How difficult is it to convert a B4 from an automatic trans. to a 5spd?

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What, if anything in specific, would you like to know ?

Wiring would be my main concern. Do I have to change some sort of major wiring harness or a module which communicates with the transmission? Also, will the 5 spd transmission out of a b3 fit the b4 with an AAH engine? (Non quattro) And one more question, do I have to change the brake booster for the hydraulic clutch?

"Wiring would be my main concern. Do I have to change some sort of major wiring harness or a module which communicates with the transmission?"

No, in fact converting to a manual completely removes the need for most of the wiring to the transmission. The only thing on the manual that requires wiring is the speed sensor (which is already there and the same one as the automatic, see pic 1), so you just plug that back into the one on the manual transmission and then the wiring for the reverse lights (pic 1) which you can just run two independent wires for. The reverse lights need to be wired up to the empty relay spot under the drivers side dash (pic 2). That empty relay spot will have the Park/Neutral Position switch relay in there on the automatic's setup, but it needs to be removed and jumpered in order for the car to start. Removing it causes no issues since it's only there for the automatic transmission and shift lock control which will be removed when you convert to a manual. You will also disconnect the computer for the automatic transmission, you can leave it in the car or take it out as it will no longer be used. The harness that goes to the transmission from the computer runs through the dash. I didn't feel like taking apart my entire dash just to get the harness out so I left it in the car and just tucked it up and to the side at both end points.

Pic 1 (courtesy of Miktip)

<a href="http://s101.photobucket.com/albums/m49/starch87/?action=view¤t=tranny.jpg" target="_blank"><img src="http://i101.photobucket.com/albums/m49/starch87/tranny.jpg" border="0" alt="Photobucket"></a>

Pic 2 (courtesy of Miktip)

<a href="http://s101.photobucket.com/albums/m49/starch87/?action=view¤t=starterjumper.jpg" target="_blank"><img src="http://i101.photobucket.com/albums/m49/starch87/starterjumper.jpg" border="0" alt="Photobucket"></a>


"Also, will the 5 spd transmission out of a b3 fit the b4 with an AAH engine? (Non quattro)"

No clue if it will work, all my parts came from a B4. But Audi uses the same components on multiple platforms so its possible.

"And one more question, do I have to change the brake booster for the hydraulic clutch?"

Nope, the only thing on the brake system you have to swap out is the fluid resevoir, the cars with manual transmissions draw their fluid from the brake resevoir. On a manual transmission car it will have an extra nipple on the bottom of the resevoir that connects to a rubber hose which supplies the clutch master cylinder with its fluid.

I have also emailed you a partially complete copy of a word document I have been working on that goes into more detail concerning the swap.