What's a Starter Relay

The starter relay is a switch that operates between the ignition circuit and the starter circuit. It allows a small current to control the flow of the high electricity. This protects the ignition switch and ensures the starter system’s current stays under control. 

Where is the Starter Relay Located

Starter relays are typically used in motorcycles, vehicles and appliances like starter relay switch refrigerators. You can find the vehicle’s starter relay in these locations:

·In the fuse box under the hood. This is the most common location. 

·On the inner fender. The starter relay is often a cube-shaped part.

What are Parts of the Starter Relay

When you turn the ignition key and start the engine successfully, these components inside the starter relay work together.

Coil Winding

Made of copper wire, the coil winding is wrapped around an iron core and receives the current from the ignition circuit. Together with the iron core, it forms an electromagnet.

Iron Core

As the ignition circuit passes a small current through the coil, the iron core becomes magnetized. The magnetic force enhances, providing the power to move the armature.

Armature

The armature plays a role in connecting and disconnecting the contacts. It’s made of a magnetic material that is attracted by the magnetic field. And one end touches the contacts, while the other is attached to a spring. The spring helps the armature return rapidly to its original position. Therefore the armature usually is designed to be lightweight, easy to magnetize, and durable enough to handle thousands of  impacts and movements. 

Spring

Once the ignition circuit is off, the iron core loses magnetism. The spring then pulls the armature back to its original position, cutting off power to the starter motor. 

Contacts

The contacts inside the relay are normally open (NO, terminal 87). The magnetic field moves the armature to connect the contact. Since these contacts experience frequent physical friction, they need to be wear-resistant and efficiently conductive.

Terminals

Typically starter relays have four exposed terminals—30, 85, 86, and 87. That‘s why the starter relays are often called “4-pin relays”. And also there’s a 5-pin version that includes an additional terminal (87a):

·Terminal 30: receives power from the battery.

·Terminals 85 & 86: Both are connected to the coil. Terminal 85 is usually grounded, and Terminal 86 connects to the control ignition switch.

·Terminal 87: The normally open contact. When the ignition is on, Terminal 30 connects to Terminal 87. When the ignition is off, they disconnect.

Besides these key parts, the starter relay also includes a yoke for support, necessary insulation layers, and the relay housing. The housing is often made of insulating material to protect the internal components from other parts of the vehicle.

How does the Starter Relay Work

A typical starter relay in the fuse box labels terminal 85 and terminal 86. When you turn the ignition key to "start", the iron core is magnetized. The armature is then attracted by the iron core and closes the NO contact. A strong current from the battery flows through the contact to the starter solenoid. The starter motor activates.

When you turn the ignition key back to the "ON" position, the current stops and the iron core loses its magnetic field. Without the magnetic force, the spring pulls the armature back to its original position. The contacts open and cut off power to the starter circuit.

If your starter relay is mounted on the fender, its terminals are labeled differently: “I" and "S" instead of "85" and "86". The "I" terminal (terminal 85) connects to the vehicle body or ground.

What does a Starter Relay Do

The basic function of a starter relay is to use a small current from the ignition circuit to control the larger current in the starter circuit. However, its role isn’t limited to just remote operation and circuit protection.

One of its versatile features is the ability to take a single input and deliver multiple outputs. For example, the 87a terminal in a five-pin relay acts as a normally closed contact that can be connected to other vehicle devices. If you engage the starter, the starter relay can cut power to certain accessories to prevent overload, like the audio system or air conditioning.

Starter Relay vs Starter Solenoid

The starter relay often cooperates with the starter solenoid to protect the starter. You may confuse these two devices as the same; however, they cannot replace each other and differ significantly in several ways.

Function

An armature of the starter relay is attracted by the coil and moves to close the starter circuit. This completes the starter relay’s action.

The starter solenoid engages the starter’s pinion gear with the engine’s flywheel and it functions  as a switch, sending battery current to the starter. Only after both actions are complete is the solenoid’s task done.

Because of these, the solenoid has a more complex structure than the relay, and its core must be made of higher-quality, more durable materials.

Current Load

According to the vehicle makes and models, the controlled current of the starter relay or the starter solenoid can vary. However, the relay’s is much lower than that handled by the starter solenoid. You can see the difference in their current ratings:

The starter relay receives about 1A from the ignition switch.

