How to Test a Contactor: Steps, Tools & Tips

What You Need to Test a Contactor?

Before you start to test, know the tools and safety tips first! Testing a contactor requires basic electrical tools and strict safety protocols to avoid electric shocks, equipment damage, and inaccurate measurements.

Item

Purpose

Digital Multimeter (DMM)

Measures resistance, voltage, and continuity; digital models offer better accuracy for quick, reliable readings.

Insulation Resistance Tester (Megohmmeter)

Optional but recommended for high-voltage applications; verifies insulation integrity to prevent leaks or faults.

Insulated Screwdrivers & Pliers

Accesses terminals safely without short-circuiting live components.

Safety Equipment (Insulated Gloves, Safety Glasses, Flame-Resistant Clothing)

Protects against electrical shocks, flying debris, and fire hazards during testing.

Contact Cleaner (Optional)

Removes dust, oil, or debris that can skew test results or hinder contact performance.

Lockout/Tagout (LOTO) Kit

Secures power sources (breakers, switches) in the "off" position to prevent accidental re-energization.

Wutai Electric supplies trusted electrical solutions globally. Explore our product range, access datasheets, and get technical support at http://www.wtaiele.com. Contact our team for custom quotes or help selecting the right contactor.

How to Test a Contactor?

Follow these simple steps, which work for most AC and DC contactors, including safety contactors.

Visual Check First

Before grabbing the multimeter, do a quick visual inspection:

  • Look for damage: Check the housing for cracks, melted plastic, or burn marks— these mean overheating or electrical faults.

  • Tighten connections: Make sure all wires are snug— loose wires cause poor contact and arcing.

  • Inspect contacts: If you can see the main or auxiliary contacts, check for blackening, pitting, or welding (burnt contacts won’t conduct power well).

  • Clean debris: Wipe away dust or oil with contact cleaner if buildup is visible— gunk can stop contacts from closing fully.

Test the Contactor Coil

The coil is what triggers the contactor— if it’s bad, the unit won’t work at all:

  1. Keep power disconnected and LOTO in place.

  2. Set your multimeter to resistance mode (ohms, Ω) – use the low range (200Ω works for most coils).

  3. Find the A1 and A2 terminals (coil terminals) on the contactor.

  4. Touch one multimeter probe to A1 and the other to A2. Write down the reading.

  5. What the reading means:

    • If the number is within the manufacturer’s range (usually 50-200 ohms— check the nameplate), the coil is good.

    • If the reading is almost 0, the coil is shorted (bad— replace it or the contactor).

    • If the multimeter shows “OL” (open line) or no reading, the coil is broken (needs replacement).

Test the Main Contacts

Main contacts carry power to your load— worn contacts cause arcing and equipment failure:

  1. Manually press the contactor’s central button to close the main contacts (this simulates the coil pulling them shut).

  2. Set the multimeter to continuity mode (it beeps when a circuit is closed) or the lowest resistance range.

  3. Touch the probes across each pair of main terminals (L1 to T1, L2 to T2, L3 to T3).

  4. What to look for:

    • A beep or reading near 0 means the contacts are good (conducting power properly).

    • No beep or a high reading (over 10mΩ) means the contacts are worn, oxidized, or misaligned. Clean them with contact cleaner, or replace the contactor if they’re badly pitted.

Test Coil Activation

This checks if the contactor engages when power is applied:

  1. Safely apply the rated control voltage to A1 and A2 (match AC/DC and voltage to the nameplate).

  2. Listen for a clear “click”, that’s the armature pulling in, and the contacts closing.

  3. Visually confirm the contacts close fully (no gaps) and open cleanly when you turn off the voltage.

  4. If there’s no click, double-check the control voltage— if voltage is correct, the coil is bad.

 

Test a Safety Contactor

Safety contactors have an extra “safety release” feature, but testing is similar:

  • Follow the same coil and contact tests above.

  • To test the safety function: Trigger the safety release (usually a button or lever on the contactor). The main contacts should open immediately, even if the coil is energized.

  • If the contacts don’t open when the safety release is activated, the safety mechanism is faulty— replace the contactor (safety contactors can’t be repaired for this issue).

Wutai Electric supplies trusted electrical solutions globally. Explore our product range, access datasheets, and get technical support at http://www.wtaiele.com. Contact our team for custom quotes or help selecting the right contactor.

When Should You Test a Contactor?

Proactive testing is key to maintaining reliable electrical systems. Here’s when you should perform contactor testing:

 

During Preventive Maintenance

Incorporate contactor testing into your regular maintenance schedule. For critical industrial equipment, this might mean quarterly checks. For commercial HVAC or less frequently used systems, annual testing is typically sufficient. Regular testing catches wear before it leads to failure.

