Field Guide · Automation Maintenance
AD16 Indicator Lights in Automation Equipment: 7 Field Problems and How to Fix Them for Good
A burnt pilot light is rarely the real problem. A practical troubleshooting guide for maintenance engineers, panel builders and buyers who keep seeing the same AD16 failures come back.
MONTOLI Technical Team | Updated October 2026 | 12 min read | Maintenance & Troubleshooting
Key takeaways
- Most "failed lamps" are actually supply, wiring or specification problems — swapping the lamp without diagnosis only buys a few weeks.
- "AD16" describes a widely copied product family, not one manufacturer's part number. Mounting size, voltage, current type, terminal style and IP rating must be matched together.
- Ghost glowing, flicker and short LED life almost always trace back to leakage current, voltage spikes, heat or the wrong replacement spec — not to a bad LED.
- Keeping a labelled spare kit of 3–4 standardised variants removes the majority of unplanned stoppages.
What's inside
1. What you are actually looking at2. The seven failures we meet most often3. A five-step routine that finds the real cause4. How to specify (or order) the correct replacement5. Spare parts: what to keep on the shelf6. Preventive maintenance schedule7. Quick reference table: symptom → probable cause → first action
A Φ22 LED indicator costs about as much as a cup of coffee in most spare-part budgets. One hour of unplanned stoppage on a mid-size packaging or assembly line usually costs more than every pilot light in the cabinet put together. Yet these lamps are treated as consumables: replaced in a hurry, without diagnosis — and back with the same fault three weeks later.
The AD16 family is one of the most widely installed pilot-light ranges on machinery built or maintained in Asia: CNC cells, packaging machines, injection-moulding lines, conveyor systems, textile machines and general control cabinets. This article collects what actually goes wrong with them in the field, why, and what to do instead of reaching for the spares box again.
Read this before you order replacements"AD16" identifies a product family — a mounting-diameter and design generation that dozens of factories build — rather than a single manufacturer's catalogue number. Two lamps both labelled "AD16-22" can differ in rated voltage, AC/DC compatibility, terminal style, IP rating and even internal construction. Never purchase on panel cut-out and lens colour alone (see Section 4).
1. What you are actually looking at
An AD16 pilot light is a small assembly, and every part of it can be the reason it failed. Modular versions allow the LED module to be pulled out from the front; sealed integral versions must be replaced as a whole unit.
| Part | What it does | Why maintenance cares |
|---|---|---|
| Lens cap | Diffuses and colours the light; usually snap-fit | Yellowing from UV, chemicals or heat reduces apparent brightness. Cracks break the IP seal. |
| LED light module | LED die plus built-in current limiting (resistor / capacitor / bridge rectifier) | This is the part that defines the voltage range. It is also the part that dies from over-voltage and surges. |
| Body / housing with thread | Fits the panel cut-out and clamps the assembly | Wrong hole size deforms the body, stresses the lens or destroys the IP rating. |
| Mounting nut + anti-slip washer | Clamps against the back of the panel | Vibration loosens it; over-torquing cracks the housing. |
| Sealing gasket | Provides the IP65 front seal | Missing, hardened or wrongly seated gasket = IP40 performance, whatever the datasheet says. |
| Terminals | Screw terminals or 6.3 mm flat-pin / solder lugs | Screw terminals back out under vibration; mismatched terminal style blocks a "same size" replacement. |
Typical ratings you will find on the nameplate — again, verify against the datasheet for your exact model:
| Parameter | Typical value for the AD16 family |
|---|---|
| Mounting hole diameters | Φ8 / Φ10 / Φ12 / Φ16 / Φ22 mm (Φ22 the most common on machines and control panels) |
| Rated voltages | 6 / 12 / 24 / 36 / 48 / 110 / 220–240 / 380–400 V, in separate AC, DC or wide-range AC/DC versions |
| Current consumption | Typically ≤ 20 mA for LED versions |
| Front protection | Up to IP65 when the gasket is fitted and the mounting nut torqued to specification (IEC 60529) |
| Applicable standards | IEC 60947-5-1, GB/T 14048.5 |
| Operating ambient | Typically −25 °C to +55 °C (storage often −40 °C to +70 °C) |
| Service life | 30 000 – 50 000 h at nominal voltage and ambient temperature |
| Panel thickness range | Commonly 1–7 mm depending on model and nut type |
2. The seven failures we meet most often
Every one of these has been seen repeatedly on real lines. The pattern is always the same: the lamp gets blamed, the lamp gets changed, the lamp comes back.
