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Why relay is used in car?

Classic Car Automotive Electrical Systems — Part 6: How Automotive Relays and Fuses Work

We’ve gone through basic theory and major components over the last several articles, so by now you should be fairly comfortable with overall circuitry. Now, we’ll cover just a few more common devices and then draw a simple circuit and follow an electron through it. Ready?


This was touched-on before but it’s worth special attention. Remember that certain devices require considerable current (amps) and that, in turn, require thicker wire. High current devices require big, heavy switches to handle the current. Unfortunately, these would be ugly and expensive, so engineers use relays.

A relay consists of a small coil of wire around a central iron core. When the actuating switch energizes the coil this core moves heavy-duty contacts together, thus allowing high current to be passed to the device. That’s how a small switch can control a high-current device.

You already know the starter solenoid is a high-current relay. Other devices that typically utilize relays are the horns, power antenna, air conditioning compressor, power seats, power windows, engine cooling fans, and power tops. Sometimes, headlights and accessory driving lights use them too. It’s important to know this because many electrical failures occur in the relays themselves!

Automotive Fuses:

Almost everything in a car is wired through a fuse. Fuses are designed to fail when too much current is drawn through the device. This prevents heating of the wires and subsequent melting of the insulation, followed usually by fire!

Fuses are simple in design. Inside a fuse is a soft wire with a specific cross-sectional thickness. This dimension dictates how many amps can be carried before the wire melts. Too many amps and the fuse fails, saving the rest of the circuit from damage. Pretty neat, huh?

Most of any car’s fuses are located in the fuse panel, but some are in-line. In-line fuses are found under the dash and in the engine compartment.

Fusible Links , another kind of fuse, are used in many cars and are almost always found in the wiring harness in the engine compartment. These are molded, single-purpose links in the wire which are designed to melt under extreme conditions (usually a crash which might crush wires together, causing a huge short circuit). Your car’s schematic will show their use and location.

Let’s look at a fairly typical horn circuit and see how various components are put together to form a working system:

Horn Relay Diagram

Notice that battery voltage travels through a high current wire (red) through the relay to the horn and also through a smaller wire (blue) through the ignition switch to the relay’s low-current coil. The first thing you should be aware of is that the horn circuit is always «hot» or «live» when the ignition switch is turned on and all that’s needed is a path to ground.

That path is completed when you push in the horn button. When the button is pushed the ground connection is made, energizing the relay’s coil «A». The coil’s iron core (in this particular design) pulls down arm, connecting high-current contacts «B». High current then flows from the battery to the horn (the horn is connected to ground because it’s mounted to the chassis of the car). See how it works?

Actually, one thing is missing from this circuit. There has to be a fuse somewhere in the circuit! The high-current wire from the battery might go through an appropriate fuse on the fuse panel or there might be an in-line fuse near the horns (it depends upon the production engineering decisions as to the most economical placement, but your schematic drawing will show its location).

Also, your car’s designers might have fused the low-current side of the relay as well. Check the schematic.

Suppose the horns don’t work. Where do you start your troubleshooting? Here’s a good procedure:
1. Check the fuses.
2. Check for voltage to the horns at the horn connector. Push the horn button or jump the wire to ground to actuate the relay. If you have voltage the horns should be operable, so search elsewhere for the problem.
3. Check for voltage at the horn button. While there, check to be sure the button’s contacts touch each other when pushed. If everything’s ok, go to the relay.
4. With someone pushing the horn button, check for voltage (on the low-current wire coming from the dash) on the relay. If there is voltage, the relay isn’t working, right?

That’s right. Now you have isolated the problem to the relay and only two things can be wrong: either the relay’s coil isn’t energizing (due to an internal broken wire) or the high-current contacts are being drawn together but no current is passing through (remember high-resistance connections?). If the coil is bad the relay must be replaced. If the contacts are charred, file them smooth.


Automotive circuits are quite simple in design. Remember always that the factory used as little wire as possible, so look at the schematic diagram to see where multiple connections are made. Remember also that your friendly electron travels through several devices on its way to doing its work so you need to systematically trace the path.

Schematic diagrams are printed way too small in the manuals. Take your drawings to a copier that enlarges and blow them up to easily-readable size. Tape all the sheets together into one big drawing an you will find that tracing electrical paths becomes very straightforward.

