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Powering relay coil with Igniton power over Main EFI relay


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Looks like it is common practice to have a main EFI relay triggered by an ignition source power and have this relay supply power to the ECU and also supply power to the relay coils for other system such as igniton coils or fuel pump. Is it necessary for all other relays to be switched on my the main EFI relay? What is the difference between using the main EFI relay over the ignition key power?

I am overhauling my whole electric system and is building a separate fuse box for the EFI system. I am using the bussmann 15303-2 fuse box where pin 86 on the relays are bussed together so in order to have a "main EFI relay" in the normal fashion, I will need to wire in another relay outside the fuse box in order to seperately supply a ignition signal to the main EFI relay and have the main EFI relay  power the other relay coils through the bussed connection. I don't want to do this as I would like everything packaged neatly inside the fuse box.

If the main EFI relay would be on and powering up the other relays when the ignition is switched on anyway (or at least providing +ve power to the coils so that they are powered up as soon as they are grounded) would it make any difference if I simply have the coils for all relays connected directly to the igniton power source? What is the reason for all other relays activated by the main EFI relay rather than the igniton key? I am deciding whether I should have the EFI relay also plugged into the fuse box with the bussed connection and power the bus with an ignition power source to switch on all relays. 

Thank you!

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One of the reasons is not so much the main input feed, but the hot side of the trigger coil on a relay is typically fed by bridging from the main input. See this diagram from the link help files on aux output backfeeding


Also, you dont typically want all 80-100amps of electrical load going through your ignition switch, so the hot side of most relays are fed from the battery directly. 

Put these 2 together, and it means you have permanent battery voltage to the hot side of both the switch part and the coil part of all of your relays. This means the coils will all be passing a limited amount of current through to the ECU even with the key off. Depending on a few things this may cause anything from minor "things dont turn off with the key", to more major "lots of current flows backwards through the ECU".

A big purpose of a main relay is to detach the hot side of the coils from battery voltage when the key turns off.  To this end, you could achieve the same thing by running the switch/bus side of the relays to the battery, but running the hot sides of the coils via the key (or the output of a relay further up the chain).

You would want to check/calculate the current you expect to draw through the ignition switch though as well as that being drawn through the "top" relay if you chain a few together. There is a relationship between how much current the coil side of a relay will draw, and how much current the switch side will support. (see OEM designs for starter wiring - there is usually a starter relay in the wiring harness even though there is another relay built into the starter, basically the wriring harness relay is there to "step up" the amount of current available to drive the coil of the relay in the starter motor). To run the typical ECU/injectors/coils the usual 40amp relays are fine, but on a complete rewire you may be dealing with lights, large fuel pump, or potentially the starter relay which draw a lot if you put them all off a single wire/fuse/relay.

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Yep, I know about the back feeding which would keep the ECU from powering down that is why the relay coils would be powered by the ignition key. Do note it is just the coil only so it will not be putting much load on the ignition key. I am mainly rewiring the engine/ECU harness only so there are only a few relays to be powered by the ignition key. A quick sketch below should explain things better.

Basically, rather than supply power to the relay coils through the EFI relay, it gets its power from the ignition switch instead. The same way the EFI relay gets its power (I didn't draw any fuses in the quick sketch but they will be there).


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