Understanding the purpose of relays in cars

An automotive relay is an electrical switch made up of copper coil (that is wrapped around a soft iron core), designed in all types of vehicles to allow the efficient control of high-power electrical devices.

Depending on the design of the electrical system or a particular model of car, these are often found in areas such as the engine compartment, fuse box, under the dashboard, interior panels and near the battery. According to www.dubizzle.com, there are various types of relays designed for use in vehicles, including:

Electro-magnetic relays

These are the most common relays found in vehicles. They use an electromagnet to mechanically operate switch contacts. They include single pole single throw relays (SPST), the single pole double throw (SPDT), double pole single throw (DPST), and double pole double throw (DPDT).

Overload relays

They are also known as protective relays and are designed to protect motors and other electrical equipment from overloads or excessive current. They monitor the current flowing through a circuit and trip the contacts if the current exceeds a preset threshold, preventing damage to the equipment.

Time delay relays

These provide a delay between the activation of the coil and the closing or opening of the contacts. They are used in applications where a delay is necessary, such as motor control, lighting, or sequential system operation.

Solid state relays

They use semiconductor devices such as transistors to perform switching operation without any moving parts. This leads to faster switching speeds, longer lifespan due to absence of wear and tear, silent operation and better reliability in harsh environments. They are often used in applications requiring high reliability, precise and lighting control, heating and cooling systems.

Hussein Ssematta, a mechanic, says when choosing a relay, a car owner should consider factors such as durability, cost, electromagnetic compatibility, environmental conditions (such as temperature, humidity, exposure to dust and vibration, among others) and appropriate contact configuration (such as normally closed/open, changeover).

Functions

Automotive relays are used to control various systems such as the flow of high current to the starter motor to enable the engine to start. They are also used during power supply to the fuel pump to ensure that the fuel delivery system operates efficiently during engine startup or while the engine is running. Additionally, in many vehicles, relays control the operation of cooling fans to regulate engine temperature and prevent overheating.

Similarly, relays are often used to control accessory power outlets and other electrical accessories in vehicles which enables the powering of devices such as radios, GPS units, and charging ports.

In vehicles equipped with power windows and central locking systems, relays are used to manage the flow of electricity to these components. They are also essential for the operation of safety-related systems in cars, such as airbags, anti-lock braking systems (ABS), and traction control systems.

How relays work

Relays work based on the principle of electromagnetism by converting electrical signals into mechanical motion to control the flow of electricity in a circuit. When for instance wipers are turned on, a small electrical current flows to the coil of a relay, which generates a magnetic field around it and attracts the armature attached to one or more switch contacts to move in response to the magnetic field.

The armature then either closes or opens the switch contacts, depending on the relay’s design and configuration. For instance, when the contacts are closed, it allows the flow of electricity through the relay; when they are open, it interrupts the flow of electricity.

The relay will then provide electrical isolation between the control signal or low-voltage side, where the coil is energised and the load circuit (the high-voltage side, where the electrical load is connected). This isolation helps protect sensitive control components from high voltages and currents present in the load circuit.

Once energised, relays can maintain their state (open or closed) even after the control signal is removed. However, when interrupted, the magnetic field collapses and the relay contacts return to their open position, thus breaking the circuit.

Signs of damaged relays

Just as the switches in your home may stop functioning after a short circuit, car fuses also tend to wear out eventually, depending on the specific relay and the way they are used. It is thus advisable to replace them promptly to prevent further damage to the controlled device or system.

One of the obvious signs of a blown fuse is if a device controlled by a relay such as headlights, horn, or power windows stops functioning. Another sign is clicking sounds coming from the relays repeatedly without the controlled device functioning.

At times, you can identify a damaged relay from a burning smell or noticeable heat emanating from it or its vicinity indicating overheating (that occurs due to excessive current flowing through the relay or poor contact between its internal components) and electrical arcing within the relay.  Other signs include trigger of the check engine on the dashboard, corrosion on the terminals or melted plastic casing.

Replacing relays

Replacing faulty relays should be done as soon as the damage is noticed. Like all components, relays have a limited lifespan and may degrade over time due to factors such as mechanical wear, electrical stress, or environmental conditions. If a relay has been in service for an extended period or has exceeded its expected lifespan, it may be prudent to replace it as a preventive measure.

Checking for faulty relays
According to www.basler.com, relays last for between 15 and 20 years. However, once a relay is found to be faulty, it should be replaced as soon as possible. Visually inspecting the relay for any signs of damage, such as burnt or melted plastic casing, corrosion on the terminals, or visibly damaged internal components. 
One can also use a multimetre to measure the resistance and continuity across the coil terminals. For instance, if the resistance is significantly higher or lower than expected, the relay coil may be faulty. 
“If testing procedures reveal a relay has failed to meet the specified resistance values, exhibits poor continuity or fails to operate properly when energised, it should be replaced. Faulty relays can lead to unreliable operation of electrical components or systems, posing safety risks and potential damage to other equipment,” Hussein Ssematta, a mechanic, cautions.
Additionally, if you overhear a clicking sound as the relay energises, this will indicate that the switch contacts have changed state and the relay is faulty.