Relay Symbols, Terminal Numbers, and How to Trace Relay Circuits

Why Relays Exist

A relay allows a low-current circuit to control a high-current circuit without routing the high current through the control device. Consider the fuel pump. The fuel pump can draw 8-15 amps depending on the application. Running that current through the ignition switch, through the instrument panel wiring, and through a PCM output would require heavy gauge wiring throughout and would stress those components.

Instead, a relay sits near the fuel pump. The PCM sends a small control signal — less than 1 amp — to the relay coil. The energized coil closes the relay contacts, and battery power flows directly through the contacts to the fuel pump through short, heavy-gauge wiring. The PCM never handles the pump's full current draw.

Relays also allow modules to control loads they are not directly wired to, allow one circuit to trigger another, and provide normally-closed outputs that require power removal rather than power application. They are everywhere on modern vehicles — every fuse and relay box contains dozens of them.

The Relay Symbol on Schematics

On a wiring schematic, a relay is drawn as two distinct parts in close proximity or enclosed in a dashed box. The first part is the coil — shown as a rectangle or a series of loops (the universal symbol for an inductor/electromagnet). The second part is the contacts — shown as a switch symbol, either normally open or normally closed.

The coil and contacts are electrically separate on the schematic. They share a label or are enclosed in the same dashed border to indicate they belong to the same physical relay, but they are on different circuits. The coil is on the control circuit side of the schematic. The contacts are on the load circuit side.

Some manufacturers show the coil and contacts separated across different parts of the schematic, connected only by the relay identification label. This is why knowing the relay's terminal numbers matters — it allows you to identify which schematic symbol connects to which terminal on the physical relay, even when they are on different pages.

Standard Terminal Numbering

Most automotive relays follow the ISO (International Organization for Standardization) terminal numbering standard:

Terminal 85: One end of the coil winding. Usually the ground side of the coil, connected to ground or a module ground output.

Terminal 86: The other end of the coil winding. Usually the power side, connected to a switched power supply or a module power output.

Terminal 30: The common contact. This is where battery power (or whatever supply the relay is switching) is connected. Think of it as the "input" to the load circuit.

Terminal 87: The normally open contact. Not connected to terminal 30 when the relay is de-energized. Connects to terminal 30 when the relay coil is energized. This is where the load is connected in most applications.

Terminal 87a: The normally closed contact. Connected to terminal 30 when the relay is de-energized. Disconnects when the coil is energized. Used in circuits where you want the default state to be powered — like some HVAC controls or failsafe circuits.

These numbers are often printed on the relay housing itself. They are also shown next to the relay symbol on the schematic. When the schematic says terminal 87 connects to the fuel pump, that tells you exactly which relay terminal and which contact to follow.

Tracing the Coil (Control) Circuit

The coil circuit is the low-current control side of the relay. It is typically powered by a switched supply — ignition on, run/start, accessory — and controlled on the ground side by a module or a switch.

To trace the coil circuit: find terminal 86 on the schematic and follow that wire backward to its power source. Note what circuit supplies power to the coil — is it ignition-switched, battery-direct, or a relay output? Then find terminal 85 and follow it to its ground path — is it a direct chassis ground or a module-controlled ground?

The relay coil energizes when terminal 86 has power AND terminal 85 has a complete ground path. If either is missing, the coil cannot energize, the contacts stay open, and the load circuit is dead. When diagnosing a relay circuit, the first step is to verify both sides of the coil circuit.

Pro Tip: You can check whether a relay coil is being energized without removing the relay. With the circuit commanded on, put your voltmeter across terminals 85 and 86 (back-probe the relay socket if accessible). You should see near-zero volts — meaning voltage is present on both sides of the coil and the coil is energized. If you see battery voltage, one side of the coil has no power or no ground.

Tracing the Contact (Load) Circuit

The contact circuit is the higher-current load side. It starts at terminal 30 (power input) and routes through terminal 87 (normally open output) to the load when the relay is energized.

Trace terminal 30 back to its power source — usually a fuse in the underhood fuse box fed directly from the battery or main power distribution. Then trace terminal 87 forward to the load — the fuel pump, the cooling fan motor, the A/C compressor clutch. Between terminal 87 and the load you may find additional fuses, connectors, or splices.

If the relay is energizing (coil circuit confirmed working) but the load still does not operate, the fault is in the contact circuit — terminal 30 has no power, the relay contacts themselves are burned or fused, terminal 87 wiring is open between the relay and the load, or the load itself has failed.

Normally Closed Relay Contacts

Not all relays use the normally open contact. Some circuits require normally closed operation — power is supplied through terminal 87a by default, and energizing the relay removes power from the load.

An example is some blower motor circuits where the relay de-energizes to allow blower operation in a specific mode. Another example is failsafe circuits — if the control module loses power or fails, the normally closed contacts keep a critical function powered without any module command.

When tracing a circuit using terminal 87a, remember the logic is inverted from a typical relay application. The load is on when the relay is off, and the load is off when the relay is on. This matters when you are testing and commanding the relay with a scan tool or a test jumper.

Testing Relays

A relay can fail in several ways: coil open (won't energize), coil shorted (draws too much current, may blow fuse), contacts welded closed (load always on), or contacts burned open (load always off despite energized coil).

Bench test procedure: Remove the relay. Set your DVOM to ohms. Measure resistance between terminals 85 and 86. You should read the coil resistance — typically 60 to 100 ohms depending on the relay. Infinite resistance means the coil is open. Near-zero resistance means the coil is shorted.

Next, check terminal 30 to 87 with the coil de-energized — should read infinite (open). Check 30 to 87a — should read near-zero (closed). Now apply 12 volts to terminal 86 and ground terminal 85. You should hear a click and feel the relay snap. Now re-check 30 to 87 — should be closed (near zero ohms). And 30 to 87a — should be open.

If the relay passes this bench test, the relay itself is good. Put it back and look for the problem in the coil circuit (no control signal) or the contact circuit (no power at terminal 30, open wiring to load).

Relay Diagnosis in Practice

When a relay-controlled circuit is dead, work through it systematically. Do not just swap the relay and call it good — that wastes parts and time if the relay is not the problem.

Step 1: Is the relay energizing? With the circuit commanded on, tap the relay with a finger. You should feel a faint vibration or click as it operates. A scan tool can command many relay-controlled circuits on and off — use it. You can also use a test light on terminal 87 with the relay removed — insert the test light lead in the terminal 87 socket position. When the relay should be on, probe terminal 30 with power and terminal 87 should show power if the relay contacts close.

Step 2: If not energizing — check power to terminal 86 and ground at terminal 85. A module that is not providing ground to terminal 85 may have its own fault, or the module may simply not be seeing the trigger condition. Use a scan tool to check for related codes and parameter data.

Step 3: If energizing but load still dead — verify power at terminal 30. Check terminal 87 for voltage when relay is on. If terminal 87 has voltage but load does not operate, the problem is between terminal 87 and the load, or in the load itself.

Frequently Asked Questions