Can the electrical brains explain wiring relays for me?

I'm not any wiring expert, but I think it should look something like this.

This picture below should make it clear how to wire a standard relay, almost all of which (even Radio Shack) will have this standard pin numbering markings. Some relays have a 5th pin, labelled 87a.. This pin is rarely used since it provides constant power to the intended device when the trigger power (pin 86) is NOT applied.

In case I can't get the picture to load, here are the standard pin configurations:
87 high power lead connect direct to load (fan, etc) (10ga or larger)
30 high power battery source.. connect direct to batt. or starter solenoid post (largest post) (10ga wire or larger)
86 "trigger" power from ignition switch or fan temperature switch
85 connect to ground

Relays are used generally for two reasons... to provide a larger amperage capacity supply wire to a high-amperage device such as a fan, and to reduce the load on the switch for that device, since the relay "trigger" circuit requires only a very low amperage current to operate.

On most of my older cars, I've wired the output from my ignition switch directly through a main relay, using at least 10 ga or larger wire from the battery and to the fuse block, to help preserve the frail contacts in the switch, which were never designed to handle the power of aftermarket accessories such as fog lights, A/C, cooling fans, etc.

For the fan, you could either have the relay trigger voltage come from the ignition switch, a separate dash mounted switch, or just direct from a temperature sensing probe.

If you do the latter, then the fan may run for awhile after shutting down the engine as the radiator heats up from residual engine block heat. I'm not certain this approach serves any real purpose, and is just saps power from our already-weak 6V batteries, and I doubt that cooling the water just in the radiator itself has any useful benefit to the engine.

Yes, provided that the current the fan requires times 2 is still within the headlight relay's ratings. Make sure this is true otherwise you'll weld the contacts eventually and the fan will never shut off.

Jack



PS, That Radio Shach Relay has a 25amp contact rating I believe which might be under rated for your application. The contact carrying current is an essential parameter that you must respect if you want the relay to survive.



If this fan is the same as the last member's question it could draw upwards of 35-40amps at 6v. JUst be sure you check the fan's requirements and use a 2 times relay contact rating to be within safe operating ranges.





















jjbubaboy wrote:

Thanks all!

Think I understand it now.

I like the idea of going from ingition switch to an overall relay to preserve the switch! Would a 6v headlight relay work for that? I would imagine it would have to be a beefier relay to handle it.

Again thank you.

Jeff