Relay Rescue

Control Board — drag the probes & troubleshoot here

TRUE-RMS DMM + CLAMP

V~ •))) AC

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VAC

in air in air

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See the Field Guide tab for the full method →

Know the sequence. What should happen on a call? Stat closes → coil energizes → contact closes → load runs.
Verify power first. 120 V on the line side, 24 V at the transformer secondary. No power = stop and fix that.
Confirm the call. Use 🖐 Operate to close the thermostat so the circuit is actually trying to run.
Split the circuit. Meter at the middle, then halve again toward where the voltage stops.
Test across each device. 0 V across a closed device = good. Source voltage across it = it's open.
Confirm, then call it. Back up a voltage read with an ohm/clamp test, then 🚩 Call Fault.

V~	AC volts. Drag both probes onto two points. Reads the difference in potential between them.
Ω •)))	Resistance / continuity. Power off the leg first. Good path beeps (~0 Ω); open reads OL.
A~	Clamp. Drag the jaw around one conductor. Reads current; 0 A means none is flowing there.

0 V across a switch/wire/contact → it's closed & good (no drop).
Source voltage (24 V or 120 V) across a single device → that device is open; the meter is reading back through the load.
Source voltage across an energized coil/load → power is getting there fine.
A few volts across a wire that should read 0 V → a loose / high-resistance connection.
0 V hot-to-common everywhere → no power; go upstream (fuse, transformer, line).

Ohm a coil: a number (e.g. ~160 Ω) = good; OL = open winding; a few ohms = shorted.
Lift a leg — click a terminal screw with the 🪛 to lift its wire before ohming; parallel paths fool in-circuit readings.
Clamp a conductor to see if current actually flows. Voltage present but 0 A = open load downstream.

Paragon 8145 defrost timer — terminal 1=line, N=neutral (& clock motor), 4=refrigeration (N.C.), 3=defrost (N.O.), X=defrost-termination input.
DTFD (3-wire defrost-termination/fan-delay): red→N, brown→X (ends defrost ≈55°F), black→fans (re-closes ≈35°F so fans don't blow warm air after defrost).
Pump-down control: the space thermostat opens/closes the liquid-line solenoid; the LP control (≈15 psi in / 4 psi out) cycles the compressor contactor; the HP control is in series as a safety.

Open switch/safety	Source V across it; load dead.
Blown fuse	Source V across it; everything downstream dead; OL on ohms.
Open coil/load	Full voltage present but 0 A and no action; OL on ohms.
Burned contact	Coil pulls in but load dead; V across the “closed” contact.
Welded contact	Load runs with coil de-energized; 0 V across the “open” contact.
Shorted coil	Bus sags; a few ohms instead of normal.
High resistance	Low voltage at the load; volts dropped across a wire/joint.
Broken wire	Parts ohm fine but circuit open; OL across the conductor.