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Over in another thread, we were discussing the hot wire plug supplied at the rear bumper for connection to a standard RV - all my past trucks had that connection leading to the truck battery thru a fused lead - all the guys in the mentioned thread apparently are seeing that same pin/lead with 12 volts at all times - I'm not!

My truck is an '02 2500, has the Camper/RV package, and has 2 pre-wired plugs - one flat style 4-pin for utility trailers and such, and the 7-pin type for RV or camper hookup...

The pin that normaly would be at 12 volt potential only shows about 2.5 volts on a sensitive test meter - seems more like a "floating" voltage rather than a stiff one - sorta like a control voltage waiting for a circuit to be completed.

The relay/fuse box underhood in my truck has a 30 amp or so trailer fuse, AND a relay marked "trailer" - and I sorta assumed that relay was somehow controlled - perhaps by the ignition - to be activated and supply voltage to the hot RV lead only when the key was on - or perhaps it is for the RV running lights? In any event, I was unable to activate that relay with the key (I didn't think to try the light switch while I had it apart), and have NOT been able to get 12 volts to show at the rear RV plug - all other functions of the plug are absolutely normal - all lights and brakes operate perfectly - and unless there's some tricky setup that only applies 12 volts to that pin when the trailer is actually connected, I'm at a loss to figure where my 12 volts are!

SOOOooo - I'm hoping someone with a wiring diagram can tell me what that underhood "trailer" relay is activated by - and what it operates, and WHERE the wire from the hot lead of the RV plug actually goes to tap into it's 12 volt source...

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YUP - John got it!<br><br>The relay is only for trailer running lights, to remove their load from the light switch - it was sorta confusing because the owner's manual said a relay like that needed to be ADDED if trailer use was planned - guess that part was for trucks not already being set up for trailer/camper use - too bad it isn't MARKED as &quot;trailer lights&quot;, instead of simply &quot;trailer&quot;... : :<br><br>The 40 amp fuse marked &quot;trailer&quot; is only for the hot wire going back to the plug for RV power source - I had visually checked mine - looked OK, then actually PULLED it and checked with an ohmmeter - checked fine, so I scratched THAT off the list - but fact was, it had a hairline open that provided enough current flow to LOOK OK on the meter - and also allow the small voltage I was measuring back at the plug, but NOT full voltage! I removed the protectice cover over the top of the fuse (after measuring it a SECOND time, and finding it then OPEN!) and carefully applied pressure to the fuse element, and saw it was fractured... Yeah, you should always use a meter to check fuses, and SOMETIMES, even THAT doesn't work! ;D ;D<br><br>ANYWAY, I dunno if that fuse actually failed due to vibration, or burned because of the voltage disparity between my trailer and truck, but since I don't need it anyway, I'll simply leave the fuse out, and all is well...<br><br>Mystery solved! ;D ;D ;D