The Start Request
Pressing the start button is a request, not a command. The car decides whether to grant it, and then the engine has to prove where it is before fuel and spark are committed.
In plain language
When you press the start button, the car checks a list before it does anything: is the key authorised, is the brake or clutch pressed, is the transmission in a position that allows starting, is there enough battery, and are the modules that need to agree actually talking to each other? Any one of those can stop the car from cranking, and none of them means the starter is broken.
If the checks pass, the starter turns the engine. Even then the engine does not start immediately: the engine-management module has to work out exactly where the crankshaft and camshafts are before it will inject fuel and fire the coils. Getting the engine turning and getting the engine running are two separate problems, which is why "it cranks but won't start" is a completely different diagnosis from "nothing happens".
- Asleep
Selected security, locking, alarm, receiver and memory circuits stay powered while everything else is shut down.
- Wake-up
A key or door event wakes the access/body system and the communication networks.
- Authorisation
The immobiliser/access system decides whether this key is authorised.
- Network handshake
Modules exchange status over CAN; local devices may report over LIN.
- Power distribution
Relays and electronic switches energise the circuits the vehicle now needs.
- Start request
The request is checked against brake, clutch, selector, voltage, network and security conditions.
- Cranking
The starter rotates the engine.
- Synchronisation
The DME must see valid crankshaft and camshaft information before it will commit to fuel and spark.
- Fuel, spark, air
The DME controls injectors, ignition coils, throttle, fuel pressure and valve timing as equipped.
- Running
Combustion sustains itself and the DME moves to closed-loop control.
- Charging
The alternator supplies the electrical loads and recharges the battery.
What the customer sees
Nothing at all when the button is pressed; a single click; cranking that never catches; or a start that only works sometimes, in a particular gear, or after several attempts.
What the module is doing
Collecting the conditions it needs — security authorisation, brake or clutch, selector position, supply condition, network status — and only then commanding the starter. During cranking the engine-management module is reading crankshaft and camshaft signals and trying to establish synchronisation before it commits to fuel and ignition.
What a technician tests
Whether the request was granted at all. If it was not, which condition was missing. If it was, whether the starter was commanded, whether it drew what it should, and how the supply behaved while it was cranking. Then, separately, whether the engine-management module saw valid crankshaft and camshaft information while the engine was turning.
The technical detail
The request passes through the access/start logic, not directly to the starter. The button is an input. The module that owns start logic evaluates the conditions and issues the command; the starter circuit itself is switched by a relay or driver under that command.
The interlock conditions exist for safety and are genuine failure points. A brake or clutch switch, a selector position sensor, or the message carrying that state across the network can each fail while everything they protect is healthy. When they do, the car refuses to crank and is entirely correct to do so.
Cranking is an electrical stress test. It is the largest current draw the vehicle sees. Supply collapse during cranking will make modules record undervoltage and communication faults at exactly the moment the car most needs them working, which is why battery condition and voltage drop under cranking load are tested before anything else on a no-start.
Synchronisation is the gate to fuel and spark. The engine-management module derives engine position and speed from the crankshaft sensor and confirms which stroke the engine is on from the camshaft sensor. Without valid, plausible information from both, it will crank indefinitely without injecting or firing. A strong, steady crank speed proves the starter and the battery; it proves nothing at all about fuel or ignition.
Timing is part of synchronisation. If the mechanical relationship between crankshaft and camshaft has moved, both sensors can be working perfectly while the module correctly refuses to run the engine, because the position information it derives no longer agrees.
Why this matters when something goes wrong
Splitting the complaint at the crank is the single most useful step. "Does not crank" sends you to authorisation, interlocks, supply and the starter circuit. "Cranks and does not start" sends you to synchronisation, fuel, ignition and mechanical condition, in that order.
Both halves reward measurement over code-reading. A no-crank complaint with a list of communication faults is usually a supply problem that produced those faults. A crank-no-start with no fault codes at all is common, because a module that has never achieved synchronisation often has nothing to report beyond the absence it is waiting on.