Problem 1 — Completely dead, while the machine still runs correctly
What you seeNo light at all. Everything else on the machine behaves normally, so the control logic is fine.
Why it happensUsually not the LED: a failed transistor output channel on the PLC, a burnt interposing relay contact, a broken common-return conductor, a terminal screw backed out by vibration, or simply the wrong voltage variant installed (a 230 V unit on a 24 V circuit will never light).
What to doApply lock-out/tag-out, prove dead, and check continuity of the return path. Then re-energise and measure the voltage at the lamp terminals, with the lamp fitted — not at the PLC channel or at a loose wire tip. If the correct voltage is present at the lamp, the lamp is faulty. If not, keep going upstream.
Problem 2 — Ghost glow: the lamp glows faintly when the output is OFF
What you seeDim red or green glow in the OFF state, sometimes so faint you only notice it in a dark cabinet. Operators start mistrusting the panel.
Why it happensLeakage current from solid-state (transistor) PLC outputs is the classic cause — even 1–2 mA is enough to make a high-efficiency LED glow. Other sources: capacitive coupling in long parallel cable runs, RC snubbers fitted across contactor coils, and diagnostic pulse tests some controllers send out.
What to doMeasure the leakage current in series. On 24 V DC circuits a parallel bleeder resistor (typically 2.2 kΩ, rated 2 W so it stays cool) normally solves it. On 230 V circuits do not improvise with a resistor — the dissipation is unrealistic; use a purpose-made RC/bleeder module, an interposing relay, or move that signal to a relay output channel. Never "solve" it with tape over the lens.
Problem 3 — Flicker or intermittent operation
What you seeLight flickers, drops out for milliseconds, or changes intensity when a contactor or motor starts.
Why it happensLoose terminal, a broken strand at the crimp inside the ferrule, modular-type contact lost when the bezel works loose, supply dips when large coils pull in, or a signal return shared with a high-current load. PWM-dimming supplies on some machines are another classic.
What to doDe-energise and do a careful wiggle test on both wires and the lamp body. Separate the signal return from the power return. Add RC or varistor suppression on the interfering coil. Re-terminate with proper ferrules and torque to specification.
Problem 4 — Premature dimming and short LED life
What you seeLamps noticeably dimmer than their neighbours after a few months, or dead after a year instead of five.
Why it happensSustained over-voltage is the main killer: 24 V DC control circuits from battery-backed supplies often float at 27–28 V, and 230 V mains regularly sits at 240–250 V. Add switching surges from contactor coils and VFDs sharing the cabinet, plus ambient heat near ovens, heaters or dryers, and the LED ages fast.
What to doLog the real voltage over a full production cycle (MIN/MAX function). Add surge suppression at the coil, check the power-supply setpoint, move the lamp away from heat sources, and choose wide-input AC/DC models where the supply is unstable. A yellowed lens that merely looks dim should be replaced, not first diagnosed as an LED failure.
Problem 5 — Water ingress, condensation and corrosion
What you seeFlickering followed by failure, white or green deposits at the terminals, moisture film visible under the lens, especially after wash-down or in humid seasons.
Why it happensIP65 applies only when the gasket is present and correctly compressed, the panel is flat and de-burred, and the nut is torqued correctly. Pressure washing blows water past seals; in cold rooms condensation forms inside the lens because there is no drainage or ventilation.
