I am in the middle of doing a tII swap in my n/a. It is an s4 n/a and I am putting a s5 engine in. My question is that when I was swapping harnesses the s5 harness has a large yellow plug that goes somewhere (obviously) but it doesnt plug into the ecu it goes to another plug down in that area. The s4 harness has the 2 ecu plugs, 1 orange plug, and a yellow plug. What do I need to do to make the s5 harness work?

 

The S4 has three ECU plugs.

You would need to do considerable re-wiring to make a S5 engine harness work in a S4 car.

You would be much better off using a S4 front cover and MOP and using a S4 engine harness. Or replacing all the interior and front harnesses with S5 versions if using the S5 ECU and engine harness.

Please use the search function (found in the upper right hand corner of the page) next time. This has been covered quite a few times.