What is a relay

A relay is an electromechanical or electronic device through which a heavy load can be switched ON & OFF with a nominal input of current & voltage. This article focuses on electro-mechanical relays used in automotive applications.

How does a relay work?

The utility and function of a relay can be best understood by thinking of a lever. A lever is inserted at a corner of a heavy load and by placing a wedge (fulcrum) under the lever, the heavy load can be easily lifted by a small amount of effort.

Similarly, a heavy electrical load drawing heavy current can be switched on & off by applying a small amount of current through a coil(solenoid). What happens is this - the coil gets magnetized with the current and attracts or repels a plunger(a rod passed through the coil). The attracting or repelling of the plunger either connects two switching contacts in the relay (also known as NO or Normally Open/Single Throw/Form A) or separates them (NC or Normally Closed/Double Throw/Form C), as required. As soon as the energisation of the coil is stopped, the main contacts go back to their original position.

In automobile applications relays of different configurations with single or multiple contacts are used for different applications.

Why is a relay required?

Apart from switching a circuit with heavy load, on & off, automotive relay is required to

1. Conserve energy

2. Prolong the life of device

3. Improve the efficiency of a device.

For example let us take the switching of two headlamps of an automobile. If the same is switched on through a normal switch at the dashboard, the voltage drop from the battery to the lamps through the switch will be enormous and the heat generated in the circuit due to resistance will be considerable. The heavy current will be drawn by the lamps which will not only heat the circuit but also drain the battery heavily.

Where should a relay be installed?

Wherever manual switching has been used for direct switching of a Lamp or Motor or any gadget exceeding 10 Amps of load, a suitable relay must be put in the circuit. It is best if the relay can be kept as near as possible to the load (but not in a hot area surrounding near the engine)

How should a relay be installed?

1. Wires connected to the manual switch should be connected to any Coil terminal of the Relay.

2. The load should be connected to any one contact terminal of the Relay (unless Positive/Negative are specified) and the other to the positive terminal of the battery (for Negative Ground circuit), with an appropriate Fuse.

Bad Car AC Relay

A bad car AC relay remains a common problem for many years, make and model automobiles. We have a lot of car air conditioning articles posted on the site that aim at getting your mobile cooling unit working again.

In many of these articles we focused on some of the worst AC problems an old car can have. This time we get to share some good news with motorists.

What if the root cause of your air-conditioning problem came from a part costing around $15? Furthermore, what if the repair involved simply plugging in a new relay at the convenience center of your automobile? If this is the malfunction, it results in a repaired AC system.

Here we'll discuss how to diagnose a failed compressor relay and point to some of the automobiles that commonly enjoy this problem. Although it's never fun when the AC stops working, it’s the absolute best case scenario when all it needs is a relay to make it work again.

The two foreign cars that jumped off the map included two of the most reliable Japanese cars. Honda automobiles like the Accord, Civic and Honda Pilot make this list.

In fact, the last Accord I worked on also needed a Honda VTec Solenoid replaced. Surprisingly, the once bulletproof Toyota product line also has a high rate of AC relay failure in the Camry and Corolla models.

Digging into the message boards I found that often these relays work intermittently before they give up altogether. It’s difficult to track down intermittent electrical malfunctions. We'll get more into this in the diagnosis section below.

There’s a common misconception that if you hear the relay click that it must be good. In the close-up picture on the right you see an image of a bad car AC relay. This relay clicked as it became energized by the ECM.

However, you can see a hairline crack in the brass contact strip that allows current to flow to the AC compressor. This means the relay clicked, but did not work as intended. This is an extreme example, but it just goes to show that you can't apply simple rules to every situation.

A quick and dirty way to test a car air-conditioning relay is to locate it and then swap it out with the same type of relay. Often car manufacturers use the same exact part number relay in many different locations. This applies to all the vehicles mentioned above.

Although it’s not a permanent solution, it remains one of the fastest ways to test it out. If you swap it out and the compressor is not kicking on you have a different problem. In isolated situations where the AC compressor clutch relay has a unique part number we test it the old-fashioned way.

Remove the component from the socket and test for a power signal and a good ground at the proper terminals. You can also use a jumper wire to jump the two terminals that pass electrical current onto the air conditioning compressor.

You have to remember that an automotive relay is nothing more than a remotely operated switch. It uses the magic of magnetism to complete the opening and closing action.

When the relay energizes it creates a magnetic field that pulls the contact closed. Turning off the power to the relay collapses the field and the switch becomes naturally open. There are many different reasons that a relay can fail to operate.

When the part itself fails, look into possible causes like corrosion buildup between the contacts. This is a repairable condition. You can take an emery board or fine grit sandpaper and clean these contacts.