A 120 V control rung has a stop button, a start button held closed, an overload contact and a coil in series. The coil will not energize. You read 120 V across the overload contact. What does that tell you?

B. The overload contact is open — it is the break in the rung

A. The overload contact is closed and fine

D. The control transformer is dead

In the same working rung with the coil energized, what should a meter read across a closed, healthy contact?

B. About half the control voltage

A PLC input LED is on when the limit switch is made. What have you proved?

B. The field device and the wiring work up to that input terminal; if the machine still misbehaves, look at the program or the input address

A. Nothing — the LED can be wrong

A PLC output LED is on, but there is no voltage at that output terminal. The most likely fault is…

C. the output module or its fuse/common supply

Your meter shows 120 V across a starter coil, but the contactor does not pull in. What next?

B. Lock out and check the coil — an open coil or a mechanically jammed armature — and check the coil voltage rating matches

C. Increase the control voltage

On a long control run a high-impedance digital meter reads 40 V on a wire that should be dead, which drops to nearly 0 V on the meter’s low-impedance (LoZ) setting. That reading was most likely…

B. induced (ghost) voltage from coupling with nearby live conductors

What is the best first step when called to a machine that “won’t run”?

B. Find out exactly what it does and doesn’t do, what changed, and what still works — then look at the drawings

Fourth year Lesson 4 of 12 · about 15 min

Troubleshooting industrial controls

What you’ll learn

What a meter reads across an open contact versus a closed one, and why.
How half-splitting finds a fault in a handful of readings.
How to use PLC input and output lights to decide which side of the PLC the fault is on.
The traps — ghost voltage, burnt contacts, coils with voltage that will not pull in — and when to lock out.

Before this: Motor control: troubleshooting lab · Intro to PLCs

A control circuit is a string of switches feeding a coil. When something will not run, one of those switches is open that should be closed (or the coil or its supply has failed). Your meter finds which one by reading voltage — and the rule that makes it fast is simple: the voltage appears across the break.

Every closed contact connects its two sides together, so it reads near 0 V. The one open contact has X1 on one side and X2 (through the coil) on the other, so the whole 120 V appears across it.

The rule: voltage appears across the opening

With one point open in a series circuit, no current flows, so nothing drops any voltage — except the opening itself. Everything to its left is connected back to X1; everything to its right is connected through the coil to X2. So the full control voltage sits across the open contact, and a good closed contact reads close to zero. A closed contact reading a few volts is burnt or loose, and it is stealing voltage from the coil.

The other way to use the same fact: keep one lead on X2 and walk the other along the rung from X1. Every point before the break reads full voltage; the first point that reads zero is just past it.

Half-splitting

Walking a long string point by point is slow. Measure in the middle instead: full voltage there (to X2) means the break is further on; zero means it is before. Then split that half. Each reading halves the suspects, so 16 possible points take at most four readings, and 64 take six.

Worked: a conveyor that will not start

120 V control. The rung runs X1 → E-stop → wire 2 → guard switch → wire 3 → stop → wire 4 → start → wire 5 → OL → wire 6 → coil → X2.
Hold Start. Meter from X2 to wire 3 (about the middle): 120 V. The E-stop and guard switch are closed; the break is after wire 3.
X2 to wire 5: 0 V. The break is between 3 and 5 — the stop button or the start button.
Across the start button while held: 120 V. The start contact is not making. Three readings, one suspect.
Lock out, verify dead, and confirm with an ohmmeter across the start contact while pressed before you replace it.

The PLC as a meter

A PLC splits the circuit in half for you. Every input and output point has an LED, and those lights tell you which side of the PLC the fault is on.

Input LED off when the device is made? The fault is on the field side: the device, its wiring, or the input supply. Meter it like any control circuit.
Input LED on but the logic does not respond? The field side is proved. Look at the program online: the address, a forced bit, a permissive not made.
Output LED on but no voltage at the terminal? The output module, its fuse, or its common supply.
Voltage at the output terminal but the device does nothing? The field wiring or the device (the coil, the solenoid, the lamp).

Traps

Ghost voltage: a dead conductor running beside live ones picks up voltage by capacitance. A high-impedance digital meter reads it; a low-impedance (LoZ) setting pulls it to nothing. Coil with voltage that will not pull in: the control circuit is fine — the coil is open, the wrong voltage, or the armature is jammed. A contact that reads 0 V across it with nothing flowing proves nothing; read it with the circuit trying to run. And forced bits in a PLC can make a program lie to you — check the force table.

