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Fuel Pump and EFI Relay Output on Modular ECUs

April 08, 2018

 

 

Modular ECUs have the ability to drive an EFI relay, and if your application doesn’t require ECU control of an EFI relay, you can use this output as a fuel pump relay control.

This output can only switch to ground, it can’t source current (that is, drive an output high). So if it’s going to be used to drive a relay coil, the other side of the relay coil needs to be connected to a 12V source.

Let’s discuss the most basic system first of all, which is where we have an ignition switch which powers up everything via a relay, and we’ll use the ECU output to drive the fuel pump relay. The power supply to the ECU must be shared with the injectors. This is because the injector drivers are switching mode, which means that they need to turn the injector off and on very quickly to control the current on a low impedance injector with no dropping resistor. The injector stays open during this time.

In this case, we connect power into the ECU on pin 7 of J2.

 

We do not connect the ignition switch input pin on the ECU. The EFI / fuel pump relay output triggers the negative of the fuel pump relay. The setting in the ECU must be set to be fuel pump relay, and you can find it under Functions -> EFI / Fuel pump relay output. If you want to disable the fuel pump relay, then you can set it to “none”.

 

To enable fuel pump set EFI relay mode to "Fuel Pump"

To disable set EFI relay mode to "none"

And if you want it to run permanently, for example to drain a tank, you can set the output to “EFI relay” and it will stay on as long as the ECU has power on pin 7.

 

To enable set EFI relay mode to "EFI Relay" 

The fuel pump relay’s function is that it primes for a fixed amount of time after the ignition key is first turned on. This duration is configured in the Functions -> Fuel Pump relay section in the software. Then the fuel pump output runs for as long as the engine is running, and if the engine stops then the fuel pump shuts off.

 

 

On some cars, the fuel pump is controlled not by a relay but by a separate fuel pump control module. Often on these, the input to the control module needs to go up to 5V to enable the fuel pump, rather than pulling low as a normal fuel pump relay output will do.

In this case, the output function should be configured as “inverted fuel pump”, and use an external pull-up resistor on that output to 12V or 5V to generate the high voltage.

 

 

If you want to use the output to control an EFI relay instead, then connect the ignition switch into pin 17. Pin 16 becomes the EFI relay coil negative. The EFI relay provides switched 12V power then to the injectors, ignition coils and also back to pin 7 on the ECU, which is the main power input. The output must also be configured as “EFI relay”

 

 

Thank you and happy learning.


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