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Old 07-06-2018, 01:36 PM   #15
Ilya
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Quote:
Originally Posted by Kingtal0n View Post
I used to think all factory ECU ground a pin for turning things on and off. It just seemed safer.

But one day I ran into a computer that was throwing 12v. so they can do either.

I don't recall which the factory ecu does but it isn't difficult to test with a $5 multi meter from Harbor freight. Also you can easily reverse the order by using another relay. Since most fuel pump high performance installs need larger 12v wires anyways, its fairly common to use the existing fuel pump 12V feed wire to simple trigger a relay near the pump that pulls on a much larger 12v wire to feed the actual pump.
The best way to tell is to not use a cheap multimeter but to look at a schematic.

Your ECU might function one way but the relay it switches might be a NC or NO (Normally Closed until energized, or Normally Open until energized) and your multimeter test will just confuse you.

Unless its a 5W lighbulb, its best to switch everything through a relay, especially devices that generate EMF such as starters, alternators, and Fuel Pumps.
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