What is the function of a contactor switch?

In automotive applications, a contactor switch functions as a high-current relay—it is used to control large electrical loads (like motors, inverters, or winches) using a low-current control signal. Primary Functions of an Automotive Contactor Switch include:

Switching High Current Loads: It connects or disconnects heavy-duty circuits (e.g., starter motors, auxiliary batteries, or electric drive systems) without needing a large switch inside the vehicle.
Remote Control of Power: A low-current signal (from a key switch, ignition, or ECU) can activate the contactor, allowing for remote or automated control of high current systems.
Electrical Isolation: When open, it electrically isolates parts of the system for safety or energy savings (e.g., disconnecting an auxiliary battery when not in use).
System Protection: In some systems, contactors are used to quickly shut off power during a fault, like overcurrent or a short circuit.

How do you know if a contactor is bad?

1. No Click Sound When Energized

What it means: The contactor isn't actuating.

Likely cause: Coil failure, broken control circuit, or no power to the coil.

Test: Apply the correct voltage across the coil terminals. If it doesn’t click, the coil may be open or damaged.

2. High Resistance Across Contacts

What it means: Contacts may be worn, burned, or pitted.

Test: With the contactor closed (activated), measure resistance across the main terminals with a multimeter.

You should see very low resistance (typically <0.1 ohms).

Higher resistance suggests internal contact damage or corrosion.

3. Contacts Stay Stuck (Welded Shut)

Symptom: The load (like a motor or battery circuit) stays on even when the contactor is de-energized.

Cause: Contacts may have arced and fused together due to overcurrent or wear.

4. Burnt or Melted Housing

Visual sign: Discoloration, melting, or smoke damage.

What it indicates: Overheating due to excessive load, poor ventilation, or internal arcing.

5. Intermittent Operation

Symptom: Contactor works sometimes but not always.

Potential issues:

Loose coil connections

Failing coil winding

Dirty or oxidized contacts

Vibration-related mechanical failure

Diagnostic Tips:

Check control voltage at the coil when the system commands it to turn on.

Check continuity of the coil using an ohmmeter (no continuity = open coil).

Listen for clicking noise when it's energized.

Inspect terminals for corrosion, loose connections, or heat damage.

What is a battery contactor?

A battery contactor is a heavy-duty electrical switch that controls the connection between a battery and the rest of an electrical system—typically used in automotive, marine, RV, or industrial vehicle applications. It allows a low-current control signal to safely and remotely switch high current battery power on or off.

To maximize safety and system longevity, we recommend pairing your battery contactor or electrical switching relay with compatible circuit protection and power distribution components:

Battery Disconnect Switches – isolate batteries during service or downtime

Electrical Circuit Breakers – protect contactors from overload or short-circuit

Power Relays – for signal-level switching and load control

Wiring Accessories – including loom, clamps, and cable ties

Whether you’re managing power in emergency vehicles, off-road equipment, or industrial applications, Waytek offers the contactor solutions you need—in stock and ready to ship.

Need help choosing the right contactor for your DC circuit or battery system? Contact our sales team at 800-328-2724 or email sales@waytekwire.com for expert assistance

Relays Resources

Littelfuse Relay White Paper

Orders

Support

About Us