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One prong is main power, usually designated 30. Power from the battery goes here. Another prong is 85. Either 85 or 86 is the trigger; I get these mixed up. The other one is the ground. 87 is the power feed out to the new pump. The new pump will also have a ground wire that goes to ground. A good ground. Use your existing pump wiring to trigger the new relay and to ground the new relay. You'll have to look at another relay under the hood to figure out 85 and 86 using a test light.
87a in the middle is unused for your application. You use it only when using the relay as a dual pole relay. Dual pole means that one thing is fed and then when the relay is triggered that thing is no longer fed and a new thing is fed. For example, 87a would supply power full time to your stock fuel pump and then when a Hobbs switch sensed 20 psi of boost pressure, it would trigger the relay trigger terminal which would then switch off the first pump and switch on the new TRex. Or some use like that.
Use heavy gauge wiring from the battery to the relay and the relay to the new pump. Like 10 or 12 gauge. Use equally heavy gauge wiring for the ground from the pump. The gauge of the rest of the wiring is not critical. Use good terminals, either M crimp or soldered with heat shrink. Marine wire from a boat store is better than auto wire from the parts shops. More and finer strands.
Philip Bradley
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