FI boxes either work or they don't... which usually meaning a fuzzy connection... easy to defined but time consuming to find... basically you have to inspect every wire and every connection like Sherlock Holmes...
BATTERY...
Start with the battery for it's the weakest link in the whole system... To
determine the condition of an Maintenance Free battery give it a
refreshing charge... wait 30 minutes... measure terminal voltage...
12.8 or higher is a good battery...
12.0 to 12.8 is a insufficient charge... recharge...
12.0 or lower... battery unserviceable...
CORROSION...
This process uses two solutions, one is regular table salt and
vinegar. Any kind of vinegar will work, from balsamic, to rice, to
white vinegars. Its the acidity and corrosiveness of the salt and
vinegar together that you want. The other solution is Sodium
Bicarbonate, or baking soda, and water. This is used to neutralize the
corrosive properties of the other solution, and to further clean the
wires.
Step 1: Strip the wires to be cleaned.
Step 2: Get 2 containers, one for each solution. They can be paper
cups, plastic, glass, bowls, whatever you can find. I have vials,
because I am a professional electronics installer and I use these
solutions out in the field.
Step 3: Get 1 tablespoon of raw salt, and put it in one of the
containers. Fill up the rest of the container with vinegar, and stir
the both together. As a general rule of thumb, put as much salt in the
vinegar as will dissolve.
Step 4: Get 1 tablespoon of Sodium Bicarbonate, (baking soda) and add
it to the other container. Fill up the rest with water, and stir well.
Add more baking soda to make it cloudy. The amount is not important,
as long as it is alkaline to cancel the acid of the vinegar solution.
Step 5: Put the stripped end of the wire in the vinegar solution, and
stir the solution with the wire. any wire you want cleaned needs to be
under the solution. Movement of the wire in the liquid speeds up the
process.
Step 6: After 2 minutes or so, the wire will look very shiny and new
in the vinegar solution. The acid and salt in the solution is etching
away the oxides, exposing the bare metal. Make sure the metal is
uniformly shiny. Leave it in longer if it is not perfectly clean
throughout.
Step 7: Once the wire is satisfactorily clean, remove the wire from
the vinegar, and plunge it into the baking soda solution to neutralize
the acid's corrosive properties. If the wire was exposed to the air,
without neutralizing the acid first, it would quickly corrode again.
The baking soda keeps it clean and shiny. Swish the wire around in the
baking soda water for about 10 seconds, and then you are done!! Shiny
new wire ready for soldering, and conducting once again!!!