Motor Won't Start? How to Test a Siemens Contactor Before You Order a Replacement

I buy industrial electrical spares for a mid-sized manufacturing site. I'm not an electrician, but I'm the person who gets the call when maintenance thinks a Siemens contactor has failed and needs a replacement part number fast.

That used to send me straight to a supplier search bar with no diagnosis attached. In early 2024 I approved a $370 contactor on the strength of “it looks burned”, plus nearly $100 in next-day freight. The old contactor was fine. An overload relay had tripped, and the new part changed nothing when it arrived. This checklist exists because of that invoice.

What follows is a practical answer to “how to test a contactor” written for the person who signs the purchase order. It takes about 30 to 40 minutes, needs a decent multimeter, and ends with a clear decision: replace the contactor, or look elsewhere. If the answer is “replace”, the final step covers how to get the ordering details right. Get someone qualified to do the hands-on work and use this as the shared checklist.

Step 1: Lock the starter out and prove it is dead

No meaningful contactor test happens with the starter live. A 480V three-phase motor starter is not the place to trust the position of a switch. Lock out the panel disconnect and tag it out per OSHA's lockout/tagout standard (29 CFR 1910.147, osha.gov), then verify zero energy. My maintenance lead taught me this order: prove the multimeter works on a known live source, test line-to-line and line-to-ground at the contactor terminals, then test the meter again afterward. That final check is the one people skip. Also remember that control power, often 24V DC, can come from a separate supply—treat it as live until you measure it dead.

Step 2: Check the overload relay before you blame the contactor

This is the step I wish someone had shown me back in 2024. In a typical Siemens motor starter, an overload relay sits between the contactor and the motor. It opens the control circuit when the motor draws too much current, which drops the contactor out. When the relay is tripped, no contactor in the world will pull in, new or old.

So before measuring the contactor, find the overload relay (Siemens SIRIUS 3RU2 on newer panels) and check two things:

• Has the reset button moved or popped out? Reset it once, deliberately. If it trips again immediately, stop and look for the cause rather than holding the reset button.
• With the starter isolated, check continuity across the overload's normally closed terminals, usually 95 and 96. They should read closed. If they read open, the overload is why your coil gets no power.

A tripped overload is a symptom, not the whole story. That's why Step 7 exists.

Step 3: Photograph the label, then look for physical clues

Before removing anything, take photos of the contactor label, the motor nameplate, and the wiring around the starter. If you end up ordering a part, those photos answer half the supplier's questions while the line is down.

Then look without touching. I'm checking for blackened or pitted main contacts (visible after the arc chamber is removed), melted plastic around terminals, loose lugs, corrosion, and that burned-varnish smell around the A1 and A2 coil terminals. Not every failure leaves visible marks, though. A coil can open internally and look perfect from the outside—that's what the meter is for.

Step 4: Test the three main contacts with the armature pressed

Set the multimeter to resistance and make sure the contactor is isolated. On a typical Siemens 3-pole contactor, the line side is marked 1L1, 3L2, 5L3 (or L1/L2/L3) and the load side is 2T1, 4T2, 6T3 (or T1/T2/T3). First, with the contactor at rest:

• 1L1 to 2T1 should read open. Same for 3L2 to 4T2 and 5L3 to 6T3.
• If a pole reads closed while the contactor is de-energized, the contacts are welded or mechanically stuck. That is a definite replacement—and a safety issue.

Then press the armature down with a dry, insulated tool if needed. You should feel the contact springs move. While holding it closed, measure all three poles again. Each should read near zero, comfortably under one ohm on my meter. You're looking for a pole that stays open or reads much higher than the other two. A single high-resistance pole can make a motor hum, single-phase under load, or trip the overload after a few minutes of running.

Step 5: Test the coil between A1 and A2

Two different failures hide here. The coil winding can be open, or the coil can be healthy and simply not receive voltage. They need different tests.

