What are the sypthoms of trigger points that have gone bad. At what intervals are they cleaned or replaced? thanks

Which points are you referring to? There's a set for ignition, as well as for the fuel injection system....

Prior to replacing the injection trigger points in my '73 350SLC the engine (3.5 M116 with D-Jetronic) would suffer intermittent misfire at low speeds, especially after a long journey at higher speeds. The misfire was injection related (use something long like a screwdriver as a stethoscope to listen to each injector) with pairs of injectors intermittently missing. Cleaning the trigger points would temporarily cure the problem. The permanent fix was a new set of trigger points (at great expense). That was years ago and I have not had cause to touch them since.

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107.023: 350SLC, 3-speed auto, icon gold, parchment MBtex (sold 2012 after 29 years ownership).
107.026: 500SLC, 4-speed auto, thistle green, green velour.
124.090: 300TE, 4-speed auto, arctic white, cream-beige MBtex.
201.028: 190E 2.3 Sportline, 5-speed manual, arctic white, blue leather.
201.028: 190E 2.3, 4-speed auto, blue-black, grey MBtex.
201.034: 190E 2.3-16, 5-speed manual, blue-black, black leather.

I am referring to the trigger points in the lower distributor tray. Thanks for your responses.