Technical Wiring with relays

Please settle a disagreement.

I use a lot of relays in my cars.
I’ve wired 6 cars with various wiring harnesses and have incorporated relays for any high draw items including headlights, fuel pump, horn, electric fan, heater and wipers if the car has them.
This is how I was taught to wire cars, and my modern ot cars have even more relays!

My friends think I’m crazy and that if you need a relay you’ve used too small of a wire. They’ll only use a relay for the electric cooling fan and nothing else.

Who’s wrong?


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They're wrong, as you know, using a relay in the light circuit re moves some of the power (heat) from the switch, and gives the lights more and steadier power making them as bright as they can be.
Good job, keep it up, it is the right way. My 27 with relays for the lights.

 

You are right.

 

Relays are the safer and more reliable way to wire anything that draws higher amps.

 

Uh, yeah - you win. Your friends, although well meaning, don't understand the big electrical picture. I'd like to ask them if they'd rewire their entire car with, say, 2 gauge wire just to make sure they're running a big enough wire. And I mean *every* wire.

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There's really only two reasons a relay should be used in a 'basic' harness...
1. To control multiple items from a single switch.
2. To control an item or items whose current requirements exceeds the switch rating.

In automotive, reason two is usually why a relay is needed. If you're using one to correct dim lights and/or low voltage, your wire size is inadequate for the load. In late-model cars with all their electronics, relays are used as most electronic circuits aren't capable of switching loads larger than several amps; enough to operate a relay, but not the device.

There's exceptions to this of course; horns almost always need a relay as the contacts in the steering wheel usually won't carry the amps needed for a horn or horns. Cooling fans that draw big amps (particularly on start-up inrush) are another. If you're using 'vintage' switches that may be iffy can be another reason. The problem with aftermarket harnesses is nearly all suppliers use inadequate wire size for fuse panel feeds, or worse yet the item load, so the 'crutch' is to use relays to remove load out of the panel and/or shorten the circuit path to reduce voltage drop.

Give me a harness with wire sized for the loads, switches rated for the item they control, and I'll wire most any car with two relays.... safely.

Look here: https://www.jalopyjournal.com/forum/threads/wiring-101.843579/ ... for a fuller explanation.

 

Yes I use them but not to excess. I don't use them to solve a problem, more like to avoid one. Like old vintage switches and Halogen bulbs. To many for the wrong reason and it can leave ya scratching your head trying to find a problem. Also there are some Good ones out there as well as some Cheep Charlie's, choose wisely.
The Wizzard

 

You are correct, most people don't know how they work or how to wire them for use. I use them quite often as well!

 

like Steve says, they are great for a situation where the switch is not designed to handle the current that needs to be switched. They are also good for a situation where the switch is a long ways from what is being controlled, and would require a long run of heavy wire, which can be reduced by using a relay that is located close to the load.

I hardly ever use relays when wiring cars. I also don't use electric fans, power windows. high current fuel pumps, high current headlights, etc.

This is a traditional forum, if you build a car the old way, you don't need to add lots of relays.

 

Like your pinto motor.

 

I used to see a lot of neat old relays at one of the old time surplus stores around me, but it's gone. Anyone know a source of the old fashioned looking 12 volt relays?

The ones I am thinking of had a clear plastic cover, with a base similar to some of the bigger old style radio tubes.

If a car has to have one or two relays, they might as well be good looking vintage ones.

 

AKA 'ice cube' relays. You can still get them, electrical outlets that carry motor control stuff should have them, although you may need to order them in for 12V coils. McMaster-Carr carries a full line also... Most of them are 2 or 3 pole though, which isn't needed on a car very often.

 

Just thought I would post this. Maybe it will be of some help to folks wiring in a relay for whatever reason.

 

Thanks everyone,
It has become a joke now that I use relays for everything.

I don’t think my friends realize that they may need a relay when they use modern components on an old car. For example, my ‘54 probably didn’t have any relays when it was 6v.
Now that it’s 12v and has an electric fan it needs a relay.
Also it’s probably safer and easier to use a relay when you don’t know the exact rating of a switch or the draw of the component.

Wiring everything with heavy gauge wire and 40amp switches to avoid using a relay seems silly to me.


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Burned up several electric fuel pumps till I learned to use a relay. Fans pumps horns door solenoids and lights need relays!!!!

 

In my '59 Ford with the after market harness, I only have a relay for the electric fan, unless something is built into the fuse block that I'm not aware of. I have no other power options, no radio, no power windows. Really only a heater, wiper and the other usual basic car electrics.

