misaligned, the train goes to the wrong place or crashes.
- Rotor = conductor’s baton: it points to which instrument (plug) gets the cue (spark) next.
- Cap = insulating hat with holes for each wire; cracked hat lets “lightning” jump where it shouldn’t.

Final cautions
- Always confirm timing specs and torque specs with your factory/service manual for your model year & engine.
- Clean environment: even a speck of oil or carbon can cause misfires.
- If you’re unsure about diagnosing pickup coil vs. module vs. coil problem, a reputable parts store can bench-test components, or replace cap/rotor/wires first as inexpensive preventive measures.

That’s the full picture: what the distributor is, how it works, what every part does, common failures, and a step-by-step beginner-friendly replacement and timing procedure. Follow the steps carefully, label wires, and verify timing with a timing light.
rteeqp73

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