the cutoff solenoid is located just behind the stop lever (looking towards the firewall. It is round and has star like screws (not exactly sure what they are called). I had mine replaced by the MB dealer so am unsure exactly how to remove and install a new one. I will post a pic a bit later (at work right now). Another thing to check to see if it is the cutoff solenoid is to watch the stop lever when you turn the key off. If it doesnt go down completely, but half, that would be a good indication that the cutoff solenoid is the problem.

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1980 300D European
"The Red Fox"
200,000 miles

2003 Chevrolet Silverado

609 Certification