Yea thats what I thought the answer would be. Sorry but I made the mistake of not framing the question in better detail. Which brought about a lot of answers which I already knew.
I had already spoken with a starter repair company. They said that it was almost impossible to predict when a solenoid would fail which is what I already suspected.
I am not 100% sure about the this starter. I can tell that it has been rebuilt but the solenoid is probably original. At 50 years old I have doubts about the longevity and performance of it. It is the lack of history with this car that has caused many a problem for me. This being my second full restoration of a 60s era Mercedes. I have learned that it is better to just rebuilt or replaced almost everything.
My goal was to find out if there were a way to measure the performance of the solenoid and find out if it was still within the specifications for optimum performance. I am sure these specifications exist. But is the equipment available to actually measure it and will Bosch supply and the specifications?
I am tempted to purchase a new solenoid and install it on the starter but I do have another starter which I may put into the car. I know the history of this starter as opposed to the one I took out of the car.
And yes I do know a lot about these cars but my Achilles' heel of auto restoration is the electrical systems. I have the damnedest time trying to figure it out or more accurately remembering how everything works electrically.
I think everyone and I appreciate all of your answers.
Take Care

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Britton McIntyre

68 280 SE coupe 'Hairball'
70 280 SL
71 280 SEL - RIP May 2010