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Upgrade your vehicle's electrical system effortlessly with this comprehensive guide
If you’re considering adding extra lights, a powerful stereo system, or any other high-power accessories to your car, a relay is indispensable. A relay is a compact electrical device that allows you to control high-power devices or accessories using a low-power switch. While it's advisable to have a professional mechanic handle any electrical work on your vehicle, wiring a relay becomes much simpler when you understand its functionality. This article provides an overview of the purpose of a relay, safe wiring techniques, and guidance on replacing or installing relays in your vehicle. Let's dive in!
Essential Information
- Connect the relay's 30 terminal to the positive terminal of the vehicle battery.
- Ground the relay's 85 terminal to a nearby grounding point on the vehicle's engine or the negative terminal of the battery.
- Wire the 86 terminal to the switch that activates the relay and controls the device or accessory you wish to connect.
- Connect the 87 terminal (and 87a terminal if present) to the device(s) powered by the relay.
Procedure
Understanding Relay Terminals
Select a relay and wires suitable for your vehicle and device's requirements. If you're replacing a worn-out relay, simply opt for an identical one. Refer to your vehicle's service manual or the relay's specifications for voltage, amperage, and part number. If uncertain, seek advice from a car dealership, auto shop, or mechanic.
- Confirm the type of relay required—SPST or SPDT—and whether the large circuit needs to be normally open (NO) or normally closed (NC) when the relay is inactive.
- For a new relay system, consult an automotive amperage chart to determine wire gauge and ensure the relay can handle the power demand of your device.
Connect the vehicle battery's positive terminal to the relay's 30 terminal. Ensure the wire's secure attachment and consider using a ground wire connector or loosening the battery's screw terminal for new wire connections. Use a wire crimper and blade connector to attach the wire to the relay.
- Wire between the battery and relay must include a circuit breaker or fuse to prevent battery overload.
- Identify relay terminals by their numbers and refer to actual terminal markings for accuracy.
Ground the relay's 85 terminal to a nearby grounding point. Choose an engine-based grounding point for most accessory relays and attach the wire using a ground wire connector. Connect the wire to the 85 terminal using a wire crimper and blade connector.
- Consider using the battery's negative terminal as a ground point if your device is not sensitive to other currents.
- This wire directs power from the relay to the car's metal frame in case of a short circuit.
Wire the switch to the relay's 86 terminal. This terminal supplies low power from the switch to energize the relay's coil. If replacing an existing relay, the wire is likely already connected. Connect the wire to the switch according to its design.
- For many switches, loop the wire around a screw inside the switch and tighten it.
- In relays without a safety resistor or diode, terminals 85 and 86 roles can be interchangeable.
Connect the device or accessory to the relay's 87 terminal. This wire carries power from the relay to activate the device. Use a blade connector and wire crimper to attach the wire to the terminal. The method for wiring the device varies based on its type and power requirements.
- For an SPDT relay with a fifth terminal (87a), connect it to the appropriate device as well.
- Once all connections are secure, the relay is ready for operation.
- In some vehicles, a 'ponytail' plug consolidates all necessary wires into one bundle. If present, plug the relay into the bundle to connect the wires.
Upgrading an Existing Relay
Remove old relays from the fuse box and install new ones of the same type. Vehicle relays are typically located near the fuse box under the hood. Manufacturer-installed relays can be unplugged and replaced with identical ones. Turn off the ignition, then remove the old relay and insert the new one.
- Some relays may have a release tab for removal. Push the tab while pulling the relay away.
- If the desired relay isn't under the hood, check under the dashboard.
Installing a New Relay & Switch Setup
Position the new relay between the switch and the powered device. Choose a location that minimizes wire length between the switch, relay, device, and car battery. If wiring is external, use a mounting bracket for automotive relays on a suitable spot in the engine bay. For internal wiring or limited space, opt for a location under the driver's side dashboard.
- Some vehicles have spare spots in the fuse bank or relay box for additional relays, eliminating the need to mount one in the engine bay.
Route wires from the switch to the relay through the vehicle’s firewall. Beneath the driver’s side dashboard, locate a rubber grommet in the firewall leading to the engine compartment. Create a hole in the rubber using a screwdriver, then thread the wires through from the passenger compartment side.
- Utilize a long wire grabber to retrieve the wires once they pass through the firewall and pull them toward the relay.
- The wire linking the switch to the relay must pass through the firewall. Additional wires passing through depend on the relay's location and the device it powers.
- The firewall separates the engine compartment from the passenger compartment.
Fix the switch onto the driver’s side door using double-sided adhesive. Refer to your switch or relay kit's instructions and your car’s service manual for proper mounting. Typically, affixing the switch to the door with double-sided adhesive suffices, facilitating easier access and reducing wire length to reach the relay.
Understanding Relay Operation
Relays function by using an electric signal from a switch to close a larger circuit. For instance, when activating offroading LED lights atop your vehicle, pressing the button sends an electric signal to the relay, energizing an electromagnet inside. This magnetic field causes metal contacts to touch, completing a larger circuit to power the lights.
- Turning the switch to “Off” de-energizes the electromagnet, causing the metal contacts to separate and open the circuit, turning off the lights.
- With this system, a low current from the switch can activate a high-current circuit.
Relays optimize power and space in automotive applications. Found throughout the vehicle’s electrical system, relays regulate high-current flows from low-current switches. This allows for thin wires connecting the switch to the relay, with heavy wires only extending to the relay from the power source, not to the device it powers. Consequently, this reduces the need for bulky, heavy cables to power the vehicle.
- Additionally, relay systems ensure power travels the shortest distance from the source to the device, minimizing voltage loss.
Types of Relays in Vehicles
SPST Relays with 4 Terminals These relays are widely found in cars, trucks, and other vehicles. They typically feature a small cubicle or rectangular shape with 4 terminals, labeled 30, 85, 86, and 87. Here's what each terminal does:
- Terminal 30: Connects high power from the vehicle’s battery into the relay.
- Terminal 85: Grounds the relay to the vehicle's body.
- Terminal 86: Brings low power into the relay from the switch.
- Terminal 87: Sends power out of the relay into the device it’s powering.
SPDT Relays with 5 Terminals Also known as “changeover” relays, these are similar to SPST relays but include a fifth terminal, 87a. Here's how they work:
- Terminals 87 and 87a each control different circuits, never simultaneously.
- SPDTs are less common but useful for functions like electric windows or trunk door motors.
Mini and Micro Relays These smaller relays in modern vehicles handle the same power as older, larger ones but in a smaller size. They're labeled 1 to 5 corresponding to DIN-standard numbers and may include built-in time delays or resistors for protection.
- Relays may have a delay between hitting the switch and energizing, adjustable in some cases.
- Some relays include a resistor or diode to protect electronics from voltage surges.
Helpful Advice
Cautionary Notes
- Handle relays with care to avoid internal damage, which could result in wiring issues or even fires. Avoid making alterations to relays for safety reasons.
- Avoid working with relays (or any electrical components) in areas containing flammable or explosive gases, such as gasoline or other fuels.
