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I'm going to install a starter assist relay for my car to boost the voltage of the ignition switch signal wire (not the motor). I need 4 pins to wire a relay for the job: 85, 86, 30 & 87. Most of the automobile relays I have found on the market are 5-pin relays. What happens if I wire the 4 pins but leave the 5th pin (usually 87A) un-wired? Is it dangerous to use a relay like that? Should I cover the 5th pin both in the relay and in the socket? Do you think a 25A relay is enough for the job?

This is the diagram: enter image description here

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  • \$\begingroup\$ We might be able to answer your question if you provide a schematic, the expected load current, and a link to the relay you're considering (ideally a datasheet). \$\endgroup\$ Oct 29, 2022 at 21:03
  • \$\begingroup\$ @Jonathan S. Thanks for your comment. I'm not 100% sure about the load current but it is often said that it doesn't exceed 10 amps. Some say it may go up to 30 amps in some occasions. I edited my question and added the diagram. \$\endgroup\$ Oct 29, 2022 at 21:46

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The 5th pin ("87A") on automotive relays is the normally-closed contact. "87" is the normally-open one, and you're correct in that that's the one you have to use. "87" gets connected to "30" once the relay is energized, while "87A" gets disconnected from "30" when that happens. (It's basically an electrically-actuated toggle switch.)

You can just ignore the "87A" pin and not connect it anywhere. Just because your relay has two contacts doesn't mean you have to use both.

It's also totally fine if you plug a 5-pin relay into a 5-pin socket but leave one of the pins unconnected at the socket. Just isolate it so that nothing can touch the bare metal.

Thanks for the diagram! All of the other wiring seems to be correct, too. The fuse is also in the correct place.

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  • \$\begingroup\$ Thanks a million for your answer. By isolating, do you mean isolating the socket wire that extends from 87A pin? Is it enough to wrap insulation tape around 87A wire? \$\endgroup\$ Oct 29, 2022 at 22:03
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    \$\begingroup\$ Anything that prevents other things from making electrical contact with the 87A pin/wire will work. Heatshrink, tape, whatever. Just make sure that whatever you use to insulate it won't come off over time or with heat. You could also physically remove the pin from the socket if that's possible. \$\endgroup\$ Oct 29, 2022 at 22:05

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