Editing Openai/69540633-2650-8006-9f7f-3526b70a3503


{{DISPLAYTITLE:Ignition System Overview STTII}}

''This conversation was exported from ChatGPT''<ref>{{cite web|title=Ignition System Overview STTII|url=https://chatgpt.com/share/69540633-2650-8006-9f7f-3526b70a3503|publisher=ChatGPT|access-date=2025-12-30}}</ref>

== gpt-5-2 ==
=== User: Need info on ignition system operation on this model. ===
Need info on ignition system operation on this model. Also a wiring diagram

=== 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.).

=== User: How many cylinders is this? ===
How many cylinders is this?

=== Assistant: That Vanguard 40 HP EFI on your Scag is a 2-cylinder engine. ===
That Vanguard 40 HP EFI on your Scag is a 2-cylinder engine.

More specifically:
* Configuration: V-Twin
* Cylinders: 2
* Each cylinder has its own ignition coil
* ECU fires the coils independently based on the crank signal

So from an ignition/diagnostics standpoint, think two separate coil drivers, not a wasted-spark or single-coil setup.

If you want, next we can:
* Identify which coil is which cylinder
* Go over firing order / crank reference
* Or talk about what a normal primary waveform looks like on this engine

=== User: How many wires are on each ignition ignition coil? ===
How many wires are on each ignition ignition coil?

=== Assistant: Each ignition coil on the Briggs & Stratton Vanguard 40 HP EFI V-Twin has 2 wires. ===
Each ignition coil on the Briggs & Stratton Vanguard 40 HP EFI (V-Twin) has 2 wires.

===== - 1 wire = switched 12 V power - Hot with key ON / RUN - Usually shared between both coils from the same feed =====
* 1 wire = ECU-controlled ground (coil driver) - ECU switches this wire on/off to create spark - This is not a permanent ground

===== - No kill wire =====
* No magneto-style ground shutdown
* No high-voltage control wire

===== - If you have 12 V at the coil but no spark, you’re chasing: - ECU not pulling the control wire to ground - Missing crank signal - ECU inhibited by a safety input - ECU power/ground issue =====
* If one cylinder has spark and the other doesn’t: - Swap coils side to side - Check ECU driver for that cylinder - Scope the control wire (should see a square-ish pull-to-ground event)

If you want, tell me:
* Are you missing spark on one cylinder or both?
* Crank/no-start or runs then dies?

I can walk you straight through the fastest test path.

=== User: I don't need all this other stuff right now. ===
I don't need all this other stuff right now. Stay focused on my questions

=== Assistant: Understood 👍 ===
Understood 👍

Each ignition coil has 2 wires.

=== User: ECU controls spark directly or is there an external ignition module? ===
ECU controls spark directly or is there an external ignition module?

=== Assistant: The ECU controls spark directly. ===
The ECU controls spark directly.

There is no external ignition module on this system.
The ECU:
* Receives the crank signal
* Calculates timing
* Directly drives each ignition coil (grounds the coil to fire it)

So the ignition path is:

Crank sensor → ECU → ignition coils

Nothing in between.

=== User: Seat switch will cut spark? ===
Seat switch will cut spark?

=== Assistant: Yes — indirectly. ===
Yes — indirectly.

The seat switch does NOT cut spark directly, but when it opens, it causes the ECU to shut off spark (and fuel).

So functionally:
* Seat switch open → ECU inhibited → no spark
* No magneto kill wire, no coil grounding by the switch

In short: seat switch can result in loss of spark, but only through the ECU logic, not by grounding the ignition system.

=== User: I knew that. ===
I knew that. Didn't need to other info

=== Assistant: 👍 Got it. ===
👍 Got it.

Answer: Yes — the seat switch will result in no spark.

=== User: Looking at inputs involved. ===
Looking at inputs involved. What other inputs will cut spark

=== Assistant: Understood. ===
Understood. Staying tight and focused.

Inputs that will cut spark (via ECU):
* Seat switch
* PTO (blade engage) switch
* Brake / parking brake switch
* Key switch (RUN/START feed to ECU)
* Crankshaft position sensor (loss of signal)

That’s the complete list of inputs that can inhibit spark on this system.

=== User: Picture of the PTO lever ===
Picture of the PTO lever

=== Assistant: !https://i\.ebayimg\.com/images/g/K64AAOSwEAthXexF/s\-l400\.jpghttps://i.ebayimg.com/images/g/K64AAOSwEAthXexF/s-l400.j… ===
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5
That’s the PTO (blade engagement) lever used on the Scag Turf Tiger series.

=== User: How is it engaged? ===
How is it engaged? Electronic or with a manual lever?

=== Assistant: Manual lever. ===
Manual lever.

On the Scag Turf Tiger II:
* The PTO is engaged by a mechanical hand lever
* The lever actuates an electric PTO clutch via a switch
* So: manual lever → electrical switch → PTO clutch

There is no electronic (ECU-controlled) PTO engagement.

=== User: Does the seat and pto switch input also prevent the starter from cranking? ===
Does the seat and pto switch input also prevent the starter from cranking?

=== Assistant: Yes — both do, depending on state. ===
Yes — both do, depending on state.
* Seat switch: Can prevent cranking and will cut spark if the ECU logic isn’t satisfied.
* PTO switch: Prevents cranking if the PTO is engaged and will cut spark if engaged during operation.

So both switches are involved in starter inhibit and spark enable, through the interlock logic.

=== User: So if it cranks over that pretty much eliminates the need to check those two inputs ===
So if it cranks over that pretty much eliminates the need to check those two inputs

=== Assistant: Yes — for cranking, they’re eliminated. ===
Yes — for cranking, they’re eliminated.

If the engine cranks, that means:
* Seat switch logic is satisfied
* PTO switch is in the disengaged state

So you do not need to check those two inputs for a no-crank issue.

⚠️ One caveat (keeping it tight):
They can still affect spark after cranking begins if a switch drops out, but they’re not the reason it cranks.

Bottom line:
Cranks = seat & PTO inputs OK for start.

=== User: Is ignition coil power provided with key on power or does the ecu need a good crank signal to power up the coils? ===
Is ignition coil power provided with key on power or does the ecu need a good crank signal to power up the coils?

=== Assistant: Ignition coil power is present with key ON. ===
Ignition coil power is present with key ON.
* The coils receive switched 12 V from the key/relay as soon as the key is in RUN/START.
* The ECU does NOT supply power to the coils.
* The ECU only controls the ground (driver) side of each coil.

So:
* No crank signal needed for coil power
* A crank signal is required only for the ECU to fire the coils, not to power them

Key ON = coil has power
Crank signal = coil gets triggered

== References ==
<references />


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