The most common mysterious problem is bad ground. Look at where the negative connects to the chassis and where the engine strap connects to the chassis. On an older car occasionally an auxiliary ground wire running directly to a mounting bolt on the starter will do the trick.

Also check glow plug function. There is a large flat fuse in the glow timer which occasionally will blow.

I love to have a buck for all the older mb diesels out there which got sold because of non functioning glow plugs!

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.