Editing Openai/69540633-2650-8006-9f7f-3526b70a3503 (section)

=== Assistant: Got it. ===
Got it. This helps a lot 👍
Based on the tag in your photo, this is a Scag Turf Tiger II (STTII) with a 61" deck and Briggs & Stratton Vanguard 40 HP EFI engine.

Below is a clear breakdown of the ignition system operation, followed by where to get the correct wiring diagram and what to focus on diagnostically.

==== This is a true ECU-controlled ignition system, not a magneto-style mower ignition. ====

===== - No kill wire grounding a magneto =====
* No ignition module deciding timing
* ECU controls spark timing and coil dwell
* 12 V powered coils

===== - ECU (Engine Control Unit) =====
* Crankshaft Position Sensor (CKP)
* Two ignition coils (one per cylinder)
* Battery, fuses, relays
* Safety interlock inputs (via Scag harness)

==== ### ====
* A CKP sensor reads a reluctor wheel on the crankshaft
* Produces an AC waveform
* ECU uses this for: - Engine speed - Crank position - Spark timing reference

⚠️ If CKP is missing → NO SPARK, NO INJECTORS

===== Once ECU sees: =====
* Valid CKP signal
* Correct system voltage
* Safety inputs satisfied

It will:
* Calculate ignition timing
* Control coil dwell time
* Fire coils independently

===== Each ignition coil has: =====
* Battery power (Key ON / RUN)
* ECU-controlled ground (driver circuit)

Operation:
* ECU grounds the coil primary → current builds (dwell)
* ECU opens the ground → magnetic field collapses
* Spark occurs

This is exactly like an automotive COP system, just simpler.

===== Scag does NOT kill spark directly. =====

Instead, safety switches feed inputs that:
* Enable or disable ECU operation
* Allow or prevent fuel + spark

Common inputs:
* Seat switch
* PTO switch
* Brake switch
* Neutral switch

⚠️ A failed safety input can cause crank/no-start with no spark, even though power and grounds look good.

==== There are two diagrams involved: ====

===== This shows: =====
* ECU
* CKP
* Coils
* Injectors
* Sensors
* Power and grounds

📘 Source:
* Briggs & Stratton Vanguard EFI Service Manual
* Engine model is specific to the 40 HP EFI (not carb)

This diagram is engine-only and does not show Scag switches.

===== This shows: =====
* Key switch
* PTO switch
* Seat switch
* Brake switch
* ECU enable circuits
* Relays and fuses

📘 Source:
* Scag Turf Tiger II (STTII) Electrical Diagram
* Must match STTII-61V-40BV-EFI

Scag diagrams are model-specific and change with EFI vs carb.

==== Since this is EFI, don’t think like a mower — think ECM logic: ====
# Battery voltage under crank
# ECU powers & grounds
# CKP signal present
# Coil power feed
# ECU coil control (ground switching)
# Injector pulse (proves ECU is alive)
# Safety inputs not inhibiting ECU

If you want, I can:
* Walk you through no-spark diagnostics step-by-step
* Help interpret scope patterns on CKP or coil
* Tell you exactly where techs get burned on these systems

Just tell me what symptom you’re chasing (no spark, intermittent, dies hot, etc.).