A 5 pin relay acts as an electromagnetic switch that lets a low-current trigger circuit operate a high-current load safely. When you wire one into an automotive, marine, or custom electronics project, you protect sensitive switches from heavy electrical draws.
Every terminal on a standard Bosch-style relay serves a specific electrical function. Knowing how pin 30, 85, 86, 87, and 87a interact prevents blown fuses, melted wiring harnesses, and failed components. This guide breaks down the exact pinouts, shows you how to integrate the circuit, and helps you diagnose common wiring mistakes.
Understanding the 5 Pin Relay Pinout
Standard automotive relays use the DIN 72552 numbering system. While manufacturers might alter casing shapes or add mounting tabs, the terminal numbers remain consistent across almost all standard SPDT (Single Pole Double Throw) relays.
- Pin 30: Common feed or input. This terminal connects directly to your power source, usually the battery positive terminal or a heavy-gauge fused power distribution block.
- Pin 85: Coil ground. This pin completes the low-current control circuit by connecting to a chassis ground or a switched ground output from a control module.
- Pin 86: Coil trigger. This pin receives the positive 12V signal from your switch, ECU, or ignition source to energize the internal electromagnet.
- Pin 87: Normally Open (NO) output. When the relay is at rest, this pin has no connection to pin 30. When energized, an internal contact closes, routing power from pin 30 out to your load.
- Pin 87a: Normally Closed (NC) output. When the relay is at rest, power flows from pin 30 through pin 87a. When the coil energizes, this connection breaks, cutting power to whatever is attached here.
If you're working with standard automotive accessories like off-road lights, fuel pumps, or electric cooling fans, you will primarily use pins 30, 85, 86, and 87. Pin 87a remains unused unless you need a circuit to turn off when the relay activates, such as in kill switches or interlocking security setups.
Step-by-Step Circuit Integration
Wiring a 5 pin relay requires separating your project into two distinct circuits: a low-amperage control circuit and a high-amperage power circuit. Mixing these up or using improper wire gauges creates severe fire hazards.
- Disconnect the Power: Disconnect the vehicle battery negative terminal before starting any wiring work to prevent accidental short circuits or sparks.
- Mount the Relay: Secure the relay body to a stable surface using its mounting tab. Keep it away from extreme exhaust heat and direct moisture spray if possible.
- Wire the Power Circuit (High Current): Run a heavy-gauge wire (such as 12 AWG or 10 AWG depending on your load) from the battery positive terminal through an inline fuse holder, then connect it to Pin 30. Next, run another heavy wire from Pin 87 to the positive terminal of your load device (like a light bar or horn). Ground the negative side of the load directly to the vehicle chassis.
- Wire the Control Circuit (Low Current): Run a lighter-gauge wire (18 AWG) from a switched power source or dash toggle switch to Pin 86. Connect Pin 85 to a clean, bare-metal chassis ground point.
- Test the System: Reconnect the battery, turn on your switch, and listen for an audible click from the relay housing. Test the load with a multimeter to verify that full operating voltage reaches the device.
For authoritative technical specifications on standard automotive switching components, you can review documentation provided by Bosch Mobility.
Preventing Inductive Kickback
When you turn off a relay, the collapsing magnetic field inside the coil generates a sudden high-voltage spike known as inductive kickback. This voltage spike can reach several hundred volts for a microsecond, slowly pitting mechanical switches or frying sensitive digital controllers and ECUs.
To stop this, look for a relay that features an internal quenching diode, or wire an external 1N4007 flyback diode across pins 85 and 86. The banded side of the diode must face the positive trigger side (pin 86), while the non-banded side connects to the ground side (pin 85). This safely loops the voltage spike back into the coil, harmlessly dissipating it as heat.
Troubleshooting Common Relay Failures
Even with a clean installation, relays can fail over time due to vibration, excessive amperage draw, or moisture intrusion. When a circuit stops working, don't just replace the relay blindly. Use a logical diagnostic workflow.
- The Relay Clicks, But the Load Doesn't Work: The control circuit is fine, but power isn't reaching or passing through the heavy load side. Check your inline fuse on pin 30, inspect the connection crimps, or test if pin 30 has a live 12V supply using a multimeter.
- The Relay Doesn't Click At All: The control circuit is failing. Test for 12V at pin 86 while the dashboard switch is activated, and verify that pin 85 has continuity to a solid chassis ground. If voltage is present on both coil pins but no click occurs, the internal coil wire is broken.
- The Relay Buzzes Rapidly: This usually happens when the control circuit suffers from low voltage or a poor ground connection. The relay pulls in, drops out instantly as voltage sags, and repeats the cycle rapidly. Clean your ground points and check battery state.
- The Relay Body is Hot to the Touch: Excessive heat indicates that the load amperage is pushing past the relay's rated capacity (typically 30A or 40A for standard automotive units), or that you have high electrical resistance due to a loose terminal crimp.
Frequently Asked Questions
Can I use a 5 pin relay as a 4 pin relay?
Yes. If you leave pin 87a completely empty and unconnected, the 5 pin relay functions identically to a standard 4 pin SPST relay. Just insulate the unused terminal spade with heat shrink tubing to prevent accidental shorting.
What size fuse should I put on pin 30?
Your fuse rating must match the maximum current draw of your load device, and it must also match the amp capacity of your wire gauge. For example, a 30A relay paired with 12 AWG wire typically uses a 20A or 25A fuse to protect the circuit safely.
Does it matter which way pins 85 and 86 are wired?
If your relay does not have an internal diode or resistor, polarity doesn't matter for the coil pins. However, if your relay includes an internal suppression diode or resistor, reversing pins 85 and 86 will cause a direct short circuit when you flip the switch, instantly blowing your control fuse.
Why do some relays include pin 87a?
Pin 87a provides a normally closed path, meaning power flows through it when the relay is resting. It's used in specialized applications like horn circuits, backup systems, or dual-speed blower motor controls where a circuit needs to remain active until a trigger cuts power.
