Is anyone familiar w/how & when the trailer (frame mounted)battery is charged?Does it just deal w/trailer lighting/brakes or does it supplement main battery?On the aux battery relay (under main battery tray) I am getting aux battery voltage on rear post, alt voltage on front post & have constant power at relay switch w/control wire unplugged.(even with key off). I assumed the relay to be internally shorted. Called ford & asked if aux battery relaywas same part # as starter relay (looks very similiar). It is not. I work on chevs & have no access to anyford data. Any thoughts?
thanks

 

In case any one runs across this,
The aux battery relay works exactly the same as a starter relay, even though ford said it doesnt. All it does is join batteries together after the vehicle is started to charge both. My relay was internally shorted.