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Wiring Fundamentals: Further Considerations

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Further Considerations


00:00 - Depending on the specification of the EFI system you're building the wiring harness for, you may need to incorporate a feature known as ECU hold power.
00:08 This feature allows for the driver to control when the ECU initially receives power but lets the ECU determine when to remove its own power supply and turn off.
00:18 This feature is used in setups where the ECU may need some time to perform functions before it turns off such as parking an idle control valve in a known position.
00:27 If this function is required your ECU documentation will have specific instructions on how it should be wired.
00:33 An important issue to be aware of when designing the power supply scheme is that of ECU back feeding.
00:38 Because of the way ECUs are constructed internally it's sometimes possible for them to connect their output channels to ground when the power supply to the ECU is removed.
00:48 This can cause problems for actuators connected on one side to 12 volt power and an ECU output channel on the other.
00:55 If the power supply to the ECU is then removed, and the power supply to the actuator is not, the ECU may now unintentionally provide a ground for the actuator, turning it on.
01:05 Particularly in the case of fuel injectors or ignition coils, this may be a dangerous or damaging situation.
01:11 Luckily the solution to this problem is an easy one and we overcome the issue by having the same relay that supplies power to our ECU also supply power to the low current switching coils of the other relays in the system.
01:24 This way, if the relay providing power to the ECU is switched off, it's guaranteed that the relays providing power to the system actuators are also switched off.

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