Why Your Turck Proximity Sensor Keeps Failing—and Why the Signal Converter Is the Real Problem

At the 140-person custom machine builder where I work, I manage the sensor and controls budget—about $230,000 a year. I’m not the design engineer, and I’m not the technician on the midnight shift. But when a Turck inductive proximity sensor comes back to me for a fourth purchase order, I start asking questions.

The question that changes everything is usually: “Did anyone measure the voltage at the sensor terminals before replacing it?” Most of the time, the answer is no.

The Surface Problem: A Turck Proximity Sensor That Gives Up

The fault report sounds simple. A fill station sees two containers as one, so it stops. Maintenance opens the cabinet, tests the sensor by hand, sees the output light click on, and assumes the sensor is fine. Then it happens again. After the third interruption, someone orders a new turck proximity sensor.

Forty-five minutes later, the new sensor is in. Two days later, the same fault. This pattern repeats (honestly, more times than I want to count) in plants that have perfectly good sensors.

Maybe you already know where this is going. It’s not the sensor.

The Deeper Problem: Power, Grounding, and the Signal Converter

In my experience, an intermittent sensor dropout is rarely a bad sensing face. It’s a 24 V DC circuit problem. The piece that hides the problem is the signal converter.

A signal converter sits between the sensor and the PLC. It takes the switching signal and makes it readable by the controller. It also filters or amplifies the sensor output. But if the incoming voltage is unstable, the converter can’t fix it. It will happily report an open circuit or a false trigger.

The diagnostic that finally made sense to me as a cost controller was simple:

• Measure the DC voltage at the signal converter input terminals, not at the power supply.
• Measure the same 24 V DC rail in AC mode to check ripple.
• Wiggle the M12 cordset while watching the reading.

In our case, the voltage at the power supply read 24.1 V. The voltage at the signal converter read 21.8 V. Under load, it dipped below 20.4 V—below the lower limit of a typical 24 V control circuit. IEC 60947-5-2 expects a proximity switch to work from 85% to 110% of its rated voltage, which is roughly 20.4 V to 26.4 V at the device terminal, not at the source.

The sensor wasn’t bad. It was being starved.

If you’re on the procurement side, this is where a parts conversation becomes a systems conversation. You can’t solve that with another purchase order.

How to Use Fluke 117 True RMS Multimeter in This Workflow

The other hidden cost is the test tool. If your team relies on an averaging multimeter to inspect a DC bus, it can miss the noise. A true RMS meter should be the baseline for control circuit work, not a luxury.

I’d argue that knowing how to use Fluke 117 true RMS multimeter is worth more than another troubleshooting seminar. In ten minutes, this meter can show you the actual AC ripple on a DC rail, reveal ghost voltages with LoZ mode, and confirm that voltage at the sensor is high enough while it is switching.

The Fluke 117 is not the only true RMS meter with a low-impedance mode. But the combination on this one is easy to explain to a busy maintenance team:

• True RMS for non-sinusoidal AC signals
• LoZ mode for floating voltages
• CAT III 600 V safety rating for industrial panels

Per Fluke’s published specifications, the 117 is a CAT III 600 V true RMS meter with LoZ. We keep one in the panel shop. I consider it an asset, not an expense.

If you already have an 88 multimeter in the shop because the crew also works on fleet vehicles, keep it. But for a circuit with a signal converter, I prefer the Fluke 117. The low-impedance input helps avoid the false “24 V” that can show up as ghost voltage on an unloaded wire.

What Sensor Intermittency Actually Costs

Here’s why I keep this checklist in our procurement files.

In 2023, I audited all sensor-related purchase orders in our cost tracking system. The line had been losing one Turck inductive proximity sensor roughly every three weeks. Each sensor cost about $110. Each replacement also generated an express shipping charge and four hours of maintenance labor. The line stopped for an average of 14 minutes per incident. At our contribution rate, each stop cost about $320 in lost output.

Rough math: $110 sensor + $70 freight + $240 labor + $320 downtime = $740 per incident. Twelve incidents per year is almost $8,900. And the sensor wasn’t broken.

The surprise wasn’t the price difference between sensors. It was how many replacement sensors were sitting in the parts drawer, unopened.

When I showed this to the maintenance manager, he said, “We already suspected that.” That’s the frustrating part. The cost wasn’t a mystery. It was a system problem wearing a sensor-shaped mask.

The Short Fix: Measure Before You Replace

The fix is not exotic. It’s a checklist and one good meter. If you ask me, the fastest way out of a repeat failure is this:

1. Measure DC voltage at the signal converter and at the sensor while it is switching.
2. Measure AC ripple at the same points.
3. Use LoZ mode to rule out ghost voltages.
4. Move the connector and cable while watching the meter.
5. If you still suspect the sensor, test it on a bench supply instead of replacing it on faith.

This is not revolutionary advice. But it’s the advice that ends the cycle. No sensor replacement, no performance promise, no silver bullet. Just a few minutes of upfront measurement and a deterministic answer.

From a procurement perspective, the premium for certainty is worth paying. In March 2024, a voltage spike took out a signal converter channel, and we paid $400 extra for same-day delivery because our line was down. That rush charge wasn’t really about speed. It was about removing the “probably.” The alternative was another night of “should be fine” and the risk of missing a $15,000 customer delivery window.

I’d rather spend $400 once on certainty than save $30 and lose a customer.

Not a Sensor Problem. A Measurement Problem.

If you’re buying a Turck proximity sensor, you don’t need a lecture on how good they are. Turck’s inductive sensor line is deep—uprox factor 1, flush, non-flush, shielded, unshielded—and most of those sensors are not the weakest link. The weakest link is usually the installation: the supply, the wiring, the connector, and the signal converter.

So before the next purchase order, ask one question: did anyone measure the voltage at the sensor terminals with a true RMS multimeter? If not, you may be paying for a part you don’t need.

There’s something satisfying about a fault that doesn’t come back. After we fixed the power issue, our line ran for four months without a sensor-related stop. That’s the payoff, and you don’t need a bigger budget to get it.