Limit Switch Testing Step by Step Guide
Automatic Reset Limit Switch Testing
Safety first: shut off power and follow lockout/tagout when doing any cold continuity tests. When performing live tests keep clear of live parts, use one hand rule, insulated tools, and PPE.
Tools required
- Digital multimeter (AC volts, continuity/ohms)
- Non-contact voltage tester
- Insulated screwdrivers / nut drivers
- Heat source for functional trip test (heat gun or hair dryer) or warm water cloth
- Small mirror/flashlight and camera for documentation
- Manufacturer wiring diagram or wiring labels
Visual inspection (before testing)
- Check for burned terminals, melted housing, corrosion, or loose wiring.
- Verify correct mounting and that the sensing element or bulb (if present) is intact and positioned per manufacturer.
- Replace the switch if physical damage, heavy oxidation, or evidence of arcing is found.
Cold (power‑off) continuity/ohm test
1. Lockout/tagout and verify zero voltage at the unit with the non-contact tester and meter.
2. Disconnect the switch from the circuit (or disconnect one lead) so you measure the switch only.
3. Set meter to continuity or lowest ohms range.
4. With the switch at ambient temperature (not tripped), expected:
o Normally closed type: near 0 Ω (continuity).
o Normally open type: OL/infinite (no continuity).
5. If reading is open when it should be closed or vice versa, replace the limit switch.
Functional trip test (cold → heat → reset)
1. Reconnect the switch if removed. Energize power only if you will observe control voltage; otherwise perform heat test with power off and check continuity changes.
2. Apply controlled heat to the sensing element using a heat gun, warm cloth, or warm water — heat gradually to avoid damage.
3. Watch/measure the switch while heating:
o For a normally closed automatic reset limit, continuity should open when it reaches its trip temperature.
o Record the behavior and note whether the trip occurs smoothly and at a reasonable time.
4. Remove heat and allow the switch to cool. The switch should automatically reset (return to closed/normal position) when the sensing element falls below the reset temperature.
5. Use a thermocouple or IR thermometer to correlate approximate trip/reset temperatures if needed.
Live voltage functional test (with caution)
1. Re-apply power and keep panels removed only if safe. Set thermostat or control to call for heat so the circuit attempts normal operation.
2. Measure control voltage across the switch terminals while system is idle and while you induce a heat condition:
o At rest (no overtemp): control voltage should pass through the closed limit to downstream controls.
o When tripped: voltage should be interrupted at the limit terminal.
3. Monitor that voltage returns (closes) automatically after cooling. If the switch latches or does not restore voltage, it has failed.
Resistance/contacts expectations
- Closed contact: near 0 Ω (typically <1–2 Ω including terminals).
- Open contact: OL or very high resistance.
- Contact voltage drop under load: ideally <1 VAC; if >2 VAC replace the switch.
Troubleshooting outcomes
- Trips but does not reset: bad automatic mechanism or internal welding — replace.
- Does not trip when heated: failed element or stuck contact — replace.
- Intermittent operation: check wiring, loose terminals, poor chassis ground, or vibration; replace if contacts are pitted.
- Correct cold continuity but fails under load: high resistance contact; replace limit.
Documentation and final steps
- Record readings: ambient continuity, trip behavior (time/approx temp), reset behavior, and any voltage drop under load.
- Replace with exact temperature rating and amperage/voltage spec from the unit nameplate.
- After repair, run at least three full cycles to verify stable operation and reassemble panels.
· As always if you need a part jump on our website or email us at colemanhvacparts@gmail.com
· By: Darin DeVries
· HVAC Part Store LLC
· Coleman HVAC Parts
· Technical Service Advisor
· Author, entrepreneur, e-Commerce