AC TMI White Paper: All About Relays

What is a Relay?

A relay is an electrical component that is commonly used in cars to control high-current circuits with low-current signals. Relays are used in many different applications in a car, such as controlling the headlights, horn, wipers, starter motor, and more. It's a kind of remote switch.

Why Should I Care? What Are the Practical Considerations?

(1) Protect headlight switch while delivering maximum power to the headlights

In early Porsches, the full current for the headlights runs through the headlight switch. They were designed for high power bulbs, such as 100w, and can fail as a result of the high current. This can by solved by a headlight relay kit, which uses only low power current through the headlight switch to active the headlight relay. It is typically located near the lights for maximum efficiency and sends high power current to the headlight.

(2) Enable the use of LED signal light bulbs

LED bulbs have much less resistance incandescent bulbs and there's often not enough to properly trigger an old style mechanical flasher relay (more below). The solution is to replace it with a newer style electronic (solid state) flasher which doesn't require as much current to switch.

(3) Keep the Car Running (!)

Some later air-cooled models, 3.2's, 964's and 993's, use a relay for the fuel pump and the car will not operate if it fails. If you have the old technology mechanical flasher we recommend that you replace it with the new technology Best-in-Class® DME Solid State Fuel Pump Relay. You can then keep the old one in your glove box as a spare.

Where is the DME Relay on My Car?

Carrera 3.2 1983–89

Beneath the driver’s seat, next to the ECU.

964-model 911 Carrera 2/4 (but not Turbo), 1989–93; 993-model 911 Carrera 2/4 and Turbo, 1993–96
Inside the main fuse box at the right-hand rear corner of the luggage compartment. The 964 Turbo (but not the 993) has a conventional fuel-pump relay located in the fuse box.

How Does a Relay Work?

When a low-current signal (usually from a switch or control module) is applied to the relay's control input, an electromagnetic field is created within the relay. This field causes a movable armature (a metal lever) to pivot, which in turn causes a set of contacts to move from their normally open or closed position.

The contacts in a relay are like tiny switches that can carry much higher current and voltage than the control signal. When the contacts move to their new position, they allow or disallow the flow of current through the high-current circuit. For example, when you turn on your headlights, the switch sends a low-current signal to the headlight relay, which then allows high-current power to flow to the headlights, turning them on.

Relays are important in cars because they allow low-current control signals to safely and reliably control high-current circuits. Without relays, the switches and control modules would have to handle the high currents directly, which can cause them to fail prematurely or even catch fire. Relays are also used to isolate sensitive control circuits from noisy or high-voltage circuits, ensuring that the controls function properly even in the presence of electrical interference.

Is a Relay Essentially an On/Off Switch?

Yes, in a way, a car relay can be thought of as an on/off switch. The relay's contacts can be in one of two positions, either open or closed. When the relay is energized (by applying a control signal), the contacts move from their normal position to the opposite position, allowing or interrupting the flow of current through the high-current circuit.

However, a relay is more than just a simple switch. It provides an isolated and protected interface between the low-current control circuit and the high-current load circuit. It also allows the control circuit to be located far away from the load circuit, as the relay can be located close to the load while the control circuit can be located remotely. Additionally, relays can have multiple contacts and be used in more complex circuits to switch multiple loads, or to switch between different circuits based on the control input.

What is the Difference Between Mechanical and Electronic Relays?

The main difference between mechanical and electronic car relays is in how they operate and respond to control signals.

Mechanical relays are the more traditional type of relay, and they use physical components to make and break electrical connections. When a control signal is applied to a mechanical relay, it energizes an electromagnet that causes a metal armature to move and switch the contacts. The contacts in a mechanical relay are usually made of metal and can withstand high currents and voltages.

Electronic relays, on the other hand, use solid-state components such as transistors, diodes, and resistors to switch the load circuit. They do not have moving parts, and instead, they rely on electronic signals to control the flow of current. When a control signal is applied to an electronic relay, it activates a circuit that allows current to flow through the load circuit.

What Are Advantages of Electronic Relays over Mechanical Relays?

The main advantages of electronic relays over mechanical relays are that they are faster, quieter, and more reliable. They have no moving parts that can wear out or break, and they can switch much faster than mechanical relays, which is important in some applications. Additionally, electronic relays consume less power and generate less heat than mechanical relays, which can be important in certain automotive applications.

However, mechanical relays are still widely used in cars and other vehicles because they are rugged, relatively inexpensive, and can handle high currents and voltages. Additionally, some automotive systems require the use of mechanical relays due to their specific electrical characteristics or to meet certain safety standards.