What to doReplace the unit — don't just dry it out, corrosion keeps spreading. Check the panel for burrs around the cut-out, fit gasket and washer in the correct order, torque the nut, and route a drip loop so water cannot run along the cable into the back of the lamp. For routine wash-down specify genuinely higher-protection models, and add a cabinet breather or heater where condensation is structural.
Problem 6 — Mechanical: wrong hole size, wobbling bezel, stripped thread
What you seeLamp sits crooked, can be turned by hand, falls out of the panel, or the thread will not tighten any more.
Why it happensThe standard cut-out for Φ22 devices is around 22.3–22.5 mm depending on the brand. Too small deforms the housing and stresses the lens; too large loses the IP seal. Step drills leave a conical, burred hole. Panel thickness outside the specified range means the nut bottoms out or cannot clamp at all.
What to doCut with the correct punch or hole saw (not a step bit), de-burr, respect the minimum spacing between devices given on the datasheet, tighten to the recommended torque only, and use threadlocker where vibration is permanent. Re-check tightness after machine relocation or transport.
Problem 7 — The replacement burns out immediately (or within a week)
What you seeNew lamp of "the same" size and colour flashes and dies the moment power is applied, or fails after a few shifts.
Why it happensSame diameter and same colour is not the same part: a 24 V unit installed where the circuit is 230 V, a DC-only unit on AC, a fast-reacting LED fitted into an old filament-lamp monitoring circuit, or a flashing/buzzer type wired like a steady lamp. Some controllers measure lamp current and report "lamp fault" when an LED draws less current than the original bulb; old lamp-test wiring can also apply unexpected voltage across the terminals.
What to doBuild a cross-reference list per machine containing cut-out size, voltage, current type, function, terminal style and IP rating. Approve one physical sample before any bulk purchase, and label the inside of each cabinet door with the exact specification.
3. A five-step routine that finds the real cause
Diagnosis takes about six minutes if you do it in this order. Skipping straight to "change the lamp" is what creates repeat call-outs.
0
Make it safe first
Lock-out/tag-out, prove the circuit dead, then verify your tester on a known live source. Use a meter rated CAT III 600 V or better. Never probe lamp terminals live on 230/400 V circuits just because "it is only a signal".
1
Describe the symptom precisely
Which lamp, which colour, steady or flashing, permanent or intermittent, what changed just before it appeared (a new programme, a wash-down, a relocated machine, a replaced power supply). Half of the root causes are visible in this answer.
2
Measure where the load is
Voltage at the lamp terminals with the lamp connected, compared with the rating printed on the housing. Then check the return path. Also record MIN/MAX over a production cycle, not just a snapshot.
3
Swap with a known-good unit
Keep two or three labelled test lamps in your toolkit. This is the fastest discriminator between "lamp is dead" and "circuit has a problem" — and it prevents you condemning a perfectly good LED.
4
Trace upstream if the new lamp fails too
The fault then lives in the PLC output channel, the interposing relay, the power supply, the wiring insulation or an EMI source — not in the pilot light.
5
Fix and record
Log the true root cause in the CMMS or maintenance log, then update the machine BOM so the next technician does not repeat the same diagnosis. This one step is what turns recurring failures into a solved problem.
Classic mistakeMeasuring the voltage at the wire end with the lamp disconnected. An open-circuit reading can look perfectly healthy while the terminal, the return conductor or the output stage cannot deliver any current. Always measure under load.
