Lights
A guided diagnosis for RV techs. Each step is a check, a reading or a question.
- What are the lights doing? 12V lighting is the simplest circuit on the coach, which makes it the best place to learn voltage drop. Expect: 12V at the fixture with the switch on, under 0.5 V total drop from the battery
- 12V at the dead fixture? Meter the fixture's two wires with the switch on. Expect: 12V across the leads
- Does the switch pass 12V? Meter the switch input and output with it on. Expect: 12V through the switch
Causes and repairs
Fixture failed
Power is there and it doesn't light. LED driver or board dead, or the bulb in an incandescent fixture. Check polarity on LED fixtures.
Tools: multimeter
Replace the fixture or bulb. Confirm polarity on LEDs.
Open ground at the fixture
Positive is present, nothing across the fixture: the ground wire is open. Many fixtures ground through their mounting screws to a metal strip or the frame.
Tools: multimeter
Find the ground path and restore it. Add a dedicated ground wire if it relied on a screw.
Switch failed
Worn contacts or a loose spade. Replace it.
Tools: hand tools
Replace the switch. Use a proper terminal.
Open wire between switch and fixture
A splice behind the wall or a wire pinched at a slide. Find the open with the meter.
Tools: multimeter
Repair with a crimp and heat shrink.
Group dead: fuse or shared ground
A whole room dead is a fuse or a shared ground point. Check the fuse first (12V both sides with the switch on). Then the common ground for that area, often a screw behind a panel.
Tools: multimeter
Fuse, then ground. See the 12V guide for a fuse that keeps blowing.
Voltage drop
Dim lights far from the battery are drop in the run. Meter from the battery positive to the fixture positive with the lights on (positive side drop), then fixture negative to battery negative (ground side drop). Whichever is big is your problem. Over 0.5 V total is too much.
Tools: multimeter
Fix the connection or splice that eats the voltage. Upsize wire on long runs. A bad frame ground shows on the ground side.
Flicker: loose connection or converter ripple
A loose connection at the fixture, switch or splice flickers when the coach moves. All the lights flickering together on shore power is converter ripple (bad or missing battery).
Tools: multimeter
Wiggle test the fixture and switch. If everything flickers on shore power, see the converter guide.
LED ghost glow or switched ground
LEDs glow faintly when off if the switch is on the ground side and there is a leakage path, or if a dimmer isn't rated for LED. LEDs that dim and brighten with the pump or furnace are showing voltage drop everyone else ignores.
Tools: multimeter
Move the switch to the positive side, replace the dimmer with an LED-rated one, or fix the drop.
Keep going on 12V and batteries
Learn it
- Volts, amps and ohms, the simple way
- Voltage drop: the test that finds bad grounds
- Practice: find the bad connection
- Batteries: lead-acid vs lithium
- Fuses, breakers and grounds
Prove it
- Back lights are dim
- Battery dies even on shore power
- Lithium won't charge on cold mornings
- Coach is dead, battery is fine
- Batteries dead after storage