access control multimeter guide
a digital multimeter (dmm) is the most critical diagnostic tool in a low-voltage integrator's kit. while not executed in a rigid sequence on every job, these tests follow a logical troubleshooting path during a dead opening service call.
an integrator will typically verify primary ac voltage 1 at the wall transformer first, ensuring power is entering the system. next, they measure the distributed dc voltage 2 at the control board and terminal lock to isolate long-run voltage drops. from there, measuring dc amperage 3 confirms the power supply isn't overloaded, while audible continuity 4 validates life-safety fire alarm (facp) relays and request-to-exit switches. finally, testing resistance 5 with the power off identifies failing solenoid coils, and consulting the compliance matrix 6 guarantees the hardware geometry meets building codes.
ac voltage (alternating current)
used exclusively for verifying primary line voltage (120vac) at the breaker or testing the output of step-down plug-in transformers (16.5vac/24vac).
dc voltage (direct current)
used to verify terminal voltage at the strike or maglock. you must measure under load (while the device is energized) to detect voltage drop over long wire runs.
dc amperage (current)
measures the exact load draw of your locking hardware. the dmm must be wired in series with the device to capture the reading accurately. wiring in parallel will blow the meter fuse.
audible continuity
vital for verifying normally open (no) or normally closed (nc) states on request-to-exit (rex) motions, relay outputs, and door position switches (dps).
resistance (ohms)
measure wire run integrity or verify the internal coil resistance of a locking solenoid to detect premature breakdown before complete failure.
code compliance matrix
need help determining if your opening requires fail-safe or fail-secure hardware for building code?
| test mode | system rating | expected baseline reading | diagnostic failure state |
|---|---|---|---|
| ac voltage (V~) | 16.5VAC or 24VAC (Transformers) | 10% to 20% higher than rated output when measured with no load (e.g., ~18VAC or ~27VAC). | reading < rated voltage or 0V (blown thermal fuse in transformer). |
| dc voltage (V⎓) | 12VDC or 24VDC (Locks & Boards) | 12vdc systems: 12.0V to 13.2V 24vdc systems: 24.0V to 26.4V |
terminal voltage drops below 90% (< 10.8V or < 21.6V) under active load. |
| dc amperage (A⎓) | System Power Draw | matches the hardware manufacturer's rated spec (e.g., 250mA or 500mA per lock). | total system draw exceeds 80% of the power supply's maximum amp rating. |
| continuity (◣))) | Relays / DPS / REX | closed circuit (nc): audible beep / < 1Ω open circuit (no): no beep / "OL" (open loop) |
a closed contact reads high resistance (> 5Ω), indicating pitted/burnt relay contacts. |
| resistance (Ω) | Lock Solenoid Coils | healthy coils generally read between 20Ω and 100Ω depending on voltage and amp rating. | reading is "OL" (severed internal coil) or < 5Ω (dead short inside the lock). |
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