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Wiring Fundamentals: What Needs Power, and When

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What Needs Power, and When


00:00 - The power supply is typically the first section of the EFI harness design I consider.
00:05 While it would be possible to begin with any of the five sections, the power supply section is the logical place to start, as it has the largest overlap with the other sections and can influence their design.
00:16 The required fuses, relays, and splicing typically make it the most complex section and it's best to get its design locked down at the beginning.
00:24 The design of the power supply section requires us to determine which parts of the system require 12 volt power and when they should receive it.
00:31 We implement the design by having a number of relays connected to the battery.
00:35 The electromagnetic coils of those relays are then independently controlled to supply power to the parts of the system that currently need it.
00:43 To design the power supply section of our wiring harness, we first need to list any parts fitted to the vehicle that require a 12 volt power supply.
00:51 This will almost always include the ECU, ignition coils, fuel injectors, fuel pump and cooling fans.
00:58 But it may include other devices such as O2 sensor heaters, external loggers, or dash displays.
01:03 On occasion you will also come across some sensors that require a 12 volts power supply.
01:09 Such as an air flow meter of a wideband O2 sensor controller.
01:11 It's important to note that we don't include in this section any sensors which are solely powered by regulated supply from the ECU.
01:19 These are considered separately in the sensors section of the design process.
01:23 For each item in the EFI system we've identified as needing power, we now need to determine when that power is actually required and what controls when it is supplied.
01:33 This step tends to naturally group the powered parts of the system into sections which then determines the number of individual relays that we need.
01:41 A common setup will have two driver controlled power stages.
01:43 The first provides power to the ECU, dash display, loggers, and any other parts we might want to communicate with or reprogram while the engine is not running.
01:53 This is commonly referred to as main power.
01:56 A second stage is used to supply power to the actuators required for the engine to actually run.
02:02 Typically ignition coils and fuel injectors.
02:04 This is commonly referred to as enable power.
02:08 Splitting the driver controlled power supply like this allows for the engine to be stopped but the ECU and displays to remain active for download and examining logs or keeping an eye on heat soak for example.
02:19 Other parts of our EFI system such as the fuel pump and radiator cooling fans, will require their power supply to be controlled by the ECU.
02:27 For each part of the EFI system that requires its power supply to be switched on and off independent of other parts of the system, we'd need a separate relay.
02:36 As an example a fairly common installation will have four power supply relays.
02:40 A main power relay, which supplies power to the ECU and dash displays, and an enable power relay which supplies power to the fuel injectors and ignition coils.
02:50 These are both controlled by the driver, either from the existing key barrel switch in the car or a switch panel which has been fitted to the interior.
02:57 The other two relays are controlled by the ECU.
03:00 One supplying power to the fuel pump, and the other supply power to a radiator cooling fan.

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