7,6,4 Way Wiring Diagrams
7 Way Wiring Diagrams:
There are two wiring codes for different types of 7-pole plug and socket sets.
Most RV's use the RV wiring code for the type plug/socket that has 6 flat contacts surrounding a center round pin. Semi's and commercial trailers use the ASME (American Society of Mechanical Engineers) type 7-pole plug sets. These plugs have 6 round contacts surrounding a center round pin.
Note: the ways to convert from 7-Pin Commercial Plug to the 7-Blade Light Duty Plug is covered inTaillight Converters.
This drawing from Mark Bruss shows the relationship of the 7-round to 7-flat plugs and how only three wires are common.
BTW, the wire colors on your truck or trailer could vary from the colors shown in the diagrams.