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Basic Relay question?

EFI Wiring Fundamentals

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On a standard 4 pin relay. Why would the terminals 87 and 85 be bridged and the 85 terminal not earthed?

My electrical battery cut out suggests to wire it up exactly like that? (See attached photo)

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Hi Justin,

It looks like from that diagram that the switching circuit is being powered from the ignition circuit, hence the jumper loop from 87 to 85. The feed from 86 is most likely taken to ground through the switch.

A common relay, like that, has two, normally independent, circuits. The first is the coil that needs to be energised - have an electrical current passing through it. The second is the actual switching part. this is why you will have (at least) 4 terminals, two for the coil, two for the switched circuit.

With both circuits there is a terminal that has to receive power and in the illustration they are bridged instead of running two separate feeds.

You don't show the rest of the circuit, it is possible the coil is grounded through the #1 at the bottom, with the diode preventing 'back feeding' to ground through the other circuits connected to the ignition - basically energising the coil in reverse, which would then close the switch when it wasn't desired.

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