The 115 300D has a relay up under the cowl and a temperature sensor/timer in the head just in front of #1 glowplug. Sounds like the sensor is bad and isn't grounding like is is supposed to -- it has a thermal swtich that is supposed to regulate how long the glowplugs operate according to temperature.

My brother's is shorted -- the plugs run until you crank the engine. Your's appears to be staying open. Check for voltage on the "back" side of the sensor and see if it is hot when the plugs are supposed to be on. The dash light is operated off the relay and the sensor -- when the sensor goes open, the light goes off.

You may also have a bad relay -- and I cannot tell you exactly how to check it. It's expensive, so I would not just change it out. Check for corrosion around it and a bad ground, too!

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!