Ac / heat

Test an AC with the Diag Center

Doors, windows, vents closed. AC on cool, fan HIGH

RETURN probe in the return air, before the coil

SUPPLY probe in a supply vent near the unit

Probe tips in the air stream, not touching metal

Wires taped so they cannot be pulled out

Tap NEW TEST on AC TEST when the compressor starts

Wait 10 to 15 minutes, then read delta-T

Photo of the screen with the result

Reading delta-T

Delta-T is return minus supply. Your bands: green 16 to 22 F, yellow 12 to 16 or 22 to 26 F, red under 12 or over 26 F.

Low can point to low refrigerant, a weak compressor, very humid air, or too much airflow across the coil. High can point to too little airflow: dirty filter, blocked return, slow fan, or an iced coil.

Humid air lowers delta-T because the coil spends energy removing moisture. Write down the weather.

These are common causes, not a diagnosis. The tool shows the symptom. You find the cause.

Power first

Measure the voltage at the unit WITH the AC running. Normal is about 108 to 132 V. Low voltage under load (below about 104 V) makes compressors run hot, cycle or trip. Fix the supply before condemning the unit.

Amp draw

Clamp meter on the compressor leg, then on the whole unit. Compare to the nameplate RLA / FLA. Typical 13,500 BTU rooftop units run roughly 12 to 16 A total. Much higher means a hard-working compressor, low voltage, or a dirty condenser.

Capacitor

Power off and discharge it first. Measure with the meter in capacitance mode. Typical tolerance is about 5 to 6 percent of the label value. Bulged, leaking or out of tolerance: replace. Dual run capacitors have three terminals: HERM (compressor), FAN and C (common).

Airflow

Check the filter, return opening, ducts, and the evaporator coil face. Fan on HIGH. Measure delta-T again after cleaning. Weak airflow is the most common cause of a poor result.

Ice on the coil

Causes: low airflow, dirty coil, low refrigerant, running in cool outside temperatures. Thaw with the fan only, then find the cause before running it again.

Symptom: check this first

Nothing runs: 12V and 120V at the unit, fuses, thermostat, control board.

Fan runs, no compressor: run capacitor, start components, overload tripped (let it cool), compressor windings, low voltage.

Compressor hums, then trips: low voltage, bad capacitor, locked compressor.

Weak cooling: filter, dirty coils, airflow, charge, humidity, supply air mixing with return air.

Trips the breaker: wrong breaker size, weak compressor drawing high amps, shorted wiring, or a failing fan motor.

Controls

Most RV AC control systems run on 12V DC. Good 12V and a good ground at the board and thermostat come first. Many no-cool calls are a 12V problem.

Refrigerant

In the US, handling refrigerant requires EPA Section 608 certification. This tool measures air temperature only. It does not replace proper gauge diagnosis.