A relay can look like a simple black cube, but one incorrect terminal connection can leave a fan running constantly, a horn inoperative, or a fuse blown the moment power is applied. This relay pinout guide explains the terminal numbers found on common automotive relays so you can identify the control side, the load side, and the switching path before making a repair.

Most automotive relays use standardized terminal numbers, commonly called Bosch-style or ISO terminal designations. The relay housing, diagram, and connector cavity markings are more reliable than wire color alone, especially on older vehicles, replacement harnesses, and custom wiring projects.

Relay Pinout Guide: Know What Each Terminal Does

A typical 4-pin relay has terminals 30, 85, 86, and 87. A 5-pin relay adds terminal 87a. These numbers describe the relay's electrical function, not where the terminals sit physically on every relay. Always read the diagram printed on the relay case and match it to the socket or pigtail.

Terminal 30: Power into the switched circuit

Terminal 30 is the common contact. In most automotive installations, it receives fused battery power and becomes the source of power sent to the accessory when the relay closes. For a high-current circuit such as an electric cooling fan, auxiliary lights, fuel pump, or air horn, terminal 30 should be supplied through a properly sized fuse or circuit breaker located close to the power source.

Terminal 30 is not always battery positive. In some factory circuits, a relay may switch ground rather than power. The terminal number still identifies the common contact, so follow the vehicle wiring diagram before assuming its polarity.

Terminal 87: Normally open output

Terminal 87 is the normally open, or NO, contact. With the relay at rest and the coil not energized, terminal 30 and terminal 87 are disconnected. When the coil receives power and ground, the relay closes the connection between 30 and 87.

This is the terminal used for accessories that should only receive power when commanded on. A radiator fan, driving light circuit, or electric fuel pump is commonly wired from terminal 87 to the load. On a standard 4-pin relay, 87 is the only switched output.

Terminal 87a: Normally closed output

Terminal 87a appears on a 5-pin changeover relay. It is the normally closed, or NC, contact. At rest, terminal 30 is connected to 87a. Once the coil energizes, the internal contact moves away from 87a and connects 30 to 87.

That makes a 5-pin relay useful when one circuit needs power by default and another needs power only when the relay is triggered. It can also be used in switching and control logic. For ordinary accessory wiring, 87a is often unused. Do not connect it just because it is present.

Terminals 85 and 86: The relay coil

Terminals 85 and 86 operate the internal electromagnet, or coil. Applying the correct voltage across these two terminals pulls the relay contacts into their energized position. In a basic 12-volt system, one terminal receives a trigger power source and the other is connected to ground.

On a conventional non-polarized relay, 85 and 86 can usually be used either way. However, many modern relays contain a suppression diode. A diode-equipped relay is polarized: terminal 86 is typically positive and terminal 85 is ground. Reversing those connections can create a direct short through the diode and blow a fuse. Check the case diagram for a diode symbol before wiring the coil.

Some relays use a resistor across the coil instead of a diode. A resistor suppresses voltage spikes but is generally not polarity-sensitive. The relay symbol tells you what is inside, which is why the case diagram matters more than guessing based on pin position.

Reading the Diagram on the Relay Case

The small schematic molded or printed on a relay is the fastest way to confirm its function. You will see a coil symbol between 85 and 86, plus a movable switch contact connected to terminal 30. The contact will point toward either 87 or 87a depending on whether the relay is shown energized or at rest.

A line from 30 touching 87a indicates a normally closed connection. A gap between 30 and 87 indicates a normally open connection. When voltage is applied to the coil, the movable contact changes position. This simple drawing prevents a common mistake: treating every 5-pin relay like a 4-pin relay with one extra output.

Relay bases can be confusing because you may be looking at the connector from the wire side while the relay diagram shows the terminal side. Do not rely on the visual layout of the blades. Identify each socket cavity by its molded terminal number, then verify it with a meter if the markings are damaged.

How to Test a Relay Before Installing It

A bench test can save time when diagnosing a no-start, non-working fan, or intermittent accessory. Start by checking the relay coil with a multimeter set to resistance. Measure across terminals 85 and 86. A typical 12-volt automotive relay may show tens to a few hundred ohms, depending on its design. An open reading usually means the coil has failed.

Next, with the relay unpowered, check continuity between 30 and 87. A standard 4-pin relay should show no continuity. A 5-pin relay should usually show continuity between 30 and 87a. Then apply fused 12-volt power and ground to the coil terminals. You should hear or feel a distinct click. With the coil energized, continuity should move from 30-to-87a over to 30-to-87.

A click alone does not prove the relay is good. Burned or worn contacts can allow the coil to operate while preventing current from reaching the load. If possible, test continuity and voltage drop through the switched contacts under load. A relay that becomes hot, chatters, or drops voltage significantly should be replaced.

Wiring a Relay for an Accessory Circuit

For most added 12-volt accessories, terminal 30 receives fused battery power, terminal 87 feeds the accessory, terminal 85 connects to the switch or control trigger, and terminal 86 connects to a solid ground. This arrangement lets a low-current switch, temperature controller, ECU output, or factory trigger operate a higher-current load without routing full load current through the control circuit.

Wire size and fuse size must match the actual load and wire run. A relay does not make undersized wire safe. If an electric fan draws more current than the wire, fuse, terminals, or relay contacts can handle, the circuit can overheat even though the relay pinout is correct. Use the accessory manufacturer's running and startup current specifications, particularly for fan motors that have a high inrush draw.

The relay contact rating also matters. A 30-amp relay may be suitable for many lighting and horn circuits but may not provide enough margin for a large electric fan, winch control application, or dual-fan setup. Continuous load, startup load, ambient heat, connector quality, and duty cycle all affect the right choice.

Keep high-current connections clean and tight. Use correctly crimped terminals, sealed connectors where moisture is present, and a relay socket rated for the circuit. Loose female terminals inside a pigtail can create resistance and heat that looks like a relay failure. If the relay or connector shows melted plastic, inspect the load, wire gauge, fuse rating, and terminal tension before replacing parts.

Common Relay Pinout Mistakes

The most common mistake is powering terminal 87 and expecting terminal 30 to become the output. Although current can sometimes pass through closed contacts in either direction, wiring should follow the circuit design and relay diagram. Supplying the fuse-protected source to terminal 30 keeps troubleshooting consistent and reduces confusion later.

Another mistake is using terminal 87a as a second powered output. In a standard 5-pin changeover relay, 87a and 87 do not receive power at the same time. If you need two outputs that turn on together, use a relay specifically marked with dual 87 terminals. Those relays have two normally open outputs, often labeled 87 and 87b or simply 87 twice, and both outputs connect to 30 when energized.

Finally, do not bypass a relay by jumping terminals without understanding the circuit. Jumping 30 to 87 can be a useful diagnostic test, but it may activate a motor or fuel pump immediately. Use a fused jumper, keep clear of moving components, and never bypass safety-related circuits without the correct service information.

When a relay, socket, or pigtail has heat damage, replace the failed component and correct the reason it failed. EDS Parts can help you match the relay style, terminal count, connector, and electrical rating needed to get the repair finished with confidence.