Fuses, breakers and grounds
A fuse that blows is doing its job. Find out why before you put a new one in.
The short version
- Blows instantly = a short. Blows after a while = an overload or a dragging motor.
- Never put in a bigger fuse. The wire is sized for the fuse that came with it.
- Auto-reset breakers on battery feeds do fail open. Check voltage on both sides.
- Most RV 12V circuits ground to the frame or a ground bus. Clean, tight grounds fix a lot.
On the job
- Fuse open? Meter across it under load: 0 V across a good fuse, battery voltage across an open one.
- Instant blow: unplug the load, new fuse. Holds = load shorted, blows = wire shorted. Ohm the wire to the frame.
- Load has 12V to chassis but 0 V across its own two wires: the ground is open.
- Drop from the panel negative bus to the battery negative post under load: under 0.2 V.
Shop tip
If a fuse looks fine but you're not sure, check voltage on both legs with the circuit on. Power on one side only = open.
Try it on a service call
Put the lesson to work on a hands-on troubleshooting case.
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
- Converter / charger
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