# How to Test a Contactor with a Multimeter (Step-by-Step Guide)
A faulty contactor can bring an entire production line to a halt. Before replacing what might be a perfectly good component, technicians should perform systematic diagnostic tests. This step-by-step guide shows you how to test any industrial contactor using a standard digital multimeter.
Whether you’re troubleshooting a motor starter, HVAC system, or lighting control, these tests will help you determine if the contactor is the culprit—or if the problem lies elsewhere.
—
## Understanding Contactor Basics
### What You’ll Be Testing
A contactor consists of three main components:
1. **Coil (Electromagnet)** – Creates magnetic field to close contacts
2. **Main Contacts** – Carry load current to the motor/equipment
3. **Auxiliary Contacts** – Used for control circuits and interlocking
### Common Failure Modes
| Symptom | Likely Cause | Test Required |
|———|————-|—————|
| Contactor won’t energize | Open coil | Coil resistance |
| Contactor chatters | Low voltage, weak coil | Coil voltage |
| Welded contacts | Overcurrent, excess cycles | Contact continuity |
| Intermittent operation | Poor connections | Terminal tightness |
—
## Required Tools
### Essential Equipment
– **Digital Multimeter** (with resistance and continuity modes)
– **Insulated Screwdrivers**
– **Safety Gloves** (rated for electrical work)
– **Lockout/Tagout Equipment**
### Multimeter Settings
– **Resistance (Ω):** For coil testing
– **Continuity:** For contact testing (audible beep preferred)
– **AC/DC Voltage:** For coil voltage verification
—
## Step 1: Safety First – Lockout/Tagout
### Critical Safety Steps
⚠️ **Never work on energized equipment**
1. **Disconnect power** at the main breaker
2. **Verify zero energy** with your multimeter
3. **Apply lockout/tagout** devices
4. **Test on known live source** before and after testing
**Remember:** Capacitors in VFDs and drive circuits can store lethal voltage even after power is removed.
—
## Step 2: Visual Inspection
### What to Look For
Before reaching for your multimeter, perform a thorough visual check:
**Contact Condition:**
– ✓ Silver-gray appearance (normal)
– ✗ Blackened, pitted, or melted contacts (replace)
– ✗ Excessive carbon buildup (clean or replace)
**Coil Housing:**
– ✓ No burn marks or discoloration
– ✗ Burnt smell indicates coil failure
– ✗ Cracked housing (moisture ingress)
**Mechanical Components:**
– Check for broken springs
– Verify contactor moves freely (manually operate)
– Look for foreign objects or debris
—
## Step 3: Test the Coil Resistance
### Why This Test Matters
The coil is the most common failure point. A healthy coil has predictable resistance based on its voltage and power rating.
### Testing Procedure
**Step 3a: Locate Coil Terminals**
– Coil terminals are typically marked A1 and A2
– May also be labeled with coil voltage (e.g., “24V DC”, “220V AC”)
**Step 3b: Set Your Multimeter**
– Select resistance (Ω) mode
– Choose appropriate range (usually 200Ω or 2kΩ)
**Step 3c: Measure Resistance**
1. Disconnect wires from A1 and A2
2. Place probes on A1 and A2 terminals
3. Record the reading
### Interpreting Results
**Expected Resistance Values (Typical Ranges):**
| Coil Voltage | Expected Resistance | Acceptable Range |
|————-|———————|——————|
| 24V AC | 15-30Ω | ±20% |
| 120V AC | 300-500Ω | ±20% |
| 220-240V AC | 1000-3000Ω | ±20% |
| 24V DC | 100-500Ω | ±20% |
| 48V DC | 400-1500Ω | ±20% |
**Reading Analysis:**
| Result | Interpretation | Action |
|——–|—————|——–|
| 0Ω (short) | Coil winding shorted | Replace contactor |
| Infinite (∞) | Open coil winding | Replace contactor |
| Within range | Coil is healthy | Proceed to contact tests |
| Outside ±20% | Coil degraded | Consider replacement |
**Example:** A 220V AC coil measures 2.5kΩ. Expected range: 1-3kΩ. **Result: PASS**
—
## Step 4: Test Main Contact Continuity
### Understanding the Test
Main contacts should have near-zero resistance when closed and infinite resistance when open.