On the job

Tips

Get the drawings and find the rung before you open the panel.
Pick a reference (usually X2 or the DC common) and keep one lead on it.
Ask the operator what happened just before it stopped — and what still works.
After a repair, run the machine through its whole cycle and watch it.

Common mistakes

Swapping parts before measuring.
Using an ohmmeter on a live circuit.
Believing a ghost reading, or dismissing a real one.
Jumpering out a safety device (a guard, an E-stop, an overload) to “test” and leaving it.
Putting hands in the panel without locking out because “it’s only control voltage”.

Check yourself

0 right of 0 answered 8 questions · pick one answer each

1.A 120 V control rung has a stop button, a start button held closed, an overload contact and a coil in series. The coil will not energize. You read 120 V across the overload contact. What does that tell you?
2.In the same working rung with the coil energized, what should a meter read across a closed, healthy contact?
3.A fault could be at any of 16 test points in a long series string. With half-splitting, what is the most measurements you need to find the open?
4.A PLC input LED is on when the limit switch is made. What have you proved?
5.A PLC output LED is on, but there is no voltage at that output terminal. The most likely fault is…
6.Your meter shows 120 V across a starter coil, but the contactor does not pull in. What next?
7.On a long control run a high-impedance digital meter reads 40 V on a wire that should be dead, which drops to nearly 0 V on the meter’s low-impedance (LoZ) setting. That reading was most likely…
8.What is the best first step when called to a machine that “won’t run”?

Answers

B. The overload contact is open — it is the break in the rung — In a series circuit with one open point, the whole supply voltage appears across the opening, because everything else is connecting each side of it back to X1 and X2. A good closed contact reads close to 0 V.
C. Close to 0 V — A closed contact is nearly zero ohms, so almost no voltage drops across it. A reading of a few volts across a “closed” contact means it is burnt or loose and dropping voltage that the coil needs.
B. 4 — Each measurement cuts the suspect section in half: 16 → 8 → 4 → 2 → 1. Four readings, against up to fifteen going point by point from one end.
B. The field device and the wiring work up to that input terminal; if the machine still misbehaves, look at the program or the input address — The input LED lights from the voltage on the input terminal, so the switch, its wiring and the supply are delivering. The problem is further along — the program logic, the address the program uses, or a forced bit.
C. the output module or its fuse/common supply — The LED shows the processor has turned the output on. No voltage on the terminal means the output circuit itself is not delivering — a blown output fuse, a missing common supply, or a failed output point.
B. Lock out and check the coil — an open coil or a mechanically jammed armature — and check the coil voltage rating matches — Full voltage at the coil means the control circuit did its job. The coil itself (open winding, wrong voltage rating) or the mechanism (jammed, debris) is the fault. Isolate and verify dead before you check coil resistance.
B. induced (ghost) voltage from coupling with nearby live conductors — A conductor running beside live ones picks up voltage through capacitance, but it cannot supply current. A high-impedance meter reads it; a low-impedance setting loads it down to nothing. A real source would stay up. (Always treat it as live until proved otherwise.)
B. Find out exactly what it does and doesn’t do, what changed, and what still works — then look at the drawings — The symptom and the parts that still work narrow the search before you open a door. The operator often knows the answer (“it stopped when the guard was bumped”).

Questions people ask

Is it safe to troubleshoot a live control circuit?

Measuring voltage on an energized control circuit is often the only way to find a fault, and it is done every day — but it is live work. It needs the right meter (CAT rating for where you are), the right PPE, the procedure your employer and your jurisdiction’s safety rules require, and a clear idea of what you will touch. Anything with your hands in it — replacing a part, checking resistance — is done locked out and verified dead.

Should I measure resistance or voltage?

Voltage first, with the circuit live, to find the section that is open without taking anything apart. Resistance and continuity only with the circuit locked out, to confirm a suspect contact or coil once you have isolated it. An ohmmeter on a live circuit gives nonsense at best and damages the meter at worst.

What if the fault comes and goes?

Intermittents are loose connections, contacts that are almost worn out, vibration, heat and moisture. Watch the PLC input LEDs or a meter while someone wiggles wiring, taps the relay or runs the machine through its cycle. Many PLCs can latch a bit when an input drops so the evidence waits for you.

Troubleshooting a control circuit (lab) — find hidden faults with a meter
Intro to PLCs — I/O and the scan cycle
The troubleshooting method — the loop on one page
Using a multimeter
Meter safety and CAT ratings
Lockout / tagout walk-through

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