John- I have had a daily driver 81 300SD since 1984. It has always started in approximately two seconds of cranking after the glow plug light goes off. For some two or three years it has vibrated a LOT at stoplights but I never worried about it. This winter it began to take one to two minutes of constant cranking to get it started - very unusual. I thought this was due to the extreme cold - nearly 45 degrees F in Atlanta - and switched to 5-30w oil; this has helped in the past. This time it did not help.

Finally I took it to my mechanic and had the valves adjusted; first time since 1984 (have 252K miles on the engine). He said they were very tight.

Long story short. It now starts again after one to two seconds again and the vibration has almost gone away. With luck you may have the same experience.

Starter - this is pretty elementary but it sounds like your solenoid may be going. Years ago these could be replaced on american cars and I assume same may be true with the MB.