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Why so many relays?!

EFI Wiring Fundamentals

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Looking into starting from scratch and rewiring a Subaru track car. With only minimal electrics.

Looking at other "pre wired" option it seems they use a lot of relays.

Am I missing a concept, or is it ok to have the main ignition relay power a bus bar that feeds the ignition, injections, ecu etc? I don't see the reason to have seperate ones.

My initial plan was to go from this bus through to the coils, injectors etc.

and only using seperate relays for the fuel pumps and fans. With circuit breakers.

Any advise of what I'm about to do wrong would be great!



My Suggestion,


This has a higher initial investment, but over a longer term I have found that they pay for themselves in reliability and functionality compared to fuses/relays/circuit breakers.

Unfortunately doesn't fit in the budget. Have a huge pile of new circuit breakers and relays etc.

just trying to make the simplest system possible.

I've built two power distribution units from scratch. One for a BMW E30 M3 about 10 years ago and one for my current 1956 Ford Truck. Both times I've had to make the decision as to what items to power from the main relay, and what to power from its own dedicated relay. You just have to use you own common sense really. There is no right or wrong as long as you don't draw too much current through any particular relay. I suppose one could argue that adding more relays introduces more potential failure points. They make solid state relays now of the same size as the old classic Bosch mechanical relays and the SS relays should be very reliable. The PDU I built for the truck was meant to be sort of a "wiring showcase" since the truck is part performance machine and part hot rod (art), but in the end it turned out too messy and I've since covered it up with a nice aircraft style aluminum cover.

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