1980 300D.
Cruise accelerates car, does not hold speed at all when setting.
Tested the actuator under hood (vacuum can type), replaced
speed sensor on speedo with donor. Replaced amp with donor
(but did not know for sure if it worked), it still did same thing.

Opened metal can of cruise amp like the link/sticky/diy says to do.
Convinced myself nothing was wrong with the cruise amp.
I have seen bad solder joints on hardware and usually can pick them out.

Was sure that re-soldering as instructed would do no good.
Reluctantly started the job with my 180 watt trigger soldering iron
that I used for blower motor brushes, radio leads, and rear window defroster.
I found Mercedes solder to need fairly high temp.

Way too much heat, after creating burn marks on the first 10 or so connections
I switched to a 35 watt pencil type soldering iron.

Touched each joint, melted it and let it cool.
If the connection looked like the solder was being sucked out I added solder.
Tried not to overdo it, sometimes I did.
Lost track, sort of, of which ones I had done and which needed doing.
Sometimes you can tell by the color.

With even more reluctance I switched in the resoldered unit, convinced I had ruined it with my work.

The surprise was when I went to set it at about 40mph and it held.
I could not believe it.

So the lesson is, resolder the board, even if it looks perfect.

__________________
80 300D 340K Owned 30 yrs
83 300SD 440K Owned 9 yrs - Daily Driver 150mi/day
02 Z71 Suburban 117,000
15 Toyota Prius 2600 miles
00 Harley Sportster 24k
09 Yamaha R6
03 Ninja 250