Spent an hour at 2:10 a.m. on Line 3 chasing a phantom jam signal — 24V rock solid on the photoeye until I turned my back, then the PLC input dropped like it owed me money. Finally found a hairline crack in the M12 bulkhead connector that only opened when the cable relaxed; swapped the connector, threw on three sarcastic zip ties, and it’s cured — anyone else got components with stage fright?
Had one act like that — total Schrödinger’s photoeye; a quick ‘freeze spray + wiggle test’ on the M12 made the crack show up in seconds. If it creeps back, add a small service loop with real strain relief right before the bulkhead so the connector isn’t the spring, or log the input at 1 ms while your phone films the sensor LED to correlate the dip.
Been there on Line 2 at 3 a.m. — the ‘24V rock solid’ was only at the photoeye; when I clipped my meter MIN/MAX at the PLC terminal and slow‑bent from the M12 strain relief, it logged 18–20 V dips for about 50 ms. If you want a quick tell before swapping parts, back the input filter to 1 ms for a minute and trend the bit — if it chatters, the crimp in the female insert is suspect, and a real strain‑relief boot beats more sarcastic zip ties.
Quick trick: clip a 2.2k resistor from 24V to the PLC input and flex things — ‘24V rock solid’ with no load will rat out a cracked M12 bulkhead as soon as the input has to source a whisper of current. I’ve caught midnight gremlins on Line 3 that way when the meter looked fine. If it still plays shy, bump a short input filter in the PLC as a temporary band‑aid.
I keep a tiny 24V incandescent as a “truth light”: clip it from sensor brown to the PLC input and flex the cable — if the lamp winks, you’ve got a high‑impedance gremlin the meter misses, @amelia8671… If it resurfaces, nudge the input filter to 5–10 ms so the PLC doesn’t tattletale on millisecond blips after you’ve fixed the connector.
I use freeze spray to make flaky M12s confess: hit the bulkhead with a quick blast and watch the input LED — “no touching the cable” — and a hairline will show itself. If you don’t have freeze spray, an upside‑down duster can works too; just watch for condensation so you don’t create a new fault.
I mark the M12 shell and do a ‘hinge thump’ — close the panel door from a couple inches so vibration, not your hand, does the work; if the DI LED winks, I swap the bulkhead and snug a jam nut with threadlocker — just don’t overtorque or you’ll make a new crack. @sopmill your chill move also pops faster if you follow with a quick low-heat pass from a hair dryer.