When the System Fails

If equipment connected to a contactor stops working, testing the contactor is one of the first diagnostic steps. For example, if an AC unit’s compressor doesn’t start, the contactor is a common culprit and should be tested immediately.

After a Power Surge or Fault

Electrical storms, power outages, or short circuits can damage contactors. After any such event, test the contactors in the affected system to ensure they haven’t been compromised.

Before Installing a Used Contactor

Always test a used or surplus contactor before installing it. This verifies it’s in good working condition and prevents the hassle of reinstalling a faulty component.

How Can I Tell a Contactor is Bad?

 

Spotting a failing contactor early prevents unexpected downtime and potential equipment damage. Here are the key warning signs to look for:

It Doesn’t Activate

If your contactor doesn’t make a distinct “click” when it should energize, and you’ve verified the control voltage is correct, the coil is likely faulty. A silent contactor that receives power is a strong indicator of an open or shorted coil.

The Load is Intermittent

If the connected equipment (like a motor or AC unit) turns on and off randomly, it’s often due to worn or pitted contacts. These contacts can’t maintain a consistent connection, causing the power to cut out intermittently.

You See or Smell Trouble

Visible arcing (sparks) between the contacts, a burning smell, or blackened, melted plastic on the contactor housing are all clear signs of severe electrical problems. These issues typically stem from overloads, short circuits, or badly worn contacts.

It’s Stuck On or Off

A contactor that stays closed even when power is removed has welded contacts—a dangerous condition that can lead to equipment damage or fire. Conversely, a contactor that’s stuck open and won’t close, despite a good coil, likely has mechanical issues like a bent armature or debris buildup.

Your Energy Bills Are Rising

Worn or dirty contacts cause increased electrical resistance, which forces the connected equipment to work harder and consume more power. A sudden spike in energy costs, without other explanation, could point to a failing contactor.

What are Common Contactor Failures and How to Fix?

Coil Issues

  • Signs: No click when energized, intermittent engagement, overheating.

  • Causes: Incorrect control voltage, shorted/open coil, voltage fluctuations.

  • Fix: Verify control voltage matches specs. Replace coil or contactor if damaged.

Contact Problems

  • Signs: Arcing, burning smells, high resistance, welded contacts (contactor stays closed).

  • Causes: Overloading, frequent switching, poor maintenance.

  • Fix: Clean lightly worn contacts. Replace contacts or contactor for severe pitting/welding.

Mechanical Faults

  • Signs: Contactor won’t open/close fully, chattering, stuck position.

  • Causes: Debris buildup, bent armature, worn springs.

  • Fix: Clean the contactor. Straighten the armature if possible. Replace for irreparable damage.

Manual Actuation Damage

  • Issue: Breaking new contactors by pressing the central button.

  • Causes: Pressing too hard, angled pressure, or model-specific design limits.

  • Fix: Use firm but gentle pressure. Avoid repeated manual actuation on compact/low-amp models.

FAQs about How to Test a Contactor

Can you test a contactor with a multimeter?

Yes. A basic digital multimeter works for all key tests: coil resistance, contact continuity, and voltage checks. Most tests take 5-10 minutes and can be done in-place.

What do A1 and A2 do on a contactor?

A1 and A2 are coil terminals. Control voltage connects here to energize the coil. AC contactors have no polarity for A1/A2. DC contactors use polarity (+ on A1, - on A2) as marked.

How many ohms is a bad contactor?

  • Coil: Near 0 ohms (shorted) or “OL” (open) means failure.

  • Main contacts: Over 10mΩ when closed indicates wear.

  • Compare readings to manufacturer specs for exact limits.

How can I tell if my AC contactor is bad?

Look for these signs: No click on activation, intermittent load operation, burning smells/marks, welded contacts (stays closed), or higher energy bills from voltage drop.

How to tell if a contactor is AC or DC?

Check the nameplate for “AC”/“DC” near coil voltage. DC contactors may have +/-. AC contactors hum slightly when energized; DC contactors run quieter.

How to read an AC contactor?

The nameplate shows rated voltage (Ui), rated current (Ith), coil voltage, and standards (IEC 60947-4-1). Terminals: L1-L3 (input), T1-T3 (output), NO/NC for auxiliary contacts.

What is the 3-minute rule for AC?

Wait 3 minutes before restarting an AC compressor after shutdown. This equalizes system pressure, preventing the contactor from starting the compressor under heavy load—reducing contactor strain.

Post time: Jan-05-2026