I have a 2000 Audi A6 (that had an ART 4.2 engine).  I replaced the engine with a 4.2 out of a 2002 S6.  There is a difference in a few of the wires to the ecu, but that was, I believe (since I have gone through that several times to make sure it is correct) that is all fine.

The problem I am having is that I cannot see the ECU with VCDS on the car.  I can get to it on the bench.  Also, the car will not start.  No fuel pump kicking in; nothing.

It seems that I do not have power to the ECU, but I can't figure out why.

I hope someone can shed some light on this.  Basically, the ECU relay is not energizing.  On pin 23, I do not get ground (which comes from the ECU to energize that relay).

Also, on the fuel pump relay, the ground signal from the ecu is not there.

Since I could not figure out how to shut off the immobilizer by myself, I sent the unit out to have the immobilizer defeated.  So, my though is that at this point it is strictly plug and play.

Any thoughts that might help me get on the right track?  By the way, I have spent an embarrassing number of hours trying to figure it out, looking all over the web and I am giving up, and hoping someone might know the answer here.

Thanks...

Is the ART ecu ME7.1? If that's the case, You got a ME7.1 ecu harness and ME7.1.1 ecu. Find the wiring diagrams for both and make the necessary changes to the ecu connector. Other option is to flash the original ecu with a S6 file.

Can You provide the part numbers from both ecus?

Thank you so much for the replies.  it is the bbd engine with the engine harness that came with that engine, as well as the ecu (559b).

ddillenger, given this, should i go ahead with what you suggested?

thanks again!

DDlillenger, you were right.  And thank you so much.  In converting the wiring to 6-speed, I forgot to connect S7 to the orange connector.  Dah.  The car started, so now on to find other issues.  Thanks again...

I am running into another issue.

The car starts.  There is no throttle response, however.

Immobilizer was defeated, and the ecu was programmed for a 6-speed transmission.

I see the ecu in the car with VCDS.

I get these codes:  P1517 (ecu power relay) and P1579 (no throttle adaptation), as well as instrument cluster; airbag; and abs.

The main issue are the P1517 and P1579.

When I attempt to adapt the throttle with VCDS, all I get is an error.

On vcds, trying to adapt the throttle body, I see on the left upper side of the program where it shows "label", shows "redir fail," so vcds is not recognizing somehow the tune.  When the ecu was redone, the fellow who helped me with it changed the part number on the ecu from "B" to "E".  I don't know what that means as far as the vcds is concerned.  But it sounds to me that because vcds is not recognizing the program on the ecu, it is not letting me adapt the throttle.

Regarding P1517, I believe J271 is okay.  It has 12 volts on the input and the output, and the small 12v red wire.  The white and black small wire from pin 23 has about a volt.

Any thoughts will be greatly appreciated.

Thank you so much.

Got the car running!   Thanks a lot for your help.  By the way, it was a wiring issue.  The ECU relay is wired differently in the 02 S6 than in the 00 A6 that I have been working on.  S29 was fed from the key.  On this car, S29 was supposed to be fed fro the ECU relay.  Not having it that way was giving me a an electrical malfunction code for the ECU and was not letting me adapt.  Once I fixed it, it called me to adapt fine.

So, for anyone considering installing an S6 engine in an earlier A6, make sure S29 is fed according to the diagram.  I assumed that S29 was fed properly and did not notice that the white pin near the ECU which is supposed to connect to that fuse, was actually not connected to the fuse, and the coilpacks were getting power from elsewhere.