### Testing Procedure
**Step 4a: Identify Main Contact Terminals**
– Line side: L1, L2, L3 (input)
– Load side: T1, T2, T3 (output to motor)
**Step 4b: Test with Contactor De-energized (Open)**
1. Set multimeter to continuity or resistance (Ω)
2. Test between L1 and T1, L2 and T2, L3 and T3
3. **Expected:** No continuity (infinite resistance)
4. Any reading other than infinity indicates welded contacts
**Step 4c: Manually Activate and Test (Closed)**
1. Press contactor mechanism manually (or apply rated voltage to coil)
2. Test between L1 and T1, L2 and T2, L3 and T3
3. **Expected:** Continuity (<1Ω) on all three poles
4. High resistance indicates dirty or damaged contacts
### Interpreting Results
| Condition | De-energized | Energized | Diagnosis |
|-----------|-------------|-----------|-----------|
| Normal | ∞Ω (open) | <1Ω (closed) | ✓ Healthy |
| Welded contacts | <1Ω | <1Ω | ✗ Replace immediately |
| Burnt contacts | ∞Ω | >5Ω | ✗ Clean or replace |
| Single pole failed | ∞Ω | L1-T1 OK, L2-T2 ∞Ω | ✗ Replace contactor |
—
## Step 5: Test Auxiliary Contacts
### Test Procedure
Auxiliary contacts follow the same testing logic:
**Normally Open (NO) Contacts:**
– De-energized: ∞Ω
– Energized: <1Ω
**Normally Closed (NC) Contacts:**
- De-energized: <1Ω
- Energized: ∞Ω
**Common Terminal Designations:**
- 13-14: NO auxiliary
- 21-22: NC auxiliary
- 53-54: Additional NO
---
## Step 6: Verify Coil Voltage (If Accessible)
### Why This Matters
A contactor with a good coil may still fail to operate if supply voltage is incorrect.
### Testing Procedure
**Step 6a: With Power On (Qualified Personnel Only)**
1. Set multimeter to AC or DC voltage (match coil rating)
2. Measure voltage at coil terminals A1-A2
3. Compare to contactor coil rating
**Acceptable Voltage Range:**
- Minimum: 85% of rated voltage
- Maximum: 110% of rated voltage
**Example:** For 220V AC coil:
- Minimum: 220V × 0.85 = 187V AC
- Maximum: 220V × 1.10 = 242V AC
| Voltage Reading | Status |
|----------------|--------|
| 198V - 242V | ✓ Normal operation |
| <187V | ✗ Undervoltage - check supply |
| >242V | ✗ Overvoltage – risk of coil damage |
—
## Step 7: Advanced Tests (Optional)
### Insulation Resistance Test
**Purpose:** Detect coil-to-ground or terminal-to-terminal leakage
**Procedure:**
1. Use megohmmeter (500V DC)
2. Test coil terminals to contactor frame
3. Test line terminals to load terminals (open position)
**Acceptable:** >10 MΩ
**Unacceptable:** <1 MΩ (moisture/dirt contamination)
### Inrush Current Measurement
**Purpose:** Verify coil is drawing expected inrush current
**Typical Inrush:** 5-15× holding current for AC coils
**High Inrush (>20×):** Indicates shorted turns in coil
—
## Troubleshooting Decision Tree
“`
Contactor Won’t Energize
↓
Check Control Voltage at A1-A2
↓
┌──────────┴──────────┐
No Voltage Correct Voltage
↓ ↓
Check wiring/ Measure Coil Resistance
control circuit ↓
┌────┴────┐
Infinite 0Ω Normal
↓ ↓ ↓
Open coil Shorted Check
(Replace) (Replace) mechanically
“`
—
## Common Misdiagnoses to Avoid
### ❌ Misdiagnosis 1: “Contactor is bad”
**Actually:** Control fuse blown, emergency stop pressed, or interlock open
### ❌ Misdiagnosis 2: “Contacts are welded”
**Actually:** Mechanical binding preventing full opening
### ❌ Misdiagnosis 3: “Coil needs replacement”
**Actually:** Low control voltage from undersized transformer
—
## When to Replace vs. Repair
### Replace the Contactor When:
– Coil resistance is open or shorted
– Main contacts are welded or severely pitted
– Housing is cracked or damaged
– Multiple failures in short time period
– Unit is >10 years old with heavy cycling
### Clean and Reuse When:
– Minor contact discoloration (use contact cleaner)
– Loose terminal screws (tighten to spec)
– Dust/debris contamination (clean with dry cloth)
—
## Conclusion
Systematic multimeter testing can diagnose 90% of contactor problems in under 10 minutes. By following this step-by-step procedure, you’ll avoid unnecessary replacements and minimize equipment downtime.
**Remember:** Always verify power is off before testing, and never exceed manufacturer ratings during troubleshooting.
—
## Recommended Testing Equipment
For professional electrical maintenance, we recommend:
– **Fluke 117 Electrician’s Multimeter** – Industry standard
– **Megger MIT1025** – For insulation testing
– **NewTrend LC1D Series Contactors** – Reliable replacement units
**[Browse Our Contactor Catalog →]** | **[Contact Technical Support →]**