I've lost count of the emergency calls that started with the same sentence: “the contactor is bad.” Most of the time, it wasn't. It was a loose termination, a marginal coil, or a control transformer sized too small. That's why I keep coming back to the same approach.
This is a field-tested checklist for how to test a contactor before you replace it. It works for a Siemens motor contactor, a small automotive relay like a BMW fuel pump relay, and even a big transfer switch such as an ASCO 200 amp transfer switch. The electrical principles are the same; the safety limits are not. I'll call those out as we go.
Here's the thing: a contactor rarely fails without warning. It chatters, it drops out under load, it refuses to pull in, or it welds closed. The test tells you which of those you're dealing with.
In my role coordinating urgent motor-control repairs, I've triaged more than 200 switching-device failures in five years. Last quarter alone, 23 emergency calls involved a “bad” contactor. In 8 of those, the contactor tested fine. The real problem was elsewhere. This checklist is how I avoid becoming the guy who replaces a $230 contactor and then discovers the issue was a loose neutral screw.
How to Test a Contactor: The 7-Step Checklist
Seven steps. Steps 1-3 are safety and observation. Steps 4-6 are electrical tests. Step 7 is where most people give up too soon.
- Step 1: Isolate and verify zero energy.
Lock out the disconnect, tag it out, and verify zero voltage at the contactor's line and load terminals with a rated voltage tester. Don't trust the indicator light. Indicator lights can lie. Check phase-to-phase and phase-to-ground.
Checkpoint: No voltage reading on any terminal. If you're inside an ASCO 200 amp transfer switch, follow its specific lockout procedure—some ATS controllers have capacitors that hold a charge.
- Step 2: Look before you measure.
Remove the cover and inspect. Look for charred insulation, pitted contacts, a deformed arc chute, loose screws, or a coil that looks blistered. A burned coil has a distinct smell. If you see that, testing further with an ohmmeter is still useful, but you're likely replacing the contactor.
- Step 3: Measure the coil.
Find the coil terminals. On a standard Siemens motor contactor, they're usually A1 and A2. Set your multimeter to resistance. Compare the reading to the rating plate. There's no universal “good” value—a 24VAC coil will read far lower ohms than a 230VAC coil. If the meter reads OL, the coil is open. If it reads near zero, suspect a shorted turn.
Checkpoint: Coil resistance should be stable, not climbing or bouncing. But a coil can read fine cold and still fail hot. That's why step 6 matters.
- Step 4: Operate the mechanism by hand.
With power still off, press the armature in slowly. It should move smoothly, and the return spring should push it back out. If it feels sticky, the problem may be mechanical, not electrical. I've fixed a “failed” contactor by cleaning dried grease off the slide rails. Not glamorous, but it worked.
This step is also a good time to check auxiliary contacts. On a Siemens Sirius contactor wiring diagram, auxiliary contacts are numbered clearly: 13-14 for a normally open contact and 21-22 for normally closed. That numbering will save you an hour if you're new to the device.
- Step 5: Test contact continuity.
Use an ohmmeter across the main contacts. With the contactor de-energized, the normally open main contacts should read OL. Press the armature in manually (or energize the coil in a safe setup) and they should read near zero. For a Siemens Sirius contactor, line-side terminals are typically 1L1, 3L2, 5L3; load side are 2T1, 4T2, 6T3.
Check auxiliary contacts too. The N.C. contact should be closed when the contactor is off; the N.O. contact should be closed when it's on. If a contact reads high resistance, don't sand it. Replace the contact kit or the whole device. Sandpaper removes the plating and makes the problem worse.
- Step 6: Test coil pickup under control voltage.
With the main power still isolated, temporarily apply the correct control voltage to the coil through the control circuit. Listen for a clean, single “clack.” No buzzing, no chatter. Measure the voltage at A1-A2 at the moment of pickup. If it drops more than 15% below the coil rating, your control transformer is undersized or there's a voltage drop in the control wiring. According to IEC 60947-4-1, a contactor must operate reliably from 85% to 110% of its rated control supply voltage.
If you're testing a BMW fuel pump relay, the same idea applies: apply 12V across the coil and listen for the click, then check continuity across the switched terminals. The relay's coil current is much lower than a contactor's, so don't use the same test rig for both.
- Step 7: Test the circuit, not just the contactor.
If the contactor checks out, go upstream and downstream. Check the overload relay, the PLC output, the pilot device, the control transformer, and every termination. I only believed this after I ignored it once. I replaced a perfectly good Siemens 3RT contactor, the second one that week, only to find a loose screw on the neutral bar. That mistake still annoys me.
Checkpoint: If a contactor tests good but doesn't stay sealed in service, suspect voltage drop under load or a pilot device with marginal contact resistance.
What This Checklist Does Not Cover
Honest limitations matter. This checklist covers electromechanical contactors, relays, and transfer switches with coils and mechanical contacts. It does not cover solid-state contactors, vacuum contactors above a few hundred amps, or the microprocessor controls in a modern ASCO 200 amp transfer switch. Those need OEM-specific test procedures, and in some cases, a thermal camera and a scope.
Also, a relay is not a contactor. A BMW fuel pump relay has similar switching logic, but the ratings and safety clearances are completely different. Don't use a contactor test procedure for a relay and expect valid results, and don't use a relay tester for a power contactor.
Common Mistakes I've Made (and Seen on Site)
Skipping the coil voltage check. I knew I should check the coil rating before ordering a replacement, but I thought “same frame, same coil.” It wasn't. I installed a 24VAC coil in a 120VAC circuit. It smoked, and we lost half a day. Now I read the nameplate every time.
Trusting a no-load test. A contactor can read perfect on the bench and still drop out under motor inrush. That's why you need the last step: simulate the actual load if you can do it safely, or at least verify the control voltage doesn't collapse when the contactor pulls in.
Cleaning contacts with sandpaper. It's tempting. It's also a quick way to remove the arc-resistant plating. Replace worn contacts. If the contactor is small and molded, replace the entire device.
Replacing the contactor before checking the wiring. This is the expensive one. I've seen a $500 service call end with a $2 part. A contactor doesn't fail by itself. It fails because something told it to close or refused to tell it to stay closed. Find that something.
Final Advice
If you're searching for Siemens contactor specifications, use Siemens Industry Online Support and the specific data sheet for the 3RT, 3TF, or Sirius device you have. There isn't a single “Siemens contactor” that fits all applications. The same goes for transfer switches and relays: use the exact schematic, not a generic one from memory.
Testing a contactor isn't difficult. It's systematic. Spend 20 minutes following these steps and you'll either find a clear failure or prove that the contactor is not your problem. Both results are worth money.
