Contactor pulls in on the button but won't seal in
Press start and the contactor pulls in, but it drops out the moment you release the button. It won't latch/hold itself in through the seal-in circuit.
Safety first
You'll be working in a live control circuit near the start/stop station. Confirm what's isolated, treat the power side as live, and follow the machine's control drawings rather than guessing the logic.
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.
Apprentice mode
Full walk-through — the why, what readings mean, mistakes & safety.
Likely causes
Ranked from most to least likely.
- 1
Seal-in / hold-in auxiliary contact not making
Most likelyThe normally-open auxiliary contact that parallels the start button (the seal-in) isn't closing or is high-resistance, so the coil loses its feed the instant the button is released.
- 2
Wrong auxiliary used for the seal-in
#2If the seal-in is wired through a normally-closed or the wrong auxiliary, the hold path never completes — common after a contactor or aux block swap.
- 3
Open stop button / series safety contact
#3A stop button, E-stop, or overload contact that's marginal can carry the brief push but open under the seal-in path, dropping the hold.
- 4
Loose terminal in the seal-in branch
Least likelyA loose connection in the hold-in wiring makes the path while you press (through the button) but not through the auxiliary alone.
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.
On the tools? Job Mode shows one test at a time with thumb-sized buttons — your place and readings carry straight over.
Enter Job ModeTrace the control drawing and identify the seal-in auxiliary that parallels the start button.
A clearly identified NO auxiliary intended to hold the coil in.
You know which contact must make — test it next.
If the drawing shows no seal-in or the wrong contact type, the logic/wiring is the fault.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
Pulls in on button, won't seal in
→ step 2 - 2decision
Does the seal-in auxiliary actually close when energised?
Yes→ step 3No→ step 4 - 3decision
Is the hold path continuous through stop/overload/series contacts?
Yes→ step 5No→ step 6 - 4result
Seal-in auxiliary not making / wrong contact — replace/adjust or rewire to the correct NO aux.
- 5result
Recheck the auxiliary wiring and terminations in the seal-in branch.
- 6result
Open stop/overload/series contact or loose terminal — repair the hold path.
Common mistakes apprentices make
- Wiring the seal-in through the wrong auxiliary after swapping the contactor.
- Assuming the coil is faulty when it pulls in fine — the fault is the hold path.
- Not realising a marginal stop/overload contact only fails on the seal-in path.
- Holding the button to 'make it work' and shipping it like that.
When to stop & escalate
If the seal-in passes through a safety contact (E-stop/overload) that's intermittently opening, stop and treat the safety circuit as the priority — don't bypass it to get the hold working. Escalate if the control logic doesn't match the drawings.
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.
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Related faults
Contactor has voltage at the coil but won't pull in
You measure the rated control voltage (e.g. 24V) across the coil terminals, but the contactor refuses to energise — no clunk, no pull-in, contacts stay open.
Contactor drops out on its own / won't stay latched
The contactor pulls in when you press start but drops out the moment you release the button, or randomly during running — the seal-in (latch) isn't holding it.
E-stop circuit won't reset / machine won't start
The machine won't start because the emergency-stop circuit won't reset — the safety relay stays dropped out, holding everything off, even with all e-stops apparently released.