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Coach dead, battery good

A guided diagnosis for RV techs. Each step is a check, a reading or a question.

  1. Battery reads 12V and nothing works. Where does the voltage stop? Meter each point in order to chassis ground: battery post, battery clamp, disconnect output, main fuse output, panel positive bus. The first point that reads low is the fault. Expect: Battery voltage at every point
  2. Press the disconnect switch. Does the solenoid click? Manual disconnects are a rotary or lever switch. Solenoid disconnects have a switch inside (often at the entry door) that pulses a latching solenoid near the battery. Expect: Click and battery voltage on both big studs
  3. Battery disconnected, ohm across the main fuse or reset breaker. Main protection is usually a 100 to 300 A fuse or an auto-reset breaker near the battery. Expect: Near 0 Ω

Causes and repairs

Battery terminal

Corroded or loose clamp. Reads fine on the post, low or nothing on the clamp. A clamp that looks fine can still have 2 V of drop under load.

Tools: hand tools, wire brush

Clean the post and clamp to bright metal, tighten, protect. Do the negative too. Load test after.

Solenoid contacts burned

It latches and the big contacts don't pass current. Meter across the two big studs with loads on: more than 0.2 V is bad contacts.

Tools: multimeter

Replace the solenoid. Clean and torque the cable lugs.

Solenoid coil or its switch

Meter the small terminals while a helper presses the switch: you should see a 12V pulse (latching types pulse each direction). Pulse present and no click: the solenoid. No pulse: the switch, its fuse, or the wire.

Tools: multimeter

Replace the failed part. Some latching solenoids can be closed manually with a lever to get the customer home.

Manual disconnect switch

Rotary switches corrode inside and get hot. Meter across it with loads on: over 0.2 V is a bad switch.

Tools: multimeter

Replace the switch. Torque the lugs.

Main fuse open

It opened for a reason: a short on the main feed, a slide or jack motor stalled, or an inverter surge. Auto-reset breakers also just wear out.

Tools: clamp meter, multimeter

Replace with the same rating. Reconnect with a clamp meter on the cable and everything off: draw should be a few amps or less. A big draw with everything off is the short to find before you leave.

Fuse holder or terminal

The fuse is fine and the holder or its lug is burned or loose. Heat marks are the giveaway.

Tools: hand tools

Replace the holder. Re-crimp a burned lug.

Open main ground

Positive arrives everywhere and nothing runs: the negative path is open. Battery negative cable, the frame ground lug, or the panel ground bus.

Tools: multimeter

Meter from the panel negative bus to the battery negative post with a load on: under 0.2 V. Clean the frame ground and battery negative to bare metal.

Battery path, not the loads

On shore power the converter feeds the 12V loads directly, so the coach works without the battery. Dead on battery alone means the path from the battery to the panel: disconnect off, the main fuse, or a battery that reads 12V open but collapses under load.

Tools: multimeter, load tester

Unplug shore power and walk the points above. Load test the battery: a battery that drops below 10 V under a small load is done.

Keep going on 12V and batteries

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