I just purchased a W116 300SD. When I turn the key the glow plug light stays on and it does not go out until I start the car. How do I fix this? Does the glow plugs stay on until the car is started or do they go out at a predetermined time? I have just been guessing how long to wait before I try to start the car. Thanks in advance for any help. I also have a W115 240D but it has a gorilla knob so I don't have any experience with a glow plug light.

I don't know specifically about your model.
If you have the Glow Plug relay on the Fender Well there is a timer inside and it is also connected to a temp sensor in the Cylinder head.

I theory the colder the coolant is the longer the timer is supposed to take to turnoff the Glow Plugs. You could be having a issue with the coolant temp sensor or the wiring.

When the Glow Plug light is not acting normally it is a good idea to check the Glow Plugs by way of the connector of the Glow Plug Wires at the Glow Plug Relay with an Ohm Meter. How to do that is in Repair Links
Fast navigation http://www.peachparts.com/shopforum/diy-links-parts-category/146034-fast-navigation-do-yourself-links.html

Or you can go to the Dieselgiant website.

Your Engine is starting normally and running normally after it starts is more important then what the GP light is doing.

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84 300D, 82 Volvo 244Gl Diesel

This info is from an 1978 SD Mercedes service book.

It has that the timer will time out after 90 seconds of no start.

What happens to a glow plug light when there is a failure:
1-Light blinking for 30 seconds when Key is in position 2 (the preglowing position). The main preglow circuit is interrupted like a burned fuse.

2-Light does not light up but engine still starts. Electrical interruption between the temp sensor and the or the preglow light (like a burnt light blub)

3-Light is blinking for 30 seconds after the Engine starts. Electrical interruption in one of more of the Glow Plugs.

There was no description of the light not turning off during the preglowing.

If I remember correctly this is slightly different from the W123 fault flashes. Also in southern CA temps on my 84 300D the Timer turns of in 30 seconds not 90 seconds.

Also I had my Glow Plug Light stop working completely but the Engine started OK and ran OK after so I knew the Glow Plugs were all working and I did not fell like pulling out the instrument panel to see if there was a burnt bulb. I drove it like that for little more then 1 year and the GP light started working again all by itself.

I know that some people can't stand it when something like the GP light does not work normally but if the Engine starts and runs normally the light is not critical.

I don't know if SDs have this but there is the Bosch GP and some Mercedes came with I believe it was Schuler GP Relays (which are expensive to replace). That would be Relays on the Fender Well.

I don't know if the Schuler Relay operates the same as the Bosch one does when it comes to the GP light showing faults.

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84 300D, 82