Quote:
Originally Posted by Joe KCMO
Old Henry,
I understand how a relay works. My question is that with a + ground relay both the primary circuit and secondary circuit can be fed from the same source. Then when you ground the primary circuit with a + ground the switch closes and supplies the negative full power to the load. Meanwhile the full load would not flow thru the horn grounding wire, although still connected.
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The horn relay works great for what it is designed for. It is exactly the same kind of relay as the starter solenoid - power coming into the relay from a constant source then switched to the load through the secondary internal switch by grounding the primary circuit. If that's what you want that works great. That is the very simplest of relays and is used for the horn and starter so that the huge load to the horn and starter doesn't all have to pass through the horn switch and starter switch. A single small wire can be run from the relay to a ground switch to activate the relay without doubling the wires to and from the switch. Such a simple relay might also be used for a fuel pump that is powered by the secondary circuit of such a relay with the ground wire of the primary going to a grounded toggle or intermittent normally open switch under the dash. But, power to the fuel pump could not be controled by the ignition switch with that kind of relay.
The kinds of relays most want in their cars is the one that is activated by turning on the ignition which then activates the relay to connect the constant source of power from the battery to the load without putting the full load through the ignition switch. To isolate the primary circuit from the secondary circuit in that way requires the 4 connections described.
BTW, the polarity of the current going through a relay doesn't make any difference how it works. Any current will magnetize the coil and close the secondary circuit regardless of which direction it's flowing through it. So, it doesn't matter whether it's positive ground or negative ground.