Date registered: Feb 2010
Vehicle: 1992 500SEL
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So I take it that the mechanical one you are disassembling is your original, and the digital is the one on Ebay? Also, the digital one is acting up on your car aswell? Interesting.
The digitals are not known to go bad, as they are now the replacement part for the W140s, replacing the older analog types which were prone to failure. If the digital is acting funny, then there is a good chance you have a wiring issue with the car.
The old mechanical ones can fry by themselves, or with some help by either boosting, or by an electrical problem. Especially if any of the positive inputs are flakey and disappear during operation, this will definitely fry it. The digitals are friendlier and you can throw anything at them, and they wont fry, they'll just misbehave, similarly to what you are seeing.
Since your digital is consistantly misbehaving, from what you described, it should be easy to track down the issue. In my FAQ page, there is a pinout of the round connectors identifying + and -. Go through it carefully and verify yours are either good or bad. If you find a problem, its a wiring issue.