It is what the name says "ADD" = "Anti-Defueling Device"
Ian
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Ian Coleman -
PRESIDENT Great Lakes Cummins Crew
1990 W350 4x4 LE std cab auto
Project Restoration
2001 2500 4x4 3rd gen head, Twin Turbo, DTT trans, Monster pump
2006 2500 4x4 Smarty JR, H2's B&W, AFE stage 1, 5" FBD exhaust