Reading a Running System
In this course
5 sections, plus a graded quiz. Click any to jump to it.
Introduction
Reading a running system means stating its charge state and likely failure point **without disassembling anything**. A tech who can do that is a tech the owner trusts. This chapter builds the reflex: read the numbers, walk the loop, state the diagnosis — then touch the system.
Why it matters: reaching for the gauges and the charge first, without checking the seawater loop, is how good techs break good compressors.
The four numbers that tell you the charge state
- Suction pressure — at the evaporator outlet.
- Discharge pressure — at the condenser outlet.
- Superheat — the evaporator outlet temperature above its saturation temperature.
- Subcooling — the condenser outlet temperature below its saturation temperature.
Superheat tells you how the evaporator is performing. Subcooling tells you how the condenser is rejecting heat. Together, they tell you the charge state without weighing the system.
Remember this line: a correctly charged system has superheat and subcooling both within
the manufacturer's specification. One high, one low — that is the charge error.
Undercharge vs overcharge
- Undercharge: high superheat, low suction, low subcooling. The evaporator is starving.
- Overcharge: low superheat, high subcooling, high discharge. The evaporator is flooding.
A 10% low charge will not show up on a pressure reading in many cases, but it will show up in a short compressor life and a high superheat at the evaporator. Weigh in, every time, to the manufacturer's specification.
Walking the raw-water loop on a live fault
When a unit trips on high pressure, work upstream first:
- Verify the seacock is fully open.
- Pull and inspect the seacock-to-strainer hose interior.
- Clean the sea strainer.
- Check the condenser for scaling or blockage.
- Only then verify the refrigerant charge.
If the raw-water loop is restricted, the condenser cannot reject heat, the discharge pressure climbs, and the high-pressure switch trips — the unit is protecting itself, not failing.
Documenting the reading
Record the four numbers at steady state, state the charge state (low, correct, or high), and justify it with the numbers. Then walk the raw-water loop and identify the most likely restriction point. This is the same workflow you will run on a live call, and it is the foundation of the refrigeration course.
Your progress
Knowledge check
3 questions · Graded on submission · Pass at 80%
Your attempt, best score, and certification status are saved to your student record.
Ready to go deeper?
Enroll in the full Marine AC track or grab the all-in-one package — pricing is shared when you enroll.