Timer relay not switching its output
A timer relay is powered but its output contact never changes state — the delayed action (start, changeover, stop) never happens, or it switches at the wrong time.
Safety first
The timer's output can start downstream equipment when it finally switches. Make sure that's safe before you sit and watch it time out.
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
Full detail — causes, the why, and common mistakes.
Likely causes
Ranked from most to least likely.
- 1
Wrong timer function or mode selected
Most likelyOn/off-delay, single-shot, and other modes behave very differently. The wrong mode (or a trigger input it never receives) means it never switches when you expect.
- 2
Missing supply or trigger/control signal
#2Many timers need both a supply and a separate trigger/start signal. If the trigger never arrives, the timing never begins.
- 3
Time set to an extreme value
#3The set time or range multiplier is set far longer than expected, so it looks 'stuck' when it's actually still timing.
- 4
Failed output contact
#4The internal relay contact has welded or failed open, so even when timing completes the load never sees the change.
- 5
Wiring to the wrong contact (NO/NC)
Least likelyThe load is landed on the wrong contact set, so the switching is happening but on terminals you're not watching.
Reports are saved on this device to reflect what you actually find.
Testing sequence
Work through one test at a time. Expected reading and what each result means.
Confirm the timer's function/mode and time range match what the circuit needs.
Mode and range set to suit the intended delayed action.
Mode is correct — check supply and trigger next.
Wrong mode or range explains the behaviour — set it correctly and re-test.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
Timer not switching
→ step 2 - 2decision
Is the correct mode/function and time range set?
Yes→ step 3No→ step 4 - 3decision
Are supply AND any required trigger present?
Yes→ step 5No→ step 6 - 4result
Set the correct mode/range for the intended action.
- 5decision
Does the output contact change state after the set time?
Yes→ step 7No→ step 8 - 6result
Restore the supply / trace the missing trigger so timing can start.
- 7result
Timer works — load is on the wrong contact or has its own fault.
- 8result
Output contact failed — replace the timer.
Common mistakes apprentices make
- Not realising the timer needs a separate trigger input as well as a supply.
- Watching the wrong contact set (NO vs NC) and concluding it isn't switching.
- Mistaking a long set time for a stuck timer.
- Assuming on-delay behaviour when the timer is actually set to off-delay (or vice versa).
When to stop & escalate
If the timer is part of a defined control sequence and the required mode/time isn't documented, confirm the intended timing against the machine's control description before changing settings.
If you're past your competence, authorisation, or the safe limits of the job — stop and hand it on. There's no fault worth getting hurt over.
Related faults
PLC output LED is on but the device doesn't work
The PLC output indicator says the output is energised, but the connected device (valve, contactor, lamp, motor starter) does nothing. The program thinks everything is fine.
Contactor chattering or buzzing instead of holding in
The contactor rapidly clicks/buzzes, pulls in and drops out repeatedly, or hums loudly without seating cleanly. Often comes with arcing noise and heat.
No control voltage in the panel
Nothing in the control circuit will operate — contactors won't pull in, indicators are dead, the PLC may be off. The control voltage that should be there simply isn't.
Learn the theory
How the gear and circuits behind this fault actually work.