4. How to specify (or order) the correct replacement
When you cannot find the exact original part number, these seven fields are enough to define a functionally identical replacement. Give all of them to your supplier, not just the first two.
| Field | Example | Why it matters |
|---|---|---|
| Mounting hole diameter | Φ22 mm | Determines panel cut-out; also limits the available models |
| Function | Steady / flashing / with buzzer | Flashing and buzzer types have different internal electronics and wiring |
| Lens colour | Red, green, yellow, blue, white | Machine safety semantics: red = fault, green = ready, amber = warning |
| Rated voltage & current type | 24 V AC/DC or 230 V AC | Wrong either way = never lights, or burns out instantly |
| Terminal type | Screw terminal / 6.3 mm flat pin / solder lug | Must physically match the existing harness |
| Front protection | IP40 / IP65 / higher for wash-down | Depends on location and cleaning method |
| Approvals & documents | CE, RoHS; UL/cUL where required | Needed for export machines and customer audits |
The LED-upgrade trapReplacing old filament signal lamps with LEDs changes the load current. Controllers with lamp-filament monitoring may report a permanent "lamp fault", and old lamp-test circuits can feed back voltages in unexpected ways. Before a fleet-wide retrofit, test one unit on each machine type and check how the controller reacts.
For overseas buyers and panel builders, add three more items to the purchase request: an English datasheet listing all the fields above, batch colour/brightness consistency (different production batches can visibly differ in hue across one panel), and the IP test report plus declaration of conformity for your target market.
5. Spare parts: what to keep on the shelf
Standardising on one family and one or two voltage variants across a whole line is the cheapest reliability measure available. Below is a practical minimum stock for a typical cell running Φ22, 24 V AC/DC signal lamps.
| Item | Typical specification | Minimum stock per 10 machines | Remark |
|---|---|---|---|
| Steady green (running / ready) | Φ22, 24 V AC/DC | 5 pcs | Most frequently replaced colour |
| Steady red (fault / stop) | Φ22, 24 V AC/DC | 5 pcs | Keep one mounted test unit in the toolkit |
| Steady amber (warning) | Φ22, 24 V AC/DC | 3 pcs | Also used for maintenance bypass indications |
| Flashing / buzzer type | Φ22, 24 V AC/DC | 2 pcs | Check wiring: usually not interchangeable with steady types |
| Small-size signals (Φ8/Φ10/Φ16) | Same machine voltage | 5 pcs | Often used on HMI doors and sub-panels |
| Mounting nuts, washers, gaskets | Match the model | 10 sets each | Cheapest item, most commonly missing when needed |
| Lens caps / legend plates | All colours used | 5 sets | Colour labels fade faster than LEDs fail |
- Label each spare bag with the full specification string, not just "green lamp".
- Keep a compact commissioning kit (Φ22 red / green / amber, 24 V and 230 V) for new installations.
- Store gaskets away from heat and ozone; aged rubber is a silent cause of IP failures.
- Re-check the stock list whenever a machine is modified or a voltage level is changed.
6. Preventive maintenance schedule
| Interval | Task | Acceptance criterion |
|---|---|---|
| Every shift (visual) | Scan the panel during operation: dead lamps, dim lamps, cracked lenses, faded legends | All status lamps readable from the normal operator position |
| Monthly | Hand-check that bezels are not loose; verify that displayed colour matches the machine state | No lamp can be rotated by hand |
| Quarterly | Torque-check terminals to datasheet value; inspect gasket compression; clean lenses with a soft cloth and isopropyl alcohol | No loose strands, no corrosion, no moisture film |
| Every 6 months | Log supply voltage MIN/MAX over one full production cycle at the lamp terminals | Reading stays inside the rated range of the installed model |
| Annually | Insulation/continuity check of signal wiring; audit actual installed voltage vs. nameplate; replace lamps showing obvious brightness loss vs. their neighbours | No mixed voltage variants inside one cabinet |
| After every wash-down | Inspect for moisture under the lens, dry, re-torque the bezel | Dry terminals, no white or green deposits |
7. Quick reference table: symptom → probable cause → first action
| Symptom | Most probable cause | First action |
|---|---|---|
| Completely dead, other functions fine | Wrong voltage variant, loose terminal, failed output channel, broken return conductor | Measure voltage at the lamp terminals under load |
| Faint glow when OFF | Transistor output leakage current, capacitive coupling, RC snubber leakage | Fit a suitable bleeder resistor or interposing relay (24 V) / purpose-made module (230 V) |
| Flicker | Loose contact, shared return conductor, supply dips, EMI from coils | Wiggle test, separate returns, add coil suppression |
| Short LED life | Sustained over-voltage, switching surges, high ambient temperature | Log MIN/MAX supply voltage; add surge suppression; check PSU setpoint |
| Moisture under the lens | IP seal lost: missing gasket, burrs, wrong torque, pressure washing | Replace unit, de-burr the panel, re-fit gasket and torque |
| Loose or crooked lamp | Wrong cut-out size or panel thickness, vibration | Re-cut with the correct punch, use threadlocker, re-check after transport |
| Replacement dies at once | Wrong voltage/current type; incompatible flashing or buzzer version | Compare all seven specification fields; approve a sample first |
Frequently asked questions
Can I install a 230 V AC indicator on a 24 V circuit (or the other way round)?
