Marine freshwater pressure pump diagnostics

Marine freshwater pressure pumps (Jabsco Par-Max, SHURflo Aqua King, Groco) are 12/24V diaphragm pumps with a built-in pressure switch. Normal operation: pump cycles on at 20-25 psi, off at 40-45 psi. Common failures: (1) Pump won't stop cycling with no faucet open, internal check valve leaking (allows accumulator to bleed backward) OR a plumbing leak (check under sinks, water heater relief valve, toilet supply). Isolate by closing the tank shutoff and watching if the pump still cycles. (2) Pump runs but produces little flow, clogged strainer at the pump inlet (all pressure pumps need a 40-mesh strainer, most people forget it) or a torn diaphragm (pump gets noisy, rebuild kit is cheaper than a new pump). (3) Pump won't start, pressure switch adjustment (small hex screw on the switch head) or dead switch. Bypass the switch temporarily with a jumper to confirm. (4) Chattering short cycles, install a small pre-charged accumulator (2-4 liter, air side pressurized to 20 psi with a bike pump) upstream of the first faucet. This is the single most effective upgrade for pressure-pump quality of life. Winterize by draining tank, running RV-safe propylene glycol antifreeze through until pink comes out every faucet.