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Ask Dave: Indoor lights only work when ignition is on. How come?

Dear Dave,
My husband, David, can’t figure out why, when our RV is plugged in for power, all of our indoor house lights do not come on at all unless we have the ignition key turned on partially or turned on completely and also have the engine on. Please help us. We live in our RV full-time and have almost no $$$ to spend. Thank you so much!!! —Christina, 1999 Georgie Boy Chevy Pursuit

Dear Christina,
I believe you might have a bad cell or short in the house battery/batteries. My first question is: Do any other 12-volt components work when you are connected to shoreline power such as a vent fan or water pump? Since you are living in the RV full-time and are probably not only plugged in but also have city water pressure, you might not notice that the other 12-volt components are not working.

Your house batteries provide power to the distribution center to automotive-type fuses and then to the various 12-volt components. When you are plugged into shoreline power, the converter should be putting 13.6 volts into the batteries until they reach 12.6 volts and then drop to 13.2 volts for a maintenance charge. This should power the house lights unless you have a dead short in the battery, but then nothing should work.

The reason the interior lights are coming on when the ignition is turned on is the battery isolation manager (BIM) has not opened the circuit from the engine or chassis battery to the positive side of the house batteries. That is designed to charge the house batteries via the engine alternator while driving.

Use a multimeter to check the house battery

I would recommend getting an inexpensive multimeter like the one I just purchased at Harbor Freight on sale for $4.95 (or on Amazon for around $10 or less) and checking the house battery/batteries. Start with the unit unplugged to see what state of charge the battery/s are in, then plug it in to see what charge comes from the converter. My guess is on the state of charge on the batteries. I doubt it is the fuse or fuses in the distribution panel as typically the BIM and charge from the engine battery when you turn the key on is downstream or before them.

If you have no charge showing when connected to shoreline power, not only are the batteries dead but the converter/charger is not working either.

I would then connect a standalone battery charger to the house battery to see if the lights work.


 You might also enjoy this from Dave 

I get a 13.2-volt reading on house batteries.

Dear Dave,
I have a 2017 Jayco Alante and I would like to know why, in testing my house batteries, I am getting 13.2 volts. I have been advised by the dealer I should be getting 13.6 volts. What is the correct number I should see on my voltage meter? —Dave

Read Dave’s answer.


Dave Solberg is a leading expert in the RV industry and the author of the “RV Handbook.”

Read more from Dave here

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