Relay Buyer's Guide
A relay is an electronically controlled switch. It acts as an intermediary, letting a small electrical current safely control a high-power component like a headlight, fuel pump, or cooling fan. Without relays, the full current needed to power these components would have to flow through your dashboard switches, creating both a fire risk and a wiring headache. This guide covers how relays work, what the key specifications mean, and how to choose the right relay for your application.
How Do Relays Work?
A relay lets a small current control a much larger one. Inside a typical relay are three parts: an electromagnetic coil, a spring-loaded switch, and a set of electrical contacts. The sequence of a relay is:
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The driver flips a switch, allowing a small control current to flow into the coil.
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The coil generates a magnetic field, pulling the spring-loaded switch closed.
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With the switch closed, the high-current circuit is complete and the component turns on.
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When the control current stops, the spring pulls the switch back open and the circuit breaks.
For example: When you flip your headlight switch, a small signal travels to the relay's coil. The relay closes its internal switch, allowing power to flow directly from the battery to the headlights through thick, dedicated wiring that never needs to pass through your dashboard.
Why Are Relays Important?
Relays should be used when powering any high-current component, stock or aftermarket, including lights, fans, fuel pumps, starter motors, audio systems, and winches. Relays solve two problems at once:
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Safety: High-current circuits need thick wiring to avoid overheating. If that current had to pass through your dashboard switches, those switches and their wiring would have to be far more robust to prevent a fire. Relays keep the high-current path short and dedicated, while the wires to the dashboard only need to support small current signals.
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Simplicity: A single thin control wire can switch a high-current circuit anywhere in the vehicle. This means you don't need to run heavy gauge wiring all the way from the battery to every switch on the dashboard.
Reading the Pin Numbers
Relays follow a standardized pin numbering system. Once you know what each pin does, you can wire any relay:
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Pin 30: Power input, connected directly to the battery or fused power source.
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Pin 85: Ground.
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Pin 86: Control signal input. Energizing this pin activates the relay coil.
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Pin 87: Normally Open (NO) output. Current flows through this pin when the relay is activated.
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Pin 87a: Normally Closed (NC) output. Current flows through this pin when the relay is at rest. Only present on 5-pin relays.
Number of Pins
The number of pins determines how many output circuits the relay controls. For most applications, a 4-pin normally open relay is what you want.
4-Pin relays have a single output, in one of two configurations:
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Normally Open (NO): At rest, no current flows. Energizing the relay closes the circuit and turns the component on. This is the standard configuration, used any time you want something to turn on when power is applied.
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Normally Closed (NC): At rest, current flows freely. Energizing the relay opens the circuit and turns the component off. Used when you want something to turn off when power is applied, a less common need.
5-Pin Relays (SPDT, or changeover relays) have both an NO and an NC output, letting the relay switch between two separate circuits. A common example is switching between low beam and high beam headlights, where the relay routes power to one circuit or the other depending on its state.
Current Rating
The current rating tells you the maximum continuous current the relay's contacts can safely handle. If you choose a relay rated below your component's current draw, the contacts will overheat and eventually fail.
Rule of thumb: Check the maximum current draw of the component you're powering, then choose a relay rated for at least that much, ideally with some headroom. Common automotive relay ratings are 20A, 30A, and 40A.
Coil Suppression
When a relay coil is de-energized, it generates a brief voltage spike. Left unmanaged, this spike can damage sensitive electronics, particularly your ECU. Relays handle this in one of two ways:
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Diode-suppressed: A diode absorbs the spike completely. The safer choice when the relay's control circuit connects to sensitive electronics.
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Resistor-suppressed: A resistor reduces the spike but doesn't eliminate it entirely. Releases slightly faster than a diode-suppressed relay.
If you're unsure which to choose, and especially if the relay is triggered by your ECU, go with diode-suppressed.
Relay Types
Mechanical Relays are the standard design, the kind you'll find in nearly every fuse and relay box. They use physical contacts and a spring-loaded switch. Reliable and inexpensive, though the moving parts will wear over time.
Solid State Relays (SSRs) use transistors instead of physical contacts. With no moving parts, they last longer and switch faster. You'll increasingly find these on modern vehicles, though they're more sensitive to heat than mechanical relays.
Conclusion
Relays are a small but essential part of any electrical system. Whenever you're adding a high-current component to your vehicle, the right relay keeps your wiring safe and manageable. For most applications, a 4-pin normally open mechanical relay rated above your component's current draw is exactly what you need. Reach for a 5-pin relay only when you need to switch between two circuits, and consider an SSR if longevity and switching speed matter for your application. You can find all the parts you need and plan your build on RideBuilder.
Additional Resources
