Self-contained marine AC unit diagnostics

Self-contained AC units (Cruisair SMX/SMXV, Dometic Turbo, MarineAire) are one-piece assemblies mounted under a berth or in a locker: refrigerant loop + water-cooled condenser + air handler in one box. Sized in BTU per hour: 5000 BTU for a small forward cabin, 10-16k for a salon, 24k for a large master. Common failures: (1) Not cooling / low output, start with raw water flow. Marine AC requires 3-5 gpm of raw water per 12k BTU. Clean the strainer, inspect the pump impeller (Cruisair uses March, replace every 3 years), check the through-hull isn't blocked with growth. Insufficient raw water flow trips high-pressure lockout. (2) Water in the pan / drip pan overflow, condensate drain clogged with slime. Blow out with compressed air OR pour a cup of vinegar down monthly to keep it clear. (3) Circuit breaker trips at startup, compressor start capacitor failed (measure with a capacitor tester, cheap replacement) or hard-start kit needed. (4) Cold air but weak flow, fan motor failing OR ductwork restriction. Check current draw of the fan; a healthy PSC motor should draw close to its nameplate amp rating, not below. (5) Frozen evaporator, restricted airflow (dirty filter, blocked return) OR low refrigerant charge. A frozen evaporator with a full charge means poor airflow, always check the filter FIRST.