Broken sensors that won’t stay fixed!

It’s another day at the plant, and the “Underside Clamp Retracted” sensor on Station 29, Op 30 is acting up again.  Seems to be intermittently functioning…the operator says that the line is stopping due to “Error: Underside Clamp Not Retracted”.

You think to yourself, “Didn’t we just replace that prox last week?”  A quick check of the maintenance log confirms it: that prox was indeed replaced last week.  In fact, that particular prox has been replaced seven times in the last six months.  Hmm….the frequency of replacement looks like it’s going up…four of the seven replacements were performed in the last two months.

What’s going on here?  Is it really possible that seven defective proxes just all happened to end up at Station 29, Op 30, Underside Clamp Retract?  Not likely!

There must be something else going on that is causing that prox to malfunction.  A likely culprit?  Wiring and connections.  It’s not unusual to find a quick disconnect cordset that has worn or damaged sockets that make intermittent contact with the pins on the back of the prox sensor.  Sometimes, the sockets are corroded, or full of dirt, grease, or oil.  The simple act of disconnecting the old prox and connecting the new prox may clean the contacts just enough for the prox to start working again (temporarily).   From the perspective of the maintenance technician, it looks like the prox was defective and replacing it “fixed” the problem, however this is a problem that will keep coming back!

Another possibility: broken wires inside the cable, particularly at the sensor end, or anywhere the cable may be strained or flexed.

When troubleshooting prox problems, the lesson is: don’t assume it’s always the prox.  It’s a good idea to keep a battery-powered prox tester and a known good quick-disconnect cordset in your toolbox.  When you suspect a bad prox, hook it up to the tester and confirm your hunch.  Be sure to inspect the connector sockets, cable, and wiring terminations.  When in doubt, replace the cable!  Cables are typically cheaper than proxes, and they are subject to a lot more wear and tear than we may think.

Many manufacturing plants maintain a “Shelf of Shame” where non-functioning products are showcased for the vendors to come and inspect.   I have sorted through piles of prox sensors in these collections on many occasions.  Some are obviously broken due to physical impact or exposure to harsh environments like high heat and weld slag.

But, checking the ones that aren’t physically damaged with a prox tester reveals a different story.  It’s surprising how many good proxes can be found in these piles of “bad” sensors!

Leave a Reply