purpose of relays

[l1ttle_d3vil]

I've got a rough idea of how relays work, but do you have to use them when wiring up fuel pumps and radiator fans etc?

or can you just wire the pump and fan up to run manually straight off a switch? atleast as a temporary solution? it is quite a lot of wiring to get sorted to get one relay wired up, as opposed to 2 wires with a switch inbetween...

just looking for a simple way to control my fans for now until I get everything + new engine permanently in place.

[thegreatestben]

I think alot of it is safety related.
If you crash and the engine stops it cuts power to the fuel pump doesnt it?

[Lloyd]

You only need a tiny wire to run to a relay to switch it on. The relay can switch big currents. Basically its so you have the relay next to the fan for example, you only need to run heavy wire to the relay and from there to the fan, leave you just thin wire to run to switch/key/whatever.

Very useful if you have lots of stuff to switch, or if you have a high current item

[Crampy]

You only need a tiny wire to run to a relay to switch it on. The relay can switch big currents. Basically its so you have the relay next to the fan for example, you only need to run heavy wire to the relay and from there to the fan, leave you just thin wire to run to switch/key/whatever.

Very useful if you have lots of stuff to switch, or if you have a high current item

Exactly. If you didn't use relays, it would eventually burn out the contacts on your switch gear, due to the arching when turning the inductive circuits on and off (motors, pumps, etc). It saves weight and cost as well, due to not needing the heavy gauge wiring going all the way through the car and not needing heavy duty switch gear.

[l1ttle_d3vil]

If you didn't use relays, it would eventually burn out the contacts on your switch gear.

over 2 or 3 years? or 6 weeks? basically I just need something so I can get my car around town on the odd occasion, and switch my fans on if I need to. It definitely isn't going to be a long-term or permanent solution.

[Caveman]

The circuit opening relay has two coils to switch the fuel pump on.

Fuel pump will run if...

The engine is cranking (STA)
There is air moving through the motor (AFM or ECU)

When starting the motor, there is little air flow but the STA signal will turn the fuel pump on.

When the motor is running and you crash and rip a fuel line, the motor will die (no fuel) and the fuel pump will turn off due to no air running through the motor.

If the fuel system remains intact when you crash and the engine stays on then the fuel pump should not stop.

Sorry for the bad english etc

Point: circuit opening relay can save your fuel pump from becoming a flame thrower in case of a crash

[Crampy]

If you didn't use relays, it would eventually burn out the contacts on your switch gear.

over 2 or 3 years? or 6 weeks? basically I just need something so I can get my car around town on the odd occasion, and switch my fans on if I need to. It definitely isn't going to be a long-term or permanent solution.

Well after what Caveman has just written, personally I'd say to hook it up to relays. It wouldn't be worth the risk. Relays aren't that expensive and they're very easy to hook up.

[Lloyd]

And saying its less wiring isn't entirely true.


Run your power feed to the relay straight from the battery and bridge it over to one of the coil terminals as well. Run the output from the relay to the fan and earth the other fan terminal to the chassis.

Now all you need is to feed an earth to the relay through one small wire to a switch, and that switch can be earthed under the dash anywhere.

So now you basically have a power wire from the battery to the fan with a relay in the line somewhere, and a thin single wire to the dash. You're running less wiring if anything

[Caveman]

And saying its less wiring isn't entirely true.


Run your power feed to the relay straight from the battery and bridge it over to one of the coil terminals as well. Run the output from the relay to the fan and earth the other fan terminal to the chassis.

Now all you need is to feed an earth to the relay through one small wire to a switch, and that switch can be earthed under the dash anywhere.

So now you basically have a power wire from the battery to the fan with a relay in the line somewhere, and a thin single wire to the dash. You're running less wiring if anything

Yep agree with this

[NZ_AE86]

Relays are there for two reasons, one - which is to allow a switch with small contacts to be used, two - allows small wiring to run from your switches to the relays. If all your switches had contacts large enough to handle the current drawn when switched on and off we would not be able to have small easy to use switches we are used to.

