Forward/reverse interlock locking out both directions
Neither direction will run — the interlock that stops both contactors closing together appears to be holding everything off, so no movement at all.
Safety first
Confirm what the interlock protects before touching it. The motor can start the moment the interlock clears, possibly in an unexpected direction.
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
One direction's contactor stuck closed/aux not resetting
Most likelyIf one contactor (or its aux interlock contact) is stuck, the interlock keeps the other locked out permanently.
- 2
Mis-wired interlock holding both off
#2An interlock wired wrongly can hold both directions out instead of just preventing overlap.
- 3
Common control fault upstream
#3A shared fault (control supply, e-stop, overload) stops either direction regardless of the interlock.
- 4
Mechanical interlock jammed
Least likelyA mechanical interlock bar between the contactors has jammed, blocking both.
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.
Check both direction contactors are fully released at rest (none stuck, aux contacts reset).
Both contactors open and their aux interlock contacts reset.
Both released — check the interlock wiring and common control.
One stuck contactor/aux is holding the interlock — free/replace it.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
Neither direction runs
→ step 2 - 2decision
Are both contactors fully released with aux contacts reset?
Yes→ step 3No→ step 4 - 3decision
Is the common control path (supply/e-stop/overload) healthy?
Yes→ step 5No→ step 6 - 4result
One stuck contactor/aux holding the interlock — free or replace it.
- 5result
Check interlock wiring/mechanism vs drawings for mis-wire or jam.
- 6result
Shared control fault — fix that first.
Common mistakes apprentices make
- Assuming a control fault when one contactor is simply stuck closed.
- Removing the interlock to get running and risking simultaneous closure.
- Not checking the common control path that affects both directions.
- Reworking interlock wiring without the drawings.
When to stop & escalate
Never disable the interlock to restore operation — it prevents a dangerous fault. If the interlock logic is unclear, confirm against the drawings before any change.
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
Reversing starter trips when changing direction
The drive runs each direction on its own but trips protection when you change from forward to reverse (or vice versa), often if the changeover is too quick.
Motor goes one way but won't go the other (e.g. down but not up)
A reversing drive works in one direction only. One command (say, down) runs fine; the other (up) does nothing, or just hums/trips. Common on hoists, doors, and conveyors.
Contactor contacts welded closed — load won't switch off
The contactor won't drop out when the coil is de-energised. The load stays powered even with the control circuit off, because the main contacts have welded together.
Learn the theory
How the gear and circuits behind this fault actually work.
Forward / reverse circuit
Two contactors run a motor in either direction; reverse swaps two phases, and an interlock prevents both closing at once.
Interlocks
Logic that prevents an unsafe or impossible combination of states — like two contactors closing together.
Safety systems (E-stops, guards & safety relays)
The protective layer that stops a machine safely — built to fail safe and never to be defeated, not ordinary control wiring.