How To Connect 5 Pin Relay: A Complete Step-by-Step Wiring Guide
A 5-pin automotive relay uses an internal electromagnetic coil to switch between two different circuits via a shared common terminal, allowing a low-power switch to safely control high-amperage accessories. Successfully connecting this component requires identifying standard ISO pin designations (Terminal 30, 85, 86, 87, and 87a) and matching them to your power source, ground, trigger switch, and load device.
Pre-Operation & Equipment Checklist
Working with automotive and industrial electrical systems requires careful preparation to prevent short circuits, blown fuses, and component damage. A standard 5-pin relay (commonly referred to as a Single Pole Double Throw or SPDT relay) is rated for specific voltage thresholds, typically 12V DC in automotive applications, with current-carrying capacities ranging from 30 amps to 40 amps. Before starting any wiring procedure, ensure you have gathered all necessary materials and reviewed basic electrical safety protocols. Disconnect the vehicle or system battery before making physical contact with live wires to eliminate the risk of electrical shock or accidental arc welding.
- Essential Gear and Tools: Standard 5-pin SPDT relay with a matching plastic relay harness socket, wire strippers, crimping tool, digital multimeter for continuity and voltage testing, heat shrink tubing, a heat gun, and appropriate gauge automotive wire (typically 14-AWG or 12-AWG for high-current paths and 18-AWG for trigger circuits).
- Mandatory Standards and Prerequisite Knowledge: Familiarity with Ohm's law, basic circuit continuity, and standard automotive color-coding conventions. Ensure components match system voltage requirements (e.g., matching a 12V relay to a 12V power supply).
- Time and Budget Benchmarks: Estimated project duration is 30 to 45 minutes for a single relay installation. Estimated budget for tools and components ranges from $10 to $30 USD.
Step-by-Step Relay Wiring Instructions
Step 1: Identify the Standard ISO Pin Designations
Inspect the bottom of the relay or the accompanying wiring harness to locate the numerical ISO terminal markings. Terminal 30 represents the common power input, Terminal 85 is the relay coil ground, Terminal 86 is the relay coil trigger input, Terminal 87 is the normally open (NO) output, and Terminal 87a is the normally closed (NC) output.
Pro-Tip: Always reference the printed schematic on the side housing of the relay body, as pin placement can occasionally vary on non-standard specialty relays.
Step 2: Wire the Control Circuit (Terminals 85 and 86)
Connect a low-amperage wire (18-AWG) to Terminal 86 and run it to your control switch or electronic control unit (ECU) trigger source. Connect another 18-AWG wire to Terminal 85 and route it securely to a clean, unpainted chassis ground or the negative battery terminal. This pairing forms the electromagnetic coil circuit that activates the relay when voltage is applied to the switch.
Warning: Reversing the polarity on standard relays equipped with an internal suppression diode or resistor can destroy the internal components instantly; ensure Terminal 85 receives ground and Terminal 86 receives positive switch voltage unless specified otherwise by the manufacturer.
Step 3: Connect the High-Current Power Input (Terminal 30)
Strip a section of heavy-gauge wire (12-AWG or 14-AWG) and crimp it securely to the wire leading to Terminal 30. Route the opposite end of this wire directly to a constant 12V power source, such as the positive battery terminal or a dedicated power distribution block.
Pro-Tip: Always install an inline fuse or circuit breaker on the wire feeding Terminal 30 as close to the power source as possible to protect your wiring harness from catastrophic short circuits.
Step 4: Route the Switched Output Load (Terminals 87 and 87a)
Determine whether your accessory requires power when the relay is activated or when it is at rest. Connect your target accessory (such as auxiliary lights, a fuel pump, or an electric fan) to Terminal 87 if you want it to turn ON when the switch is engaged (Normally Open). Alternatively, connect it to Terminal 87a if you want power flowing continuously until the relay is triggered, which cuts the circuit (Normally Closed). Crimp and seal all connections using adhesive-lined heat shrink tubing to prevent moisture corrosion.
