Why Is My Timing Relay Not Timing Correctly? Causes and Fixes

Why is my timing relay not timing correctly? Most timing faults trace back to one of four causes: the function/range switch is set wrong, the supply voltage falls outside the rated band, the wiring doesn't match the delay type (on-delay vs. off-delay), or the unit has drifted past its rated repeat accuracy (typically ±0.2% to ±1% of set value, per IEC 61812). The consequence ranges from a nuisance — a conveyor starting a second early — to a real hazard, such as a star-delta transition closing too fast and bridging both contactors. This article walks through range/function checks, voltage and wiring faults, accuracy math, trigger-pulse issues, and drift from heat and age, in the order a panel technician should check them.

Start With the Function and Range Switch, Not the Timer Itself

Before assuming the relay has failed, confirm what it's actually set to do. Multifunction units such as the Schneider Zelio Time RE22 or ABB CT-MFD combine several delay modes and several time ranges in one housing, selected by small rotary or DIP switches on the front face. A unit set to "off-delay" will never fire on power-up the way an on-delay function does — that isn't a fault, it's the wrong mode for the job. Check three things in order: function selector (A/on-delay, B/off-delay, interval, star-delta, etc.), range selector (0.1 s vs. 1 s vs. 10 s decade), and the dial position within that range. A dial reading "5" on the 1-10 s range times out ten times faster than the same dial position on the 1-100 s range. This single mismatch accounts for a large share of "the timer is broken" service calls.

Supply Voltage Instability and Multi-Voltage Timers

Electronic timers derive their delay from an internal oscillator or RC network referenced to the supply rail. Multi-voltage models accept a wide AC/DC band (commonly 24-240 V AC/DC on one part number), but a sagging or noisy supply — a control transformer loaded near its VA rating, a shared 24 V DC bus dipping under motor inrush — can shift the oscillator reference and stretch or compress the delay by a small but measurable margin. On older pneumatic and motor-driven timers, voltage sag has a bigger effect: a motor-driven timer running slow because its drive motor is fed low voltage will time everything long, consistently, not randomly. What we see in the field: intermittent voltage-related drift is far more often a loose terminal or an undersized control transformer than a defective timer.

Repeat Accuracy vs. Setting Accuracy — Why the Delay Varies Cycle to Cycle

Two different numbers get confused here. Setting accuracy is how close the actual delay is to the dial marking the first time you set it — a coarse analog dial might read 10 s but actually deliver 9.5 s. Repeat accuracy is how much that same unit varies from one cycle to the next once it's set, and it's the tighter, more meaningful spec for sequencing applications. If a technician reports "the delay changes every time," that's a repeat-accuracy complaint, not a setting-accuracy one, and it points to component drift or supply instability rather than a miscalibrated dial.

Run the numbers before condemning a unit. A timer rated ±0.5% repeat accuracy set to 30 s should stay within ±0.15 s cycle to cycle. If field measurements show a 2-3 s swing, the timer itself is out of spec or damaged — check the accuracy math from the timing range and accuracy guide before assuming the worst.

Wiring Faults That Present as a Timing Fault

A large share of "timing" complaints are wiring problems in disguise. Two are worth checking first. On a true off-delay timer, the delay only starts once the trigger input is removed while the auxiliary supply stays live; if that trigger wire is tied to the same terminal as the supply, the unit loses power at the same moment the delay should start and it either fires instantly or not at all. On a control-signal off-delay (no internal energy storage), removing supply and trigger together does the same thing — the timer has nothing left to count with. The fix is separating supply and trigger onto distinct terminals per the datasheet's 2-wire vs. 3-wire connection diagram, covered in the timing relay wiring guide. Second, check contact routing on multifunction units: many have two independent change-over outputs, and connecting the panel to the wrong one produces behavior that looks exactly like a bad delay setting.

Star-delta applications carry the highest stakes for a wiring error. If the transition (dead-time) output is bridged out or miswired to close instantly, the star and delta contactors can overlap for a moment — a direct short across two phases. Confirm transition wiring against the star-delta motor starting guide any time a starter trips on overlap rather than simply mistiming.

Minimum Trigger Pulse and Recovery (Reset) Time

Two timing parameters get ignored until something breaks: minimum trigger pulse width and recovery time. If the input signal is shorter than the datasheet's minimum pulse width, some units won't register the trigger at all — the relay appears to randomly skip cycles under fast PLC scan rates or bouncy pushbutton inputs. Recovery time is the reset interval a timer needs after completing (or being interrupted mid-cycle) before it will accept a new trigger reliably; retriggering inside that window can produce a shortened or corrupted delay on the next cycle. This shows up most in pump alternation and sequencing applications with tight cycle times — worth checking against the pump alternation and sequencing article if cycles are running back-to-back with little dead time between them.

Temperature, Load and Long-Term Drift

Electronic timing relays hold their rated accuracy within a stated ambient range, usually -20°C to +55°C or similar. Push a DIN-rail enclosure past that — a control cabinet in direct sun, or stacked tight against VFDs and soft starters dumping heat — and the RC/oscillator reference drifts, typically stretching the delay slightly as internal resistance and capacitance shift with temperature. This depends on the specific component grade inside the unit, so two timers from the same family can drift by different amounts under the same heat load. Contact loading matters too: a relay switched consistently near its rated current, cycled hard, ages its output contacts and its internal electronics faster than one running well under rating. On older pneumatic timers, the bleed orifice and diaphragm wear mechanically — delay time creeps upward over years of use, not something a control-voltage timer experiences.

Some panel builders keep pneumatic or motor-driven timers running in legacy equipment long past the point they'd spec an electronic replacement, mostly because rewiring the panel costs more than the drift is worth — but that math changes once the drift starts tripping downstream interlocks.

Conclusion

Work the checklist in order: function and range switch first, supply voltage second, wiring topology (2-wire vs. 3-wire, correct output contact) third, then accuracy math against the datasheet, then trigger pulse and recovery time, then temperature and aging. Most "defective timer" reports resolve at step one or two. For a full bench and in-panel test procedure, or to review selection criteria before swapping to a different range, see the timing relay selection guide and the timing relay engineering guide. Stock replacements from the timing and control relays collection when a unit is confirmed out of spec rather than miswired.