Limit Pack Diagnostic Drill
// Can't advance until you can answer with certainty — that's the rule
The Light Switch Analogy
// Air pushes a door → door turns a knob → knob flips a switch → PLC watches the light
// Click PLAY to watch the chain work correctly
FAULT 1 — AIR
Door Never Moved
Regulator off, solenoid dead, sticky piston, low supply pressure. Valve didn't move — so nothing downstream changes. The limit pack is innocent. Check air first.
FAULT 2 — MECHANICAL
Knob Is Loose
Cam loose on shaft, set screw backed out, sugar binding on coupler. Valve moved but the cam didn't rotate. Switch never got hit. PLC saw nothing.
FAULT 3 — SIGNAL
Switch Is Broken
Cam hit the switch. Switch didn't change state. Micro-switch failed internally. Everything mechanical worked — signal chain broke at the last inch. Ohm the switch.
// WHAT YOUR HANDS DO vs WHAT YOUR BRAIN IS ASKING
| What You Do (Hands) | What You Think (Brain) |
|---|---|
| Meter voltage at limit pack supply terminals | "Is the pack even alive? Does it have power?" |
| Command valve OPEN — watch actuator physically | "Did air actually move metal?" |
| Remove cover — stroke valve again — watch cams | "Is the motion reaching the switches?" |
| Ohm the switch while cam physically hits it | "Did the cam really change the switch state?" |
| Watch PLC input bit on HMI live | "Does the PLC agree with what I'm seeing in the field?" |
| Check sequence permissive on HMI | "Is feedback the thing blocking the next step from running?" |
If you can't answer one row with certainty — don't move to the next row.
Certainty, then advance. Uncertainty means stay and dig.
Certainty, then advance. Uncertainty means stay and dig.
// THE SEQUENCE UNBLOCK CHAIN
Power exists
→
Air moved metal
→
Cam rotated
→
Switch changed
→
PLC saw it
→
Sequence advances
Break any link in this chain → sequence hangs. Your job is to find the broken link, not rewrite the chain.
// BURN THIS INTO MEMORY — 6 Questions, One Per Step