 

Sorry, I'm with them. A properly wired system needs fuses, but not relays. My early cars have horn relays and starter relays and not much more and they run fine. My later cars have more relays, and more often than not, the relay becomes the source of failure, not the prevention thereof.
Let's not forget that you still have to feed the relay the same size power supply before and after the load, and then run lighter gauge trigger wires to your switch. Every connection is a potential failure, plus the innards of the relay as such.

I've seen guys place fuel pump relays in the back of the vehicle, requiring just as much heavy gauge wire to feed the relay as the pump would use without the relay. Come to think of it, even if the relay was in the engine compartment and the pump at the tank, you'd still need the same amount of heavy gauge wire. What is being helped in this scenario? Saving the switch? Well for goodness sake the relay itself is a switch and there are plenty of high current switches to manage a fuel pump.

I admit that certain scenarios where the load is closer to the battery than the switch (but ironically the fuse block power source is under the dash), like headlights, horns, cooling fans, could benefit from a relay, but putting a relay on most loads is a waste of time and money, makes for a non-traditional build, and creates more problems than it solves.

 

Why does any 54 or 59 Ford have an Electric Fan? The Wizzard

 

Here's my contribution to "Fun with Relays". I don't use relays in circuits under 10amps, but do enjoy the flexibility that 5 pin relays give you. Below is what I came up with for the hi/low beams on my '32. The headlight switch is from a European OT car and will not pass full current and I wanted something besides the usual floor switch for hi beams. Easy to wire and the hi beam switch is small and blends in with the dash.

 

Is the 2nd relay really even necessary? Think you could eliminate it?

 

The relay diagram Mr T body posted is how I made the Hotronics window switch that uses the Stock handle operate my Specialty Power window motor. No way was I going to put a Rocker switch in here.

 

If you don't use both relays then when going to High beam both elements would stay on. For me on the windows I have power on one relay full time but till the handle is moved neither relay is grounded. Power goes to the wire for up or down relay by going to ground through window handle switch.
The Wizzard

 

If your electric pump is High draw you need a different pump. My Holleys and before when you could still get them the Mallory pumps that I was using only drew or draw about 3 amps.

That said, I have used relays on anything that draws more than 15 amps for a long time. About he only high draw component that I have not used a relay on is a starter it has the relay built in.

That won't settle your argument as you friend don't know me but if it makes you feel better then I'm your huckleberry.

 

For hi beams it is necessary. Below is the dash I'm putting in my '32 street/race car. The hi beam switch is the second arrow from the left and the hi beam indicator light is to the right of that. Much better than a big ol floor switch.

 

it's surprising how quick you can go on a pump that only draws a few amps...and has a long wire feeding it power, through a couple different switches and no relay. Like, high 9s in a street driven car.

 

I always wondered why a fuel pump relay was 30 amps. Someone tried to explain it to me once but I didn't get it.

I would guess that one of those aeromotive dual pump setups would draw quite a bit but I haven't researched them. We did finish a blown 502 last year with an electronic hat and the in tank for it draws 15 amps. But I just use a damned old carburetor or two my fuel supply requirement is just not that high.

 

There is no need to use a relay if the wiring and switches are capable of handling the load. If you want / need to source a power supply that will supply a higher current to run an accessory then a relay is the way to do it.

Using a relay when not needed just complicates the wiring for no good reason, but if you are matching old style switches with modern accessories that use a lot of current, using a relay could certainly save a problem down the road.

The car in my avatar uses a horn relay, dual h/lamp relay, cooling fan relay and a pair of relays for the door locks. All other circuits are on their own.

 

Just used switches for a 6V car. LOL

I have been known to use headlamp relays, but not always depends on the application. I like them on Quartz lamps they increase the life of the bulb by assuring good power supply. Most people don't bother to properly mount the relay, it should be as close to the component as possible.

I did make a startling discovery a few years back, *I had a modern build that kept coming in with burnt out AC clutch. I replaced a couple in the run of the summer in the car ( easy money right?) then decided that good mechanic work was required. Did some research and discovered that a relay was the missing part of the equation, replaced the clutch and wired in a relay ( as close as possible to the AC pump) and have not replace one since.

*I did not do the install, when no one else can fix it my driveway is its last stop.

 

The 292 y block I put in has a long waterpump and the stock fan hit the rad. I already had the electric fan so it is temporary until I find a short waterpump and fan that fits.



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The less the better.
Relays are cool for dual switched input activation or deactivate a circuit.
Cool for switching polarity on Window and seat motors.
Or light duty / antique switches.
Other than that it's not needed