Just wanted to post this in case it could help anyone else. I have a 1998 Formula with 240k kms (150k miles). It has an A4 and for the last couple months it would bog down and sort of stutter every other time I would come to a full stop. Car was smooth at every other speed but would bog whenever I came to a stop and sort of run rough until I started moving again. The battery guage had been jumping around for a long time and whenever the engine would bog at a stop, the battery guage was bouncing a lot between 10 and 15V. I just assumed based on the age of the car and the battery gauge bouncing that it was time to replace the alternator. I read up on the truck alternator swap and ordered a KG3 unit on ebay for 40bucks shipped from a 2003 Tahoe.

Finally got around to installing it this past weekend. It wasn't overly hard but I don't want to be doing it again any time soon. Put the car up on two 2x6's under the front tires (another 3-4" would have made life easier). Someone on here posted a tip to use a coast hanger with a 1/8"hook on the end to pop the 4-pin connector, this worked like a charm. Two minutes to find a coat hanger, cut and bend the hook, and then another 20 seconds to pop the connector off. A flash light to get some light to see where the connector is and the rest was easy (check images of connector on Google so you know what you're looking for and see how it engages). Went under the car next to get the back 13mm bolt off. This wasn't as bad as I thought it would be. Used a wrench to crack it and turn it bit by bit. Access is hard, but came out within 5 mins. Popped the two 15mm bolts off from the bracket. Gave some love taps with a hammer and rod to from the top side get the alternator moving, and back under the car to start the wrestling match to get the alternator to drop. Got access to the battery cable on the back of the alternator and ran into the same issue as other posters where the whole post started to spin instead of just the nut. I ended up using a screwdriver to wedge the cable and stop it from turning and the nut broke loose and came off. Squeezed the alternator between the sway bar and fan and chucked it aside. Took the rear support bracket off next as I don't need it now.

Truck alternator was a tight fit to get back in. Took a good 30 minutes of pushing and turning and falling on my head before I finally forced it between the sway bar and radiator fan. Got it into position and then realized I needed to push back the centering bushings that are on the main alternator bracket. I used a socket that cleared the diameter of the bushing and a 3" bolt and washer that slipped through from the inside of the bracket a nut on the other end to squeeze the bushing back out. This would have been much easier prior to putting the new alternator in position but I wasn't about to take it back out. Once the bushings were reset, the rest was fairly easy. Tapped the alternator into position, bolted it back up, went by feel to get the 4 pin connector plugged in. And that's when I realized the battery post on the new alternator is not the same size as the old one. The OEM alternator uses a 8mm stud and the KG3 uses a 6mm stud. I'm surprised this wasn't mentioned anywhere before. Looked around the house but my jars of nuts and bolts are skimpy when it comes to metric. Ended up pilfering a nut from my headlight assembly just to get the car going again. One of the power steering hoses is still lightly touching the alternator but I'll tackle that later.

Fired her up, and she ran well with no bouncing around of the battery guage. Drove to work the next day and car was smooth as can be, no bogging down at stop lights. Thought problem was fixed but then when I was driving home at night, the car was bogging down worse than before at every stop and the battery guage would dip drastically. So now that I ruled out alternator, I was thinking about other possible causes such as shorts that were related to my brake lights. Then it clicked in my head. I had one of those leaking taillights on the passenger side. I bought a new used one on ebay but I never checked that one. Needless to say, I pulled over and checked it. It was full go water. I popped it off and drained what felt like 1 liter of water out of it. Put it back on and the car has been fine ever since! Been driving for 3 days and car is dead smooth at stops.

I'm pretty sure the old alternator was starting to fail because the battery guage was flickering away for a long time, but I think the bogging down was being caused by the water in the tail light. Anyways, just wanted to share some things I learned while doing the KG3 upgrade.

 

What's funny is my alternator gauge would bounce around when I had a brake light out.

I also did the truck alternator upgrade, I remember removing the old one being the hardest part.

My alternator came with the bolt on it, so I wouldn't have known it's a different size.

 

2 more days of driving and car has been running great with no issues. Can't believe the taillight water was the culprit.

Now time to do the rear axle bearings and seals. Got me 2 new 10 Factory shafts for $33 each on Amazon, now just waiting for them to arrive.

 

Damn $33? I'm going to have to jump on that too

 

Swapped my brake light bulbs and this took care of my similar idle issue. Can't believe how brake light bulbs can affect how the car runs.