Hi Jimmy,

Here are some things I've put together that are described but a bit scattered around in the Bentley manual. Maybe a lot to swallow at once, but chew it over and save for reference.

Some basics:
Crank Position Sensor/Sender (usually CPS on BrickBoard)*
aka Impulse Sender (Bentley)
aka RPM Sender (vernacular)
* Only on '89 up. Timing pulse comes from distributor on pre-'89

ECU, as commonly used here means the FI system Electronic Control Unit ICU refers to the Ignition Control Unit. Both "CUs" and their external components are powered from the 25A blade fuse from '83 thru '90. After that, fuse #6 is the main fuse. From '86 thru '90 fuse #6 is not used at all on U.S. 240's.

The Fuel Pump Relay (FPR) is actually two (2) relays in a single case:
• Main/System relay clicks ON when Key is turned ON (Power to ECU, etc.)
• Fuel relay clicks ON at the same time, then clicks OFF after a second.
• Fuel relay clicks on again during cranking and running.

Engine Start-Run Sequence:
1) During starter cranking, the CPS sends timing pulses to the Ignition Control Unit (ICU)

2-a) The ICU propagates these timing pulses to the Power Stage (aka Ignition Amplifier/Module) to initiate spark at the coil.

2-b) At the same time, the ICU also propagates these pulses to the FI ECU, allowing FI operation (no ICU pulses means no Fuel).

3-a) The System relay (previously energized at Key On) sends +12V to the AMM, ECU, Injectors, and the Fuel relay coil + terminal.

3-b) When ICU pulses are received, the FI ECU "energizes" the Fuel relay (completes the coil ground side) to run the fuel pumps.

As long as this sequence works, the engine runs until the Ignition is switched off, which in turn shuts down the FI system.

No-Start Analysis:
Determining whether or not there is spark at the plugs (2a), and whether or not the plugs are getting gas (3c), will allow some analysis of this sequence.

For example, if spark fails due to a bad Power Stage/Amplifier (2a), there will be a no-start, but with gas-wet plugs, because the FI ECU didget the needed timing pulses (2b). On the other hand, if 2b fails (practically never) the symptoms will be Fuel-related i.e., a no-start with spark at the plugs, but plugs remain dry.

• Jumpering the LEFT side contacts of fuses 4 and 6 will bypass both the 25AMP fuse and the Fuel relay—and will power up both pumps as soon as the key is on, for diagnostic start/run testing.
--
Bruce Young, '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.