A6 C5 ECU blowing fuse when shift into Drive

[Allroads_Lead_Somewhere]

Hi All I've searched every forum imaginable, and I am stumped by this problem. Just replaced turbos, car ran fine for 800 or so miles. Suddenly when I shift tiptronic trans into DRIVE, the ECU fuse (#29 30 amp) blows. replaced fuse, Scanned ECU, got a P0305. Started engine, car barely ran shuddering and misfiring at idle, and had something like 300 misfires in cyl. #5 within 20 seconds. As soon as I shifter car into DRIVE again, the ECU fuse blew, yet again. I replaced #5 Coil, car idles fine as long as the car is in "PARK", with zero misfires UNTIL i put the car into "DRIVE", then I get serious misfiring in Cyl. #5, but it doesn't blow the ECU fuse anymore. I did, however, notice that the #5 coil pack seems to get extremely hot. To sum it up. Car will idle perfect with zero misfires, as long as the transmission is in the "P" (park) position. But, as soon as I put the car into "D" (drive) the car starts seriously misfiring in cyl. #5. I can put the car in "R" (revers) and the car drives just fine with zero misses and plenty of power. I am stumped! Maybe something to do with tip trans? What would cause a short in coilpack # 5 when car is shifted into DRIVE, but not while idling in PARK, or shifted into "R" (reverse) ??? I could really use some help on this, as this is my daily driver. PLEASE HELP! Thank you in advance!

[Tuppidsay]

Well this one seems like fun, I would first start the car and then go wiggle the harness around the engine and see if it will act up, if nothing happens there i would lift the carpet in front of the passenger seat and take a look at the trans computer. I've found them flooded before (no signs of water on the carpet) causing all sorts of interesting issues.

[Allroads_Lead_Somewhere]

Originally Posted by Tuppidsay

Well this one seems like fun, I would first start the car and then go wiggle the harness around the engine and see if it will act up, if nothing happens there i would lift the carpet in front of the passenger seat and take a look at the trans computer. I've found them flooded before (no signs of water on the carpet) causing all sorts of interesting issues.

Thank you for your quick reply. After writing my thread, I thought about the TCU under the carpet, so I went out and checked. It's dry as a bone and doesn't appear to be out of order.
The problem I see with wiggling the harness, is that the engine doesn't misfire while idling in park, "P", so I would doubt that the issue can be replicated while idling in "P". The misfires occur only when shifted into drive. I'm afreai that I'm gonna fry the new coilpack and start blowing the ECU fuses again.

[Tuppidsay]

my thought behind jurking on the harness is that when you put the car into drive the motor tilts up, possibly moving the harness and causing a chaff or something like that. maybe if thats the cause you can pinpoint the area.

[Tuppidsay]

well digging threw the wiring diagrams fuse 29 is only for the coil packs. maybe try swapping the connectors on the Power output stages (the 2 things bolted to the top of the air filter box. Move connectors from 1 to the other and vice versa. then start the car and see if the problem moves to the opposite side of the engine. if so you may need one those.

[Allroads_Lead_Somewhere]

Originally Posted by Tuppidsay

my thought behind jurking on the harness is that when you put the car into drive the motor tilts up, possibly moving the harness and causing a chaff or something like that. maybe if thats the cause you can pinpoint the area.

Actually, that seems like a great idea. I'll do just that and post the results. Thanks again for your prompt input!

[Allroads_Lead_Somewhere]

Originally Posted by Tuppidsay

well digging threw the wiring diagrams fuse 29 is only for the coil packs. maybe try swapping the connectors on the Power output stages (the 2 things bolted to the top of the air filter box. Move connectors from 1 to the other and vice versa. then start the car and see if the problem moves to the opposite side of the engine. if so you may need one those.

Would they be the "ICM's" that sit on before the Mass Flow Sensor? Okay, so if i switch them, and the problem moves to the passenger cylinder bank, then I've found my problem? Sounds good to me! Thank you for the ideas, you guys are great!

[Allroads_Lead_Somewhere]

Just an update. I tried moving around and jerking on the harness while engine was running and transmission in "Park" position. I could not duplicate the problem that occurs when I shift into "D". Also, I swapped the connectors at the ICM's that sit atop the airbox, the problem remained the same at #5 cyl. coilpack, P0305

[Allroads_Lead_Somewhere]

Thank you to those who replied with their ideas. The problem turned out to be a short in the #5 coilpack harness at the connector to the coilpack. I peeled back the black shrink tubing to find that the 4 wires that supplied voltage and signal to the coilpack were missing most of the insulation, which was dry, cracked and falling off the wire. Four of the six coilpacks had compromised harnesses, so I cut the wires, slid on new shrink tube insulation and silver soldered every wire back together. I would have preferred a new harness, but I was pressed for time and I think this will do just fine.
Thanks again for all your help!

[Allroads_Lead_Somewhere]

To Tuppisday, after reading over your post again, it looks like you had the problem nailed from the beginning. When you suggested that I move the harness around, while the engine was running, you had the problem solved. I did just that and that's how I found the short in the #5 Coilpack harness connector. So thank you very much and I appreciate your knowledge!!!