Omron 8 Pin Relay Wiring Diagram
Technical Specifications and Pinout Comparison
| Terminal ID | Pin Designation | Wire Gauge (AWG) | Typical Function |
|---|---|---|---|
| Terminal 30 | Common Feed | 12 - 14 | Direct connection to high-amperage power source (Battery/Fuse Box) |
| Terminal 85 | Coil Ground | 18 | Path to chassis ground for the internal electromagnetic coil |
| Terminal 86 | Coil Trigger | 18 | Signal input from dashboard switch, ignition, or ECU |
| Terminal 87 | Normally Open (NO) | 12 - 14 | High-current output that energizes only when the relay is triggered |
| Terminal 87a | Normally Closed (NC) | 12 - 14 | High-current output that stays energized until the relay is triggered |
Common System Failures and Field Fixes
- Symptom: The relay clicks rapidly or buzzes when the switch is activated.
- Root Cause: Insufficient voltage or a poor ground on Terminal 85, often caused by a loose ground connection or an undersized wire starving the electromagnetic coil of necessary current.
- Actionable Fix: Clean the chassis ground connection to bare metal, secure the bolt tightly, and verify that the trigger wire provides a steady 12V under load.
- Symptom: The accessory operates in reverse (it is on by default and turns off when the switch is flipped).
- Root Cause: The accessory load wire was mistakenly plugged into Terminal 87a instead of Terminal 87.
- Actionable Fix: Relocate the accessory power lead from the Normally Closed (87a) terminal to the Normally Open (87) terminal.
- Symptom: The relay clicks, but the connected accessory fails to turn on.
- Root Cause: A blown inline fuse on the Terminal 30 power feed, or a loose crimp connection preventing high current from reaching the load.
- Actionable Fix: Test continuity across Terminal 30 and Terminal 87 using a multimeter while the relay is engaged, inspect the inline fuse, and recrimp any loose wire terminals.
Frequently Asked Questions
What is the difference between a 4-pin and a 5-pin relay?
A standard 4-pin relay features a single output terminal (Terminal 87) that operates exclusively in a Normally Open state, meaning power only flows when the relay is triggered. A 5-pin relay adds an extra terminal (Terminal 87a), providing a Normally Closed pathway that allows the relay to toggle power between two distinct circuits or accessories.
Can I leave Terminal 87a empty if I only need a standard on/off switch?
Yes, Terminal 87a can be left completely disconnected and insulated with heat shrink tubing if your application only requires a standard Normally Open switching function. Leaving this pin unpopulated will not interfere with the normal operation of Terminals 30, 85, 86, and 87.
Do I need a diode-protected relay for my installation?
Relays equipped with an internal suppression diode or resistor are highly recommended when connecting to sensitive automotive computers or ECU trigger outputs. The diode safely absorbs the high-voltage spike (inductive kickback) generated when the electromagnetic coil collapses, protecting downstream electronics from damage.
How do I test if a 5-pin relay is broken?
You can test a relay by using a digital multimeter set to continuity mode across Terminals 85 and 86 to verify coil resistance, which should show a clear numerical reading rather than an open circuit. Next, apply 12V power to Terminals 85 and 86 to listen for an audible click, and use the multimeter to confirm continuity switches from Terminal 30-87a over to Terminal 30-87.
What size wire should I use for wiring a 5-pin relay?
Use heavy-gauge wire, typically 14-AWG or 12-AWG, for the high-current circuit paths connected to Terminals 30, 87, and 87a to safely handle high amperage loads. For the low-current trigger and ground paths connected to Terminals 85 and 86, thinner 18-AWG wire is fully sufficient since minimal current passes through the control coil.
Mastering automotive electrical wiring requires precision and quality components to ensure long-term reliability and safety. Equip your workspace with the proper terminals and testing tools today to complete a professional, factory-grade relay installation.