With a little practice and patience you will no longer fear your car’s electrical system.

© 2023 Second Chance Garage, LLC. All Rights Reserved. Reproducing any material on this website without permission is prohibited.

How to check a relay switch

The Video Course teaches you everything about modern cars.

If a component that is fed with electricity through a relay (See How car electrical systems work ) terminal of the battery to the feed terminal on the component, thereby bypassing the relay and supply wiring.

If the component still does not work, it is faulty; if it works, then the supply is faulty and the fault will be in the relay or the connections to it.

Trace the supply wire back to find the relay — this is a small metal or plastic box which usually has four spade terminals and is located near the battery .

Check that a supply wire has not become detached from a terminal. Check each terminal for corrosion, especially the thin wire from one terminal which goes to earth on the car body — probably fastened under a screw or bolt near by.

Remove the screw and clean the terminal and the underside of the screw head.

The relay has one thick cable coming from the positive (+) pole of the battery. A second thick cable goes from the relay to the component. A thin wire runs from the control switch on the steering column or dashboard , while a second thin wire goes to an earthing point.

Use a circuit tester to check whether current is reaching the relay. Clip one wire to earth on an unpainted part of the car and probe the feed terminal on the relay.

If the tester lights, there is power arriving at the relay. If it does not light, check the connection at the battery.

If the tester lights, turn on the switch inside the car which controls the component and use the tester again to check for power on the thin wire leading from the switch to the relay.

If there is no power, use the lamp to check the input and output terminals on the switch. This will tell you if current is reaching the switch from the battery, and if the switch is passing the current when switched on.

If there is power at the relay, use the tester on the relay earth terminal the second thin wire. No current flowing to earth means that the relay unit is faulty and must be replaced.

If the relay is earthing properly, leave the control switch on and use the tester on the relay terminal which feeds the component.

If there is no power, the fault is in the relay again — probably the contacts are burned or stuck in the open position.

Burned contacts can also fuse together, so that they stick in the closed position, so the component does not switch off. In either case, replace the relay.

Some relays have small pin connectors and plug into an enclosed socket.

Remove the suspect relay and replace it with another of the same type. If the component works, the original relay is faulty.

If the component still does not work, check the terminals in the base of the relay connector block with the circuit tester probe. For the tester bulb to light there must be a good contact at the test points. That is the reason for the sharp probe, and for the sharp teeth on the clip.

The probe is useful for poking under the plastic covers of spade terminals and snap connectors without the need to disconnect them.

Sometimes it is convenient to use the probe to prick through the insulation of a wire if other access is difficult.

Apart from the circuit tester, another useful aid is a test lead — a 10 ft (3m) length of wire with a crocodile clip at each end. This allows you to make direct connections from the battery to components which are some distance away, for example the rear lights, rear-mounted electric fuel pump and fuel-tank sender units.

A typical charging circuit

The battery is earthed to the body by a short, heavy cable or by a braided wire strap.

On most cars the negative battery terminal is earthed. From the positive terminal another heavy cable goes to the starter solenoid switch, which feeds current to the starter along a third heavy cable.

A wire leads from the live side of the solenoid (not through the switch itself) to the ignition switch .

Another wire leads from the live side of the solenoid to the ammeter (if fitted) on the instrument panel. Thus, the ammeter is always live, and always shows whether any power is being discharged. This circuit is then completed to the generator , so that current in the opposite direction causes the ammeter to show how much the battery is being charged.

From a point after the ammeter, another wire (not shown) goes to the lighting switches and to the fuse box, where it supplies power for circuits not controlled by the ignition switch.

An ignition-controlled circuit

If the car circuits could be accidentally left ‘live’ when the car is not running, the battery would be discharged unnecessarily. For this reason, most circuits are operated through the ignition switch. (The exceptions are those which might be needed for safety — chiefly headlamps, sidelights and emergency flashers.)

From the ignition switch a plain wire runs to the fuse box, where it is connected to the fuses of all those circuits which come on with the ignition.

From each fuse a plain wire runs to each of the circuits, picking up a trace colour after its first connection.

