Definitive solution to repair cruise control module 5GA 004 397-16
My wife and I both have 1999 A6s, so I had two modules to view. I use my cruise control all the time, my wife almost never. My cruise control started exhibiting the normal problem - it would kick out after a half hour of use, and would not work again until it was off for some period of time. So, I pulled both my module and my wife's, and looked at both.
As others had mentioned, at first nothing appeared amiss. So, I took them on the porch and had a good look in very bright sunlight. And then I found it...
The problems are due to solder fracture!
Look carefully at the solder joints for C7, C9, and C11. These are Wima red capacitors. If you get the light right, what you will see a very very faint ring around the solder joints, centered about 0.5mm distant from the lead.
To repair the board, you first need to remove the resin coating that covers the solder. You can use a burnishing tool or a small screwdriver. Get as much off as you can. Then, heat the leads, melt the solder, and add a bit of new solder. This fixed both of my modules, and I've gotten about 8 hours of problem-free use out of my cruise control since the repair!
I surmise that the reason for the problem is that these capacitors are placed directly on the PC board, and have no expansion/contraction capability. Thus, as the seasons change (or cabin temp changes), the leads push and pull on the solder. At some point, the solder fractures, making an intermittent connection, and one that is sensitive to temperature. [I use to see this type of failure a long time ago when I was a field service engineer.]
This failure will occur whether you use your cruise control or not - its due to ambient temperature changes not usage based changes [thus the identical failures on my two 10 yr old units.]
For googlers: how to repair 5GA 004 397-16 5GA004397-16
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Sorry to bring this back from the grave, but this helped me solve my cruise control woes! I'll help by adding a few pictures to help other people (most notably the pins of the capacitors that need to be re-soldered):