Someone mentioned the fuel pump switching off when the car stops - this has nothing to do with the relay. It will stop because the ECU will stop triggering the relay after a preset time.

As for how long it will take to burn out you switch, that will depend on how big your switch is. HRT is on to the best way to do it. Put a fuse in the feed from the battery too.

There are other relay uses for special purpose relays but very few cars use them.

[l1ttle_d3vil]

Run your power feed to the relay straight from the battery and bridge it over to one of the coil terminals as well.

thanks for the info HRT, but you lost me with this bit. why bridge it over the coil too? won't the coil have power constantly from the battery then too?

and generally the relays i've wired up have batt + earth, and are switched by an ignition power. can the relay instead have 2 batt sources, then switched by an earth like you've said? or do the 12v+ sources need to be one batt + one ignition?

hope this makes sense, you've probably answered my question already but its too early for me

and out of curiosity; burning the switch out, does that go for any little switch with 12v going through it? as in when people put a kill switch inline with the + terminal of the coil, or a switch inbetween the fuel pump and relay output?

[Lloyd]

Switch in line with fuel pump or coil shouldn't be an issue as they aren't being used when you turn the switch on so there isn't a big surge of current suddenly going through so there is no big spark arcing across the contacts.


As for power to the coil as well as the feed to the relay, yes it will always have power to the coil. It has nowhere to go however as you haven't given the other side an earth so the coil doesn't actually have any current flowing through it so the coil isn't engaged. As soon as you feed that other side an earth, then the relay will switch.

Just remember, you don't have to feed something a power feed to make it go. It needs power and earth to go, so there is no real issue with having constant power to something and simply feeding it an earth

[Caveman]

Relays are there for two reasons, one - which is to allow a switch with small contacts to be used, two - allows small wiring to run from your switches to the relays. If all your switches had contacts large enough to handle the current drawn when switched on and off we would not be able to have small easy to use switches we are used to.

Someone mentioned the fuel pump switching off when the car stops - this has nothing to do with the relay. It will stop because the ECU will stop triggering the relay after a preset time.

As for how long it will take to burn out you switch, that will depend on how big your switch is. HRT is on to the best way to do it. Put a fuse in the feed from the battery too.

There are other relay uses for special purpose relays but very few cars use them.

ummm it has everything to do with the relay, without it the system would not work.

I think hes meaning circuit opening relay system vs a switch in the cabin for the fuel pump

[NZ_AE86]

ummm it has everything to do with the relay, without it the system would not work.

I think hes meaning circuit opening relay system vs a switch in the cabin for the fuel pump

The fact is it has a relay in the circuit has nothing to do with it turning off when the engine stops!
The relay is only there to take the current, if it is controlled by a switch on the dash it will keep running until that switch is off regardless of the engine being stopped or running.

[Caveman]

ummm it has everything to do with the relay, without it the system would not work.

I think hes meaning circuit opening relay system vs a switch in the cabin for the fuel pump

The fact is it has a relay in the circuit has nothing to do with it turning off when the engine stops!
The relay is only there to take the current, if it is controlled by a switch on the dash it will keep running until that switch is off regardless of the engine being stopped or running.

no im comparing running a switch directly from power to the fuel pump and ditching the relay system. vs running the circuit opening relay controled by the starter and ECU (or AFM in some cases)

maybe I miss lead in my last post

[NZ_AE86]

Sweet! Yeah we are all on the same mark now!

[Adoom]

I'll add something in here too.
Depending on the state/gauge of the wiring in your car. If you took power from an existing terminal on the fusebox then to a switch then to the pump/fan/whatever. It will work. But if the wires are long and wussy with a small switch there will be more resistance which turns your electrical current into heat, this cooks your wussy wires and switch. It also means the pump/fan isnt getting full power and its going slower......
Wire in a relay(just use the wussy switch to turn it on) and use some big manly wires with F all resistance to supply the current that the pump/fan uses. It will go better/faster MORE POWER!!!!

GO THE STARVIA!!

As an example: plug some air horns into the original wires for the factory horn beep beep. Then wire up a relay instead. It louder...isnt it?