12 volts not working

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pa bell

Active member
Aug 11, 2012
Buckeye Lake, OH
I cannot power any lights or equipment in my TT unless I am on Shore power or hooked to the truck. I have put a  new battery in with the same results. I am stumped for an answer any ideas?
What make, model, and year is your rv? Does it have a battery disconnect switch? On travel trailers, there is usually a circuit breaker in the positive line, usually near the battery/tongue area. It may be hanging in midair, mounted to the frame, or in a junction box. It may need to be reset or replaced. It looks like this:
The 12 V trouble is with a 2015 MXT303 toy hauler. It does not have a disconnect switch that I know of. I will look for that relay who wrote about.
On my Sunnybrook trailer, that circuit breaker was hidden in a home style junction box underneath the front floor.  I had to crawl under the trailer and trace the positive battery wire going into the box and remove the cover before I found that little bustard.
The simple answer is that your battery is not connected to the rest of the 12v system. The tougher question is Why? If no disconnect relay, then you have lost a wire connection to the battery, either between the battery Positive post and the converter/charger, or the battery negative post and chassis ground. Gonna have to hunt to figure out what it is.
I did find the relay mounted to the frame. It was mostly disintegrated. As I can have 50 amp service, am I right to assume that I should be buying a 50 amp breaker?
50 amps refers to the 120/240 volt side of life.  it has nothign to do with the 12 volt.

What converter do you have.. I don't need to know that by the way.  Most converters have a model number in this format xxaa  IE: mine is a 9180..  The first two are the model line (9100) the last two are the max output (80) so I have an 80 amp breaker.

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