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FFCobra Master Craftsman
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O.K., I am planning the wiring. Carbed car, no AC, no heat, no radio, just the basics. The wiring harness will have flasher relays with it. Where else are relays REALLY needed? I see some pictures with relays all over the place. When is a Relay really needed or required, when is it overkill? What is the criteria to determine if a relay is needed? Thanks,
Doug
 

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You need a relay on the headlights, otherwise the headlight switch runs really hot, and pops the internal circuit breaker and the lights go off at the worst times.

It is also useful to have a relay on the electric cooling fan for the radiator.

Bill
 

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Horn.... if ya have twin Fiamms etc for the compressor...
Headlights.... if ya running 100+w Hellas....

They are basically used to stop a big electrical load going thru the switch on the dash. Far better to have low load switches on the dash than huge things to handle high loads...
Just think how big the starter switch would have to be if there wasn't a starter solenoid...

RickG, knowing just enough about electrics to be dangerous....
 

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Like Bill and Rick said, use relays to keep from having to get high amperage toggle switches.

Also consider, one low amperage switch can control several high amperage relays at the same time. Think about headlights (low and high), running lights, and dash lights all needing to come on from a single switch. You do not really want to have to run all of that current through one switch.
 

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On a real basic car, I say relays for hi-low beams, tail lights, brake lights, horn, electric fan and fuel pump if running an electric one. The headlamps and tail lights will be brighter, the fan will run at a higher rpm with less stress on the dash switches. If you wire the relays ground switched and a dash switch fails all you have to do is ground the switch lead and the device will work. A quick fix with an aligator / jiffy clip lead. Sure beats the alternative on a dark lonely road.

Michael S.
 

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Nine years, wirediet 92 donor wiring, fuel pump relay only.

Donor ignition switch, light switch and high/low beam switch. Fan runs through original MK I Cobra dash toggle and horn toots directly through dash button. Never a blown fuse.

Donor alternator and starter work fine with Holley equiped 345/302. 30+ thousand miles with lots of open track.

Roger
 

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Implied above...relays/selonoids allow you to run smaller gauge wires, too.
 
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