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As requested here is a step by step procedure for this fix.

Cruise Control Solder Fix
By Kevin Braun (Summeronthechain@yahoo.com)

Please take all necessary precautions. If you are not comfortable around circuits please leave this to someone who is. I have designed this write up so that even a person who is new to electronics and soldering should be successful, but bear in mind that shit happens.

Description:
This fix is designed to cure inoperable or improper functioning cruise control operation in the 1997-1999 D2 Audi A8. The main part of this write-up may also apply to similar vintage Audi A4, A6, and some Volkswagen models as well.

Problem:
Cruise control no longer activates, shuts itself off after a short period, or requires multiple attempts at turning it on before it activates.

Cause:
Over time the factory soldering heats and cools with the ambient temperature and can break the soldering on the board due to contraction/expansion. This causes an intermittent connection between the circuit and the capacitors.

Other Possible Causes:
Some other known causes of cruise malfunction on the A8 are:

-A leak in the vacuum lines running between the brake switch and cruise servo, most notably at the T fitting along the driver's side fender.

-The F125 switch located underneath the car near the front driver's side halfshaft can also cause the cruise not to work. Over time the connection gets corroded with water and oil and can lose continuity. The remedy for that is replacement or cleaning of the connector itself.

-The cruise control stock itself. This is the entire assembly on the left side of the steering wheel. Some of the terminals can become corroded to a point where they no longer make contact. Some people have remedied this by prying the switch apart carefully and cleaning the contacts with an appropriate solution or replacement of the switch.

Tools needed:
*Solder
*Solder gun
*10 mm open end wrench
*Phillips screwdriver
*Small flat head screwdriver

Please disconnect the negative lead on the battery before starting.

1. Remove the control box cover.
Open the hood. On the left (passenger) side near the cabin you will see the control box and the plastic trim surrounding it. The plastic trim is held on with some clips at the front and a few Phillips head screws.



Once you remove the trim, finish removing the several screws that hold the lid on the box. There is one screw that is buried under the cowl towards the center of the car, but is easily accessible through a hole in the cowl plastic.



Gently slide the cover to up and to the front of the car. You now have exposed the Cruise control module, as well as a number of other critical computer components. These components include the TCU (transmission control module) and are very sensitive to condensation/water so please be careful.

2. Remove the cruise control module.



It is the smaller silver/aluminum colored box. There is one 10 mm nut holding it on. Remove the nut and lift the module out. Be careful not to pull hard on the module as the wire harness is still connected.



Remove the wire harness by prying outwardly on the two tabs. It is stubborn but will come eventually. I used the small flathead here to help it along. Take care not to break off the tabs.


3. Open up the module.
On the end of the module with the harness connector, pry up gently on the 4 aluminum tabs with the flat head screwdriver.



You should be able to wiggle the flat black piece out. It is not connected to anything inside the module.



Once you have it opened up you need to slide the circuit board out. Try with your fingers first. Take caution not to damage any of the circuitry. Mine was in pretty solid so I had to use a pliers to gently tug it a bit. Be very careful in doing so.

4. Solder.
Once the circuit board is out look at the C7, C9, and C11 circuits. These are capacitors. Make sure you are looking at the correct ones.



Then flip the board over and locate their leads. Don't feel bad if you can't find a crack in the solder, I couldn't find one in mine either. These are what you will be re-soldering. There is a thin coating on the back of the board. Using your small screwdriver, lightly scrape as much as you can of it off the 6 solder points.



Heat up your soldering gun. If you have never soldered before I suggest you play around with the solder a bit to learn how it flows and reacts as it heats and cools.

Re-solder the points. Try to be as neat and tidy as you can. Also make sure you heat up the lead and the original solder. Minimal additional solder should be needed.

It should only take a minute for the solder to cool.

5. Rebuild and replace module.

The rest should be pretty explanatory so here is the gist of it:
1.Slide circuit back into module box
2.Replace black end cover
3.Bend aluminum tabs back
4.Plug wire harness back in
5.Replace module box back into position and secure the holding nut
6.Slide control box cover back into place and replace screws
7.Put plastic trim back in place
8.Go test it out!

Good luck and God bless the D2!

• Downloadable PDF of write-up.