I don't get why Honda design the main relay this way...

Fuel pump relay coil power is from Pin2 (ST switch) and Pin6 (ECU 12V feed) which is controlled by IGN1 (Pin5). Why are there two sources to feed fuel pump coil?



When only IGN1 is on, fuel pump can be controlled by ECU A7,8.

When IGN1 and ST is both on, fuel pump can be controlled by ECU A7,8.

When only ST is on, fuel pump cannot be controlled because it gets power from IGN1.



And I believe when you crank (ST is on), the IGN1 is also on. It is impossible that when ST is on, IGN1 is not on.



Then what is the point of having IGN1 and ST control the fuel pump 12V feed? I don’t get it. Honda…

 

you dont think its to prime the pump for a set time?

 

Looking from the schematics, once IGN1 is on, fuel pump is controlled by Pin1 (ECU A7,8). Pin2 (ST switch) seems like play no role in controlling the fuel pump..... Did i miss something?

 

 

 

 

Thanks for the explanation. But looking at the relay schematics, how does the relay work as described?

"One relay supplies battery voltage to ECU, power the injector, and power the second relay" => I understand that. Pin 5 and Pin 3 power the coil and Battery Voltage flow from Pin7 to Pin6 which powers ECU and second relay's coil.

"The second relay is energized for 2s when ignition is switched on" => I guess that is controlled by ECU pulling down Pin1 for 2 seconds to allow battery voltage flow from Pin6 thru second relay's coil. That is understandable.

".... and when the engine is running which supplies power to the fuel pump" => When engine is running, ECU pulls down Pin1, current flow from Pin6 to Pin1 to activate the coil. Pin5 supplier power to fuel pump. That makes sense too.


So it is Pin1 (ECUA7,8) is controlling the fuel pump all the time.... Then what the hell is the function of Pin2 from crank signal???

 

Your assumption is wrong. Starter position and ign1 position on the ignition are not tied together, they're separate .

And the reason is so you can crank the starter longer than 2 seconds to get the engine started, if needed.