Pull up a starting wiring diagram and figure out what should be getting 12volts, what is a load. Make sure you check these things:

1. Good battery
2. good connections that arent dropping volts where no volts should be dropped.
3. Starter solenoid
4. Igntion switch
5. neutral safety switch
6. corrison on any connections
7. check for opens, shorts

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1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)