A $890 Mistake That Changed How I Diagnose Siemens Contactors
I need to be honest: In September 2022, I swapped out a perfectly good Siemens Sirius 3RT2 contactor.
Wasted $890 in parts, labor, and a 1-week production delay. The real culprit? A corroded auxiliary contactor on the same rail. The main contactor was fine.
After that, I created a diagnostic checklist. We've used it 47 times since then (as of January 2025). It caught 42 bad contactors and—more importantly—stopped me from replacing 5 good ones.
Here's the playbook I wish I had in 2022.
The Core Problem: Everyone Jumps to 'Replace the Contactor'
Most maintenance teams (including mine, two years ago) follow the same flawed logic:
- Motor stops
- Check for voltage at coil
- Voltage present? Must be a bad contactor
- Replace it
This oversimplification ignores three common failure points that mimic a bad main contactor:
- Failing auxiliary contactor: The main contactor's aux contacts (used for interlocking or status feedback) can fail, but the main power contacts are still good. You see a "status missing" alarm and assume the main contactor dropped out.
- Control circuit corrosion: Especially in high-humidity environments, the coil terminals can corrode. You measure 24V DC at the source, but the contactor coil gets 19V. It chatters or won't pull in.
- Overload relay trip: The contactor looks dead because the overload relay is tripped. Replacing the contactor doesn't fix the overload trigger.
(This was my error in 2022—I had a tripped overload relay on a Sirius 3RU2. Thought the contactor was dead.)
My Siemens Contactor Diagnostic Checklist
I keep a laminated copy of this in my tool bag. Here it is (note to self: I should digitize this).
Step 1: Visual & Auditory Check (30 seconds)
Before touching a multimeter, listen and look:
- Can you hear the contactor pull in with a distinct 'clack'? A weak or buzzing sound indicates low coil voltage or mechanical wear.
- Is there any visible arcing or burning around the main contacts (visible through the arc chamber on Sirius 3RT2 series)?
- Check the auxiliary contact block (Siemens 3RH series). Are any of the actuators stuck or displaced? I once found an aux block that had popped partially off its rail—causing intermittent feedback faults.
Step 2: Coil Voltage at the Contactor (2 minutes)
Measure at the contactor terminals, not at the source. I measure between A1 and A2 on the Sirius series.
Most buyers focus on 'is there voltage?' and completely miss voltage drop. If your control circuit shows 24V DC at the PLC output but only 19V DC at the coil, you have a wiring or power supply issue.
(I learned this the hard way. The question everyone asks is 'is the contactor bad?'. The question they should ask is 'is the contactor getting proper voltage?'.)
Step 3: Check the Overload Relay (1 minute)
Press the 'Test' or 'Reset' button on the Siemens Sirius 3RU2 overload relay. If it was tripped, the contactor won't operate. Many people (myself included) forget this simple check.
Step 4: Auxiliary Contact Function Test (3 minutes)
If the main contactor pulls in but the system reports a 'contactor feedback' fault, test the auxiliary contacts. Use your multimeter on the normally open (NO) terminals. When the contactor closes, the aux contacts should also close. If they don't, the aux block is the problem—not the main contactor.
Step 5: Verify the Motor Circuit (5 minutes)
With the contactor pulled in, measure continuity across each power pole (L1-T1, L2-T2, L3-T3). A high resistance reading indicates pitted or burnt contacts. A perfect reading is near 0 ohms.
When You Actually Should Replace the Contactor
If you complete the checklist and the contactor fails Step 5 (high resistance on main contacts) or physical issues from Step 1 (serious arcing, mechanical damage), then yes—replace it.
But let's be specific about what to replace it with. As of Q1 2025, the direct replacement for a failing 3RT2 is typically a 3RT2... but check the specific rating. A 50-amp application needs the correct frame size (S00, S0, S2, etc.).
What was best practice in 2020 (just check the catalog number) may not apply in 2025. Siemens has updated some Sirius models. Always cross-reference the current Siemens datasheet (siemens.com/sirius) before ordering. I've caught three mismatches this way in the last year.
The Exceptions: When My Checklist Can Fail You
I have to be honest: this checklist isn't perfect for every situation.
It doesn't cover:
- Intermittent faults that happen once a week. My checklist catches a 'stuck off' or 'stuck on' fault. If the motor stops randomly every Tuesday, you need a datalogger.
- Rare applications like 8 pole lighting contactors. The wiring logic is different. The diagnostic approach is similar, but I haven't documented enough failures (yet) to be confident.
- Inverter-fed motors. If the contactor is after a VFD, the failure modes are different (overheating from harmonics). This checklist assumes a direct-on-line motor start.
For those cases, I'd recommend data logging the coil voltage and auxiliary contact status over a 24-hour period. That's how we caught our intermittent issue in early 2024.
The bottom line: In my experience, roughly 1 in 10 'bad' contactors isn't actually bad. The extra 4 minutes of diagnostics saves you $250+ in parts and prevents unplanned downtime. Simple.
