I'm going to install a aux fuel tank, and need to connect a sensing wire to the OEM tank sending unit wire harness. I crawled under the truck to look for the OEM sending unit wire harness, but couldn't find it. Any help guys? 2008 LMM

Are you meaning the wiring for a stock OEM aux fuel tank that is an option on Cab and Chassis setups and not referring to stock wiring for main tank? If looking for the other not sure. Not looked for it. But if stock non cab and chassis come with it then look back towards the passenger side rear wheel area or maybe in front of the fuel cooler about where the cab and bed meet. If present should be one for sending unit and one for a fuel transfer pump.

Brian

I'm looking for the Purple wire contained within the sending unit wiring harness on the OEM fuel tank.

I'm adding an aux tank in the bed of my 3500 dually, and the computer located within the cab has a wire that needs to connect to the OEM tank sending wire, so that it knows when it is low, and then transfers fuel from aux to OEM.

The instructions (ATTA brand tank), say that for GM vehicles, it is a purple wire.

I do not know the wiring of that unit your adding but if it refers to the wire on the sending unit itself then from a book on my 2003 2500 Avalanche which is GMT-800 platform it is a purple wire off the sending unit. In the book on my 07.5 which is GMT-900 platform it is a Tan/Black wire. Only 2 wires on a sending unit one goes to ECM and the other goes to ground I do believe. So I believe that is the one your gonna tap. Wish my 2003 showed Duramax setup but have a feeling the color is changed in the new model or at least the Bosch ECM maybe.

If you could scan the instuction skematic it might help and post here. Could not find install docs on ATTA site.

Brian