I went down the road of trying multiple CCU units on my 87 300D. I ended up following the instructions by forum member James Dean, and was able to replace the capacitors inside the unit. Mine went from flaky operation to perfect operation. I also re-soldered all of the points on the bottom of the circuit board. There is a good chance that flaky-operation could link back to cold solder joints and/or bad capacitors. The boards on these are pretty simple as far as electronics go. I don't have time to look it up right now but if you search CCU solder or something like that I bet you will run across his posts.

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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)

good idea. did that to my w123 but i didn't know the w124's had cold solder joint issues.

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83 300CD- sanden, dual p/f condensers, 160a alternator, ect
91 300TD- 722.6, #22 head, 3.5L IP, w140 manifolds, ect

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)

any ideas on the dash swap? will it work?

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83 300CD- sanden, dual p/f condensers, 160a alternator, ect
91 300TD- 722.6, #22 head, 3.5L IP, w140 manifolds, ect