I should add:

If it is the CPSU, the car will crank and catch for a fraction of a second before shutting down.

The insulation on mine had worn away and exposed the stranded wires -- which were also a bit corroded. When it got hot outside, the heat interrupted the signal between the crankshaft end of the sensor and the ignition control unit end, and the engine would shut down -- usually when the car was stopped.

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2008 E350 4matic / Black/Anthracite

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Gone but not Forgotten:
2001 E430 4matic, 206,xxx miles, Black/Charcoal
1995 E320, 252,xxx miles, Black/Grey
1989 260E, 223,00 miles, Black/Black