Quick answer
Sub hum or protect with the engine running is commonly RCA run tight against alternator charge leads or a weak amp ground. Reroute shielded RCAs away from power bundles, ground the amp to bare metal close to the amp, and confirm remote turns on only with ACC.
Problem
- You are wiring sub RCA and remote turn-on from the headunit.
Symptoms
- Sub stays off; amp in protect.
- Hum when engine running.
Cause
- Grounding topology lets alternator charging current modulate the RCA reference.
- Remote turn-on sequencing couples with headunit mute or delay behaviours.
Fix Step-by-step
- Establish the amp ground first - Short lead to bare metal; sand paint; torque to spec. Common mistake: pretty factory bolt through painted sheet - high resistance ground.
- RCA routing - Down the rocker opposite big power; cross existing bundles at 90°. Ute with amp under rear seat: avoid parallel runs with alternator charge lead through the cab tunnel.
- Remote (+12 V turn-on) - Meter 0 V key off, +12-ish ACC on. Weak headunit output → fused relay coil from remote, contact from battery+.
- Long canopy / tray runs - Use proper shielded RCA or balanced DSP feed - cheap RCA on a 4 m run on a diesel often hums.
- Smoke-test gains - Headunit sub level −6 to flat; amp gain minimum; verify no protect LED engine off, then idling.
- Hum only with engine running - Reroute RCAs first; if it stays, treat as charging/ground topology (see whine article).
Typical install mistakes
- RCA shield brushing seat rails or custom plate steel.
- Remote tapped to power-antenna lead - sub wakes with radio only and pops oddly.