No. A 230 V unit on 24 V will simply never light, because its internal current limiting is designed for a much higher voltage. A 24 V unit on 230 V destroys instantly — usually with a visible flash. This single mismatch is responsible for a large share of "immediately dead replacements".
Do AD16 indicator lights care about polarity?
It depends on the version. Wide-range models marked AC/DC contain a bridge rectifier and can be wired either way. Models marked for DC only usually must be connected with the correct polarity — reverse them and the lamp stays dark, and some versions can be damaged. Check the marking on the housing before wiring.
Why does my indicator glow dimly even though the PLC output is OFF?
Leakage current. Even 1–2 mA from a transistor output or from capacitive coupling in a long parallel cable is enough to make a modern high-efficiency LED visibly glow. On 24 V DC, a parallel bleeder resistor (around 2.2 kΩ, 2 W) is usually sufficient; on 230 V use a purpose-built module or an interposing relay instead of an improvised resistor.
What hole size should I drill for a Φ22 model?
Commonly around 22.3–22.5 mm, but it varies by manufacturer — check the datasheet of the exact model you are fitting. Use a punch or a hole saw rather than a step drill, and de-burr afterwards: burrs lift the gasket and destroy the IP seal even when the diameter is correct.
The lamp is marked IP65 — is that valid everywhere on the device?
Ratings of this kind normally refer to the front face only, and only when the gasket is correctly fitted and the mounting nut tightened to the specified torque against a flat, clean panel. The terminals at the rear are typically at a much lower protection level, which is why water running down the cable into the back of the lamp is a frequent failure path.
Can old filament signal lamps be replaced directly with LED types?
Not always. LED types draw far less current, so controllers monitoring lamp continuity may report a permanent lamp fault, and old lamp-test circuits can behave unpredictably. Trial one unit per machine type first, and choose LED types designed to work with monitoring circuits where those exist.
How long should an AD16 LED indicator actually last?
Typically 30 000–50 000 hours at nominal voltage and room temperature, which is several years of continuous duty. If your lamps fail much sooner, treat it as evidence of an installation problem — over-voltage, surges, heat or moisture — rather than as poor lamp quality.
Stop replacing the same lamp every month
MONTOLI supplies low-voltage control and signalling components for machinery builders, panel builders and maintenance teams — indicator lights, push buttons, and the matching breaker and contactor ranges that go into the same cabinet. If you are standardising a line or chasing a repeat failure, our technical team can help you match the specification instead of guessing.
- Cross-match an unknown or discontinued indicator against the seven specification fields
- Datasheets, drawings and samples for export projects
- Standardised spare-part kits built per machine or per production line
Note: all numerical values in this article are typical values for AD16-family pilot lights. Rights to final interpretation belong to the manufacturer's datasheet for your specific model — always confirm before ordering or installing.
