Inverter
A guided diagnosis for RV techs. Each step is a check, a reading or a question.
- Inverter complaint? Pure sine inverters (Magnum, Xantrex, Victron, Go Power) share the same logic: DC in, AC out, a transfer relay, and a remote with fault codes. Expect: DC input within the inverter's range (about 10.5 to 16 V on 12V), 120 VAC out, fault light off
- DC in, and is the inverter on? Meter the DC terminals at the inverter under load. Check the DC fuse or breaker between the battery and inverter, and the remote's on/off. Expect: 12 V or more at the inverter DC terminals; 120 VAC at the output
Causes and repairs
DC fuse or cable
The big ANL or class T fuse at the battery, a corroded lug, or the disconnect. A fuse that blew with no short is often a fuse that was undersized for the inverter's surge.
Tools: multimeter
Replace the fuse with the correct rating for the inverter and cable. Torque the lugs. Measure drop under load: under 0.5 V end to end.
Inverter dead or latched in fault
Try a hard reset per the manual (most: hold the power button 15 seconds, or disconnect DC for a minute). Check the remote cable. If it stays dead with good DC, it has failed.
Tools: none
Hard reset. Replace the remote cable. Replace the inverter if it won't wake.
Output breaker, GFCI, or sub-panel
Most inverters feed a GFCI outlet or a small sub-panel. The output breaker on the inverter, the GFCI, or the sub-panel main.
Tools: multimeter
Reset the inverter's output breaker and the GFCI. Meter at the inverter output terminals, then at the sub-panel.
Remote fault codes (Magnum wording; others similar)
Low Battery: DC under the cutout, charge it, clears at 12.5 V. High Battery: over 16 V, a charger running away or a 24V bank on a 12V inverter. Overtemp / Tfmr Overtemp: hot, check the fans and ventilation, reduce load. AC Overload / FET Overload: too much load or a short on the output. AC Backfeed: shore power got onto the output, a wiring error. Stuck Relay / Internal Bridge / Internal Relay: hard reset, then service. Dead Batt Charge: battery too low to start charging (under 7 V on 12V), charge it another way. High AC Volts: shore or generator over 130 V. No Comm: remote cable.
Tools: multimeter, clamp meter
Fix the cause, then soft reset (power button) or hard reset (DC disconnect) per the manual.
Shuts off under load
Low battery sag (meter the DC terminals the moment it trips), an undersized battery bank or cable (big drop), or the load exceeds the rating (microwave plus coffee maker).
Tools: multimeter, clamp meter
Watch DC voltage at the inverter under the load that trips it. Under about 11 V: battery or cable drop. Voltage fine: overload; check the inverter's rating and the load's real draw with a clamp.
Transfer relay
On shore power the inverter should pass AC through its transfer relay in a few seconds. No pass-through with good shore power is the relay, an AC input breaker on the inverter, or 'High AC Volts' holding it off.
Tools: multimeter
Meter AC in at the inverter. Reset the input breaker. Listen for the relay. Hard reset. Replace the inverter if the relay is stuck.
Idle draw
An inverter left on draws 0.5 to 2 A doing nothing. Over a weekend that is the battery. Many have a search or power-save mode.
Tools: clamp meter
Turn it off when not needed, or enable search mode. Check the remote isn't holding it on.