The starter solenoid’s coil carries 20A-40A, significantly higher than the relay.

Location

You can find that a starter solenoid is mounted directly on or very close to the starter motor. That’s because it needs to reduce power loss through the circuit and allows mechanical engagement with the starter and the engine.

The starter relay is usually located in the fuse box, using thinner wires to carry control signals and enable remote operation.

How to Tell If the Starter Relay is Bad

If the starter relay is of poor quality or lacks durability, it can easily overheat. The parts, including the contacts, wiring and other components, may wear out prematurely and lead to the relay  malfunction. You may notice the following symptoms:

Bad Starter Relay Sound

The starter doesn’t engage, but you hear a distinct clicking or buzzing sound. It occurs when there is severe wear or dirt buildup. The relay tries to connect the NO contact repeatedly, but the current can’t flow through the circuit. 

Engine Won’t Start

You try turning the key repeatedly, but the engine doesn't crank. Either the battery or the starter relay may be the cause and you need to further diagnose. It may be due to worn contacts, damaged wiring or the aging of the relay.

Starter Keeps Running

The starter circuit remains closed continuously. The relay fails to perform its function, which can eventually damage the entire starting system.

Intermittent Starting

Normally starting the engine starts quickly and reliably. But due to a faulty relay, you might experience occasional ignition failures. It occurs when the contacts are dirty or the relay overheats. 

How to Check Starter Relay

If you notice any of the above signs and judge the relay might be faulty, please prepare the following tools before further tests:

·Insulated tools and insulated gloves

Avoid direct contact with the terminals using your bare hands.

·A properly charged 12V battery.

·A multimeter set to the ohmmeter (Ω) setting. 

We’ll use resistance values to check the relay and circuit condition.

·Two jumper wires and a metal scraper.

Step 1: Park the Vehicle Safely

Park your car in an open area with little traffic. Put the transmission in neutral to prevent the vehicle from moving during testing and avoid accidents.

Step 2: Remove the Relay

Based on your vehicle’s make and model, the relay may be located in different positions-fuse box, fender or fuse panel.

If it’s in the fuse box, there are labels “Starter”/“Start” on the box. Find the correct location of the starter relay, and then simply pull it out by hand.

If it’s on the fender or elsewhere, use a wrench and socket to remove the nuts and screws holding the relay in place.

Step 3: Inspect the Relay

After removing the relay, check its casing for signs of melting, and examine the terminals for corrosion, dirt, or oil.

If not, you can judge that the external parts are all right and check for other possibilities.

If the casing is melted or terminals are corroded, replace the relay immediately to avoid further damage to the starting system.

If there’s only dirt, proceed to Step 4. Then clean the terminals with a metal scraper and test again to see if dirt causes the issue.

Step 4: Test the Starter Relay Coil (Off-Vehicle)

Connect the multimeter probes to terminals 85 and 86.

Normal: Resistance between 50–120 Ω.

Abnormal:

Infinite resistance (OL or 1 on the meter) means open coil

Near zero or 0 Ω, the coil circuit is shorted.

Step 5: Test the Relay Switch (Off-Vehicle)

Connect the multimeter to terminals 30 and 87. When the relay is off, the resistance should be infinite.

Use the 12V battery to offer exterior power and connect the positive & negative to terminals 85 and 86 to simulate real operation.

Good starter relay: You’ll hear a click as the armature moves and the contacts close. The multimeter should show near 0 Ω.

Faulty starter relay: Internal parts are damaged or contacts are burned. The multimeter should show infinite resistance.

Based on the results above, you can identify the real reason. Or the issue may lie in the external wiring or a fuse. 

How to Replace a Starter Relay

Once the relay is damaged, it's generally recommended to replace it directly with a new starter relay. If you have some repair experience with auto parts, you can replace it yourself—depending on the relay’s location and difficulty of access. Doing it yourself would only cost you the price of the part.

FAQs

How Much is a Relay Switch for a Car

The total replacement cost typically ranges from $50 to $75 (including labor).

How to Check Starter Relay Fuse

First safely park the vehicle, disconnect the battery, and locate the starter relay.（ follow the initial steps for "How to check starter relay"）

By checking the fuse appearance, you can look for burn marks or blackening to tell if the fuse is blown. If you confirm the relay is faulty, simply replacing it is a quick and easy solution.