The glow plug circuit on these cars keeps the glow plugs energized until you let go of the plunger on the dash. Pulling the plunger out to engage the starter does disconnect the "salt shaker" on the dash, but the actual glow plugs stay energized.

The most common problems with getting these things running are usually fuel, the glow plugs themselves and wiring to them, the engine oil, the generally and unfortunately for all of us, weak starters, as well as the condition of the injectors. A Diesel needs to turn pretty fast when the engine is cold to really reap any "heat of compression" to sustain ignition of the fuel. A low battery charge, overly heavy viscosity engine oil (cured by synthetic engine oil) or tired starter, will result in the heat of compression being absorbed in the block, making achieving the conditions for sustaining ignition of slightly coagulated fuel a genuine challenge.

The glow plugs need to come to temperature quickly and stay good and hot during the starting cycle. Old or poorly connected plugs result in the dash "salt shaker" glowing brightly, while the plugs themselves may be less than really hot. The injectors, if they are not issuing a fine mist at the right moment (timing issue, due to chain stretch and general wear of injection pump components) will conspire to make ignition impossible by squirting blobs of fuel instead of the mist needed to ignite reliably. Continuing to crank will, at best, warm up the block a bit so some of the heat of compression is still in the air charge. At worst it can result in fuel collecting int he combustion chambers and eventually preventing the starter from turning the engine over at all.

In general, in the winter when my car was afflicted with a problem, I would park on a hill and, when it was time to go, let it roll up to a brisk walking pace, with the glow plugs glowing, put it in second and drop the clutch. The speed of the engine under those conditions started it up every time.

Good luck,

Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)