1) Theory — what an ignition coil does
- Purpose: an ignition coil is a transformer that converts the low-voltage battery (primary) into the high-voltage pulse (secondary) needed to jump the spark plug gap and ignite the air/fuel mix.
- Construction: primary winding (low turns, heavy wire), secondary winding (many turns, fine wire), iron core. Switching of the primary current by the ECU/ignition module collapses the magnetic field and induces the high-voltage pulse in the secondary.
- Control & timing: the ECU/ignition module times the primary current off according to crank and camshaft position inputs so spark occurs at the correct crank angle. Modern vehicles use coil‑on‑plug or coil packs with individual drivers and feedback to the ECU.
- Failure modes: open or shorted windings, internal shorting causing weak/high-resistance secondary output, intermittent connections or broken secondary towers, thermal/age degradation, or cracked housings causing arcing. Electronic driver faults can also prevent primary switching. Failures produce weak/no spark, misfire, or intermittent misfire.

2) How a bad coil affects the whole car and the gearbox
- Engine torque disturbance: misfires reduce or cut torque on one or more cylinders; that creates uneven engine torque output and RPM fluctuations.
- ECU responses: the engine ECU detects misfires and may reduce fueling/timing or go into limp mode; it also sets codes (P030x for misfire, coil codes, etc.).
- Transmission interaction: automatic transmissions rely on steady torque and engine speed for timing shifts and clutch application. Torque dips and RPM aberrations can cause harsh shifts, delayed shifts, flare or slip, or force the transmission control module (TCM) into default shift strategy. Some vehicles reduce torque via limp-home modes that change shift behavior. Fixing coils restores stable torque and corrects many transmission shift complaints that were actually caused by engine misfire.

3) Diagnostics (theory + ordered checks)
1. Read and record trouble codes (OBD-II): identify misfire cylinders (P0301–P0304, etc.) and coil or ignition codes.
2. Live data: observe RPM, misfire counters, fuel trim, and any correlation between load/RPM and misfire.
3. Swap test (theory): swap suspect coil with another cylinder; if misfire follows the coil, coil is likely bad. This isolates coils vs spark plug or injector issues.
4. Resistance test: measure primary and secondary DC resistance versus factory spec (primary typically low ohms, secondary thousands of ohms); out-of-spec indicates failure. Note: some modern coils have electronics so resistance alone can be inconclusive.
5. Oscilloscope: view primary waveform and secondary spark pattern under load for correct dwell, current ramp, and spark energy.
6. Spark check: under safe controlled conditions, verify presence and quality of spark with a proper spark tester and correct gap.
7. Inspect wiring/connectors and ECU driver outputs if coil shows intermittent fault but bench tests normal.

4) Preparation and safety (ordered)
1. Park on level ground, engine cold. Remove ignition key.
2. Disconnect negative battery terminal to prevent inadvertent cranking and protect electronics. Wait 1–2 minutes for capacitors to discharge.
3. Gather tools: appropriate sockets, torque wrench, dielectric grease, replacement coils (OEM or equivalent), new spark plugs if wear is present, compressed air or blower, small pick for connectors.
4. Clean area around coil towers to avoid debris dropping into spark plug wells.

5) Removal and replacement — ordered step-by-step
1. Remove any engine covers or intake components blocking coil access.
2. For each coil to be replaced, do them one at a time to avoid mixing connectors:
a. Label or note the cylinder number.
b. Disconnect the coil electrical connector: depress clip and pull straight out. Check connector for corrosion/damage.
c. Remove the coil retaining bolt(s) using the correct socket. Keep bolts separate.
d. Pull the coil straight up with a slight twisting motion; if stuck, apply penetrating safe force—don’t pry on adjacent components. Inspect coil boot and terminal.
e. Inspect the spark plug in the well. If spark plug is fouled, damaged, or beyond service limit, replace it. If reusing, clean the well and verify gap and torque spec.
3. Fit the new coil: apply a very small amount of dielectric grease to the inside of the coil boot (not on the terminal) to prevent moisture and aid seating. Insert coil straight down onto plug until fully seated.
4. Reinstall the retaining bolt(s) and torque to the manufacturer’s spec (if unknown, typical small coil bolts ~8–12 Nm / 7–11 ft‑lb; use factory spec where available).
5. Reconnect electrical connector until it clicks. Ensure it’s fully seated and that locking tab engages.
6. Repeat for each coil that will be replaced. Do one cylinder at a time if swapping/diagnosing.

6) Reassembly and commissioning
1. Reinstall any engine covers or intake components removed.
2. Reconnect negative battery terminal.
3. Clear diagnostic trouble codes with a scan tool (or run the engine and clear codes via OBD tool) so you can see if codes recur.
4. Start engine and observe idle, listen for misfires, and check for warning lights. Use live data to confirm misfire counters are zero and fuel trims stabilize.
5. Test drive under normal conditions and observe shift quality. Verify any previous harsh/delayed shifts have been resolved.

7) How this repair fixes the fault — the theory connection
- Restores spark energy: replacing a weak/open coil restores correct high-voltage output so spark plugs ignite the charge each cycle reliably.
- Eliminates misfires: consistent ignition prevents dropped combustion events, stabilizing cylinder contribution and engine RPM.
- Stabilizes torque: with each cylinder firing properly, engine torque becomes smooth and predictable; the TCM receives stable inputs and can execute normal shift schedules.
- Removes cascaded faults: clearing misfires prevents negative effects on catalytic converter, prevents damage to downstream sensors, and removes ECU/TCM limp strategies.
- If connector or wiring were at fault, replacing coil and reseating connectors removes intermittent connections that caused transient misfire and associated driveability/transmission symptoms.

8) Final notes (concise)
- Always use the correct coils and spark plugs for the engine; match OEM specs.
- If misfire persists after coils and plugs are confirmed good, continue systematic diagnosis: fuel injectors, compression, cam/crank sensor accuracy, wiring, or ECU driver faults.
- Torque values, resistance specs and plug gap vary by engine—use the factory service manual for exact figures.

End.
rteeqp73

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