I dropped in a 86' 6.9L into my 88' F-250 that had a 7.3L. The wiring harness on the 7.3L has a 12 pin connector and the 6.9L has an 8 pin connector. Any thoughts or suggestions on how I convert the 12 pin to fit the proper connections on the pin and what are the extra wires? Any wiring diagrams would be great.

 

A haynes or chiltons manual should have the wiring diagrams you need. Good luck finding what you need.

 

Here's a diagram for the later models. I can't find one for the 86.
http://www.autozone.com/images/cds/g...3d800a3a46.gif

You should be able to look at the two engines and see what is different. Is there a reason why you cannot unbolt all the electrical stuff off the old 7.3 and bolt it on the 6.9 block? Then the wiring would match?