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Start-Run Sequence
1) During starter cranking, the Crank Position Sensor sends timing pulses to Ignition Control Unit (ICU)
2-a) The ICU uses these CPS pulses to trigger the Power Stage (aka Ignition Amplifier), which triggers spark from the coil.
2-b) At the same time, The ICU also propagates the pulses to the FI ECU, to allow FI operation.
==>(no ICU pulses means no FI operation)<==
3-a) The Fuel Injection (System)* relay (previously energized at Key On) powers the AMM, IAC, ECU, Injectors, RSR** (Radio noise Suppression Relay), and the Fuel (pump) relay coil + side.
* The System relay is in the white case with the Fuel relay.
** RSR powers Injectors AND Coil primary (Blue wire) on turbo engines
3-b) When ICU pulses are received by the FI ECU, it "energizes" the Fuel relay by grounding the relay coil (– side) to run the fuel pumps.
When all these things work, the engine runs until the Ignition is switched off, which in turn shuts down the FI system.
Determining whether or not there is spark at the plugs (2a) — and whether or not the plugs are getting gas (3b) — makes a good no-start "starting" point.
For example, if 2a fails due to a bad Power Stage/Amplifier, there will be a no-start with gas-wet plugs. If 2b fails (practically never) symptoms will be Fuel-related: a no-start with spark at the plugs, but plugs remain dry.
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Bruce Young, '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.
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