I couldn't find the thread I was originally thinking of, but this link is a pretty good discussion on the internals and should give you the basic idea of what the W124 CCU internals look like. The capacitors are so cheap there really is no excuse to avoid replacing them while you have it apart if you are checking for cold solder joints. I only suggest this because in my case every CCU unit I tried from the junkyard didn't work properly haha.

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97 S500 (90,000 miles) - wife's car
87 300D (298,000 miles and a replacement #14 head)
94 Suburban K2500 - need something to haul firewood
83 300SD (343,000 miles) (sold)