First, disconnect one side of the coil wiring so the rest of the control circuit doesn't fool the meter, and read resistance between A1 and A2. An open circuit—infinite resistance—means the coil winding is burned open. The practical fix on a SIRIUS contactor is a new contactor, not a rebuilt coil.

If the winding gives a stable reading, the exact ohm value matters less than comparing it with a known-good coil of the same type. Then, with qualified help, energize the control circuit and measure voltage directly at A1 and A2 while someone operates the start button. For our example, the coil label should say 24V DC, and the reading should be close to that. If voltage arrives but the contactor doesn't pull in, the coil or the mechanical assembly is bad. If voltage never arrives, the problem is in the control chain, not the contactor.

Step 6: Don't skip the auxiliary contacts

Auxiliary contacts ride on the same mechanism and are easy to ignore because they look small and harmless. On a typical IEC block, a normally open auxiliary is wired as 13/14 and a normally closed one as 21/22. Test them the same way you tested the main contacts: press the armature and watch the continuity change. A contactor that only runs while the Start button is held down often points to a faulty NO auxiliary that should be sealing in the coil circuit. That problem may be fixed by a new auxiliary block or by replacing the contactor, depending on how the aux block is mounted—but it is not automatically a main contact failure.

Step 7: If the contactor passes, test the motor before ordering anything

If the contactor passes all the checks above, don't order a spare “just in case” and leave the line waiting. Test the motor side. Basic multimeter checks include comparing the resistance of the three motor windings and checking from each winding to the motor frame for a low-resistance path to ground. Those catch many hard failures. For intermittent winding faults or deteriorated insulation, you need an electric motor tester or an insulation resistance tester—a megger—which most plants have or can borrow from a motor shop. A motor with failing insulation can trip the overload relay repeatedly. The contactor is often just the messenger.

Step 8: If replacement is confirmed, order by part number, not by memory

Now you're allowed to order. The part number comes from what is actually installed plus the motor nameplate. Before you call a distributor, collect these:

• Motor data—if the motor nameplate says 7.5 kW (10 HP), note the full-load current as well. Contactors are selected by current and utilization category, not kW alone.
• Coil voltage—24V DC in this article's example. If the coil reads 110V AC, order 110V AC; the two are not interchangeable.
• Poles—a motor started across three phases takes a 3-pole contactor. If the circuit also switches neutral or a second load, it may be a 4-pole unit.
• Auxiliary contacts—how many NO/NC does the circuit actually use? If the old contactor carried an added auxiliary block, that is a separate order line.
• Overload relay—SIRIUS overloads are separate devices, so if it is damaged, order one with the motor's current range.

For the specific purchasing query “Siemens 7.5 kW contactor, 24V DC coil, part number”, the answer for our three-pole application has usually been in the SIRIUS 3RT2 family. One order number that keeps appearing in our stock for that combination is 3RT2025-1BB40. I believe it is the 7.5 kW / 24V DC coil version, but I don't trust my memory of the suffix, and Siemens order numbers get revised. Confirm it against a current data sheet. The ratings follow IEC 60947-4-1, and the selection table expects your motor's utilization category (AC-3 for normal motor starting), full-load current, and coil voltage. Current data sheets are on Siemens Industry Online Support (support.industry.siemens.com), and a distributor can do the same verification while confirming stock and ship date.

The mistake I now refuse to make twice

I once watched two expedited deliveries arrive in the same week—the first with the wrong coil voltage, the second correct—because nobody had read the old coil label before ordering. The freight bill stung, and the downtime stung more.

So here is the opinion that guides our purchases now: when a line is down, pay for certainty. Not for the most expensive contactor on the shelf, but for a confirmed part number, a confirmed ship date, and a distributor who answers the phone. The $80 saved by choosing an unverified listing with an “estimated” delivery date can turn into a lost shift.

When a line is down, an unproven cheap part isn't a saving—it's a second wait. The certainty is what you're buying.

If the contactor is genuinely bad, this checklist got you to the exact part. If it isn't, you avoided a box of useless spares and know where to look next. That's what testing before buying is worth.