Quick answer
If your sub hums or sits in protect with the engine running, you've usually run the RCAs tight against the alternator charge leads, or the amp ground is weak. Reroute shielded RCAs away from the power bundles, ground the amp to bare metal close to the amp, then check the remote only comes on with ACC.
Problem
- You're wiring sub RCA and remote turn-on from the headunit.
Symptoms
- Your sub stays off and the amp sits in protect.
- You hear hum with the engine running.
Cause
- If your grounding layout is poor, alternator charging current modulates the RCA reference.
- The remote turn-on sequence couples with the way your headunit mutes or delays.
Fix Step-by-step
- Establish the amp ground first: run a short lead to bare metal, sand the paint back, then torque to spec. Common mistake: a tidy factory bolt through painted sheet gives you a high resistance ground.
- RCA routing: run them down the rocker opposite the big power, and cross existing bundles at 90°. If your amp sits under the rear seat in a ute, avoid parallel runs with the alternator charge lead through the cab tunnel.
- Remote (+12 V turn-on): meter it at 0 V key off, and around +12 V with ACC on. If your headunit output is weak, drive a fused relay coil from the remote and take the contact from battery+.
- Long canopy or tray runs: use proper shielded RCA or a balanced DSP feed. You'll often hear hum from cheap RCA on a 4 m run on a diesel.
- Smoke-test gains: set the headunit sub level −6 to flat, amp gain at minimum, then check you get no protect LED with the engine off and again at idle.
- Hum only with the engine running: reroute the RCAs first. If you still hear it, treat it as charging or ground layout, see the whine article.
Typical install mistakes
- The RCA shield brushing seat rails or custom plate steel.
- Remote tapped to the power-antenna lead, so your sub wakes with the radio only and pops oddly.