Relays in Auto Electrics a basic guide

In this auto electrics Tutorial, I’m going to be looking at Relays, their use and function in vehicle electrical circuits. If you’re not familiar with relays, you soon will be. The average vehicle is packed full of them and they’re extremely useful. Hopefully what follows will give you more than enough information to comfortably understand and use them.

To begin, if you’ve got absolutely no idea about auto electrics, then see my Auto Electrics 101 tutorial.

What is a Relay?

A relay is simply an electronic switch. Their purpose is to switch electrical circuits on and off. They come in different sizes and shapes and usually have 4 or more metal lugs or terminals on them.

Here’s a really simple circuit. There’s a battery, a bulb and a switch.

It’s easy to see how when the switch is closed the bulb lights up.

The problem here is, of course, that someone has got to close the switch. What a Relay does is electronically close a switch for you.

Inside a Relay

Relays hold tiny electromagnets. When you place power into them, they created a magnetic field which either attracts or repels a metallic strip that forms the arm of a switch. Historically the standard electronic symbol for a Relay was something like this:

It’s easy to see the switch. The coil represents the electromagnet. Imagine that magnet attracting or repelling the arm of the switch above it and you’ve grasped the workings of a relay.

For this purpose, I’m going to draw my Relays a little differently.

Hopefully it’s still obvious which is the magnet and which is the switch. The four large dots represent the terminals of the relay, the small metal lugs that protrude from the plastic shell.

Energising A Relay

To energise the electromagnet inside the relay, it’s necessary to connect electric power to the two magnet terminals, in this instance causing the internal switch to close, like this:

So if we wire bulb and a battery to these switched terminals of the relay, it’s easy to see how the relay can be used to switch the bulb on and off.

If you’ve just freaked out, don’t. it hasn’t suddenly become complicated honestly. Take your time and look what’s going on. The only thing that’s changed from the previous diagram is the addition of the bulb. The bulb is using the relay as an on/off switch, and this is a relay in its most basic form, what’s called a Four Pin Normally Open Relay… four Pins for four terminals and normally open because the switch is open until power is applied to it.

Energise the relay and the switch is closed, lighting the bulb. Disconnect the power to the relay and the bulb and electromagnet turn off.

Open and Closed Relays

There are different types of relay. We’ve already mentioned the Normally Open relay, so the first obvious variation is the Normally Closed Relay. At first glance there’s no obvious difference, but the difference lies with the internal switch.

You’ll notice in the above diagram that I’ve introduced what we’ll call a master switch into the circuit just before the relay. This master switch will determine whether or not the relay is energised. We can see clearly that when the relay is not energised, when it’s not powered, in a Normally Open Relay the internal switch is open, meaning it cannot power anything (like the bulb). The Normally Closed Relay the internal switch is closed, so this will power the bulb.

Why the difference? Why have a normally open and normally closed? Because sometimes it’s useful to have one circuit powered when another is inactive. Imagine a vehicle burglar alarm. you would not want this active whilst the engine was on and you were driving. But when all the other electrics are off and the vehicle is stationary, then that’s an ideal time for the alarm to become active, so a normally closed relay might do the job nicely for us. no power energising the electromagnet (engine off) allows the alarm circuits to become powered.

Five Pin Relays

The next variation is a five pin relay. As the name suggests, with these there’s simply another terminal involved. However this extra terminal gives us a relay that has both a Normally Open and Normally Closed points. This is extremely useful obviously. Personally I tend to only buy five pin Relays as there’s virtually no price difference and a 5 pin can do everything a 4 pin can and a bit more.

For instance, let’s use a 5 pin relay to light bulbs in two separate circuits:

To begin with, notice that the master switch is closed and the relay is energised. The first four pins are acting like a Normally Open relay and the internal relay switch is only closed because the electromagnet is energised. This allows power from the battery to travel through the relay to Bulb A. However Bulb B is connected to the 5th Pin and there’s no power there, so Bulb B is unlit.

If the master switch is opened, everything changes:

As the electromagnet is not energised, the internal switch springs open. This now contacts the 5th pin. This means that power from the battery can flow through the relay and light Bulb B. The 4th Pin no longer receives power and thus Bulb A is unlit.

