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Non technical language Hmmmm. Have to be a little technical, but will try.
The ECU is what is called a microcontroller. This is basically a small computer with dedicated electrical connections for measuring voltages or supplying voltages on command. The simplest case is being able to see if a sensor is at ground or battery voltage. More complicated will be like the temp sensor, where it must measure a range of voltage.
The computer code is a set of instructions which will monitor all the sensors and make decisions based on the sensor inputs. example: If the key is on and the crank sensor is generatig pulses, the code will then set a ground signl to allow the fuel pump relay to energize. The schematic does not tell you anything about the many input sensors that go into the decision making process.
I too am a fan of using what I call using the "lowest technology to get the job done". However computers and electronics are getting so reliable that they can outpace their mechanical counterparts. The bad news is that they can and do fail without notice. With a carburator, you at least had some warnig that things were getting out of hand. Also as you are discovering it takes more and more information and knowledge to understand how they work. You can still get a lot of good things done based on observations and information posted here. There is also a good bit of "cause and effect" troubleshooting information in the Bentley manual ('83 to '93 240). Not a bad investment for owners of other years and even the 740 owner.
Good luck with your adventure.
Dan
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