mine would not start recentley. It just went click click click and stopped. turned out to be the reverse lockout switch... so just jiggle the shifter and put it from P to D a couple of times and mine started... hope this helps
(this was the first time in 249K miles it wouldnt start except for when I left the interior lights on while away for vacation and the battery obvioulsly drained

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1978 w123 300D astral silver "Benzie"
1980 w123 300CD orient red "Benzie 2"
1994 w124 E500 "Big Black"
1998 w163 ML320 "Ole' Faithfull"
1969 Datsun 2000 roadster