Adding a relay
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Elder User
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From: Ocala
Adding a relay
I am adding a fuse and relay to my distributor box but I have no clue how relays work. I think I may have it figured out but thought I'd ask you guys first. It's a four leg relay 30, 85, 86, 87. I am adding it for an aftermarket fuel pump. I want it to be on as soon as I turn the key. I know which leg is the fuse, out to pump, and ground. Now the ignition in where do I hook that one? Do I run a jumper from the fuse to that leg too? Any help appreciated.
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Joined: Jan 2001
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From: Virginia
#30 goes to the fuel pump fuse.
#87 goes to the fuel pump
#85 or #86 goes to ground
#85 or #86, whichever one you didn't use for the ground, goes to the ignition. This requires no fuse, since it will be fused by the ignition source.
That's the whole idea behind a relay. Another totally separate circuit is controlling a different circuit. In this case, the ignition circuit, is controlling the larger fuel pump circuit. The relay is the bridge between the two circuits, letting them work together, but keeping them electrically separate.
#87 goes to the fuel pump
#85 or #86 goes to ground
#85 or #86, whichever one you didn't use for the ground, goes to the ignition. This requires no fuse, since it will be fused by the ignition source.
That's the whole idea behind a relay. Another totally separate circuit is controlling a different circuit. In this case, the ignition circuit, is controlling the larger fuel pump circuit. The relay is the bridge between the two circuits, letting them work together, but keeping them electrically separate.
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Joined: Apr 2003
Posts: 673
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From: Maryland
#30 goes to the fuel pump fuse.
#87 goes to the fuel pump
#85 or #86 goes to ground
#85 or #86, whichever one you didn't use for the ground, goes to the ignition. This requires no fuse, since it will be fused by the ignition source.
That's the whole idea behind a relay. Another totally separate circuit is controlling a different circuit. In this case, the ignition circuit, is controlling the larger fuel pump circuit. The relay is the bridge between the two circuits, letting them work together, but keeping them electrically separate.
#87 goes to the fuel pump
#85 or #86 goes to ground
#85 or #86, whichever one you didn't use for the ground, goes to the ignition. This requires no fuse, since it will be fused by the ignition source.
That's the whole idea behind a relay. Another totally separate circuit is controlling a different circuit. In this case, the ignition circuit, is controlling the larger fuel pump circuit. The relay is the bridge between the two circuits, letting them work together, but keeping them electrically separate.
Dave, would you recommend a diode across the coil connections of the relay?
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Posts: 56,846
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From: Virginia
A diode would suppress the kick back voltage from the coil of the relay. This kickback is not a problem unless the coil is driven by a solid state board or computer of some sort, were the kickback would damage the circuit.
The coil is so small in these relays, the kick back voltage is not a high enough level to cause problems in regular 12 volt circuit. But in certain circumstances, the factory uses diodes such as across the A/C clutch coil, and that is usually driven by a relay and a low pressure switch. But the clutch coil is so large, the kick back is substantial, and can shorten the life of the contacts in the relay and the switch.
The coil is so small in these relays, the kick back voltage is not a high enough level to cause problems in regular 12 volt circuit. But in certain circumstances, the factory uses diodes such as across the A/C clutch coil, and that is usually driven by a relay and a low pressure switch. But the clutch coil is so large, the kick back is substantial, and can shorten the life of the contacts in the relay and the switch.