Labeling The Pins

To the best of my knowledge, the Pins on 4 and 5 pin relays are numbered (and lettered) in a universal way. These labels are:

  • 30 — the common switch connection
  • 85 — electromagnet coil
  • 86 — electromagnet coil
  • 87 — Normally Open (NO) Pin
  • 87a — Normally Closed (NC) Pin

Power Handling

Whilst the switching aspect of relays is useful, probably the main feature of them is their ability to handle large amounts of current. That’s why your average vehicle is littered with them. It’s all about the Power.

Generally it takes very little current to energise a relay, and where there’s very little current, the average human can safely handle things. For instance a switch on your dashboard. What you don’t want is massive, metal melting power surging millimeters beneath your fingertips. But the tiny amount needed to energise a relay is nice and safe.

Let’s use the example of a car horn. These noisy devices can often draw a lot of current. The one on my vehicle is rated at 20 Amps. That’s easily enough power to kill a human being.

In the above image, we can see on the left a very bad idea. A high current horn is switched directly from the battery. Yes there’s a fuse, but click that switch with a wet finger and you might just do yourself a lot of harm.

The image on the right shows the correct practice and this is the one used by vehicle manufacturers. The switch is used to energies a relay and this only uses a small amount of current and is therefore safe. Meanwhile all the dangerous current is being processed through the relay, operating the horn and isolating you from potential harm.

Different Rating For Different Jobs

Spend a few minutes on eBay or similar sites looking at relays and you’ll be amazed how many different sorts there are. Gernerally speaking choosing a relay comes down to:

  • What power is needed to energise the relay
  • What power the relay is expected to handle
  • What switching job(s) are required.

As most vehicles run on 12 volts, auto relays are energised by 12 volts of power. The amount of power they need to handle varies according the the jobs they’re doing. Starter motors, for instance, draw hundreds of Amps of power. Lights draw very few. As heavy duty relays are more expensive, it makes sense to choose an appropriate relay for each individual job.

If you look at the picture above, you can see an auto relay. It tells us that it requires 12 volts DC (Direct Current) to energise, which is what most vehicle batterys are. It also tells us it can safely handle between 30 and 40 Amps. So if you wanted to power a 20 Amp circuit, this relay would be ideal, as that’s well under the maximum load of the relay. If you wanted to power a circuit of 50 Amps, you’d need a more substantial relay.

Example — Real Life Application

One of my subscribers once asked me how they could connect a 3rd brake light to their vehicle. This could be achieved by simply splicing into the existing circuit brake light circuit and adding another, as shown in the image below on the left. However this this is actually a bad idea.

The correct way to do it is to use a relay, as shown above on the right.

Why? Difference does it make?

In truth a single bulb probably wouldn’t make a great deal of difference, but this is about good and bad practice. When it comes to auto electrics, unless you have a very good knowledge of what’s going on in the circuit you’re splicing into, you really shouldn’t overtax it by adding in more components. At best this could blow a fuse but it could potentially lead to burned out wiring and electrical fire, and all because you’ve added a new component into a circuit that had been finely balanced by the manufacturer.

The safer option was to still splice in, yes, but only to use the power in that circuit the energise a relay. The likelihood of this overtaxing the original circuit is minimal. Now, however, a new fused power supply can be introduced and this can safely power a new light or even a hundred new lights — that’s the point! The inclusion of a relay means the older, correctly rated circuit is not in danger of being overloaded and the new circuit is independent, fused and safe.

Helpful Relay Tips

Here’s a few handy relay tips:

  • Relays often come with holders. This makes wiring them up much easier.
  • If you’re not using a holder, you’ll either need to solder wires to each terms – which makes removal difficult – or you’ll need Spade Connectors. If you use Spade Connectors:
    • Make sure they’re a nice tight fit.
    • Squeeze with pliers if necessary. You don’t want them coming off.
    • Ideally purchase ones that are fully insulated. If you can’t get them, cover everything well with insulation tape. If the come loose, you could easily have a live wire flapping around and that’s never good.


    That’s about all I can think of saying on the subject of relays. They’re a safe, practical electronic switch widely used by manufacturers. There are a far greater array available than I’ve really covered here but they tend to be for specialised applications. The above tutorial is meant to cover the basics.

    To help the above sink in, why not try my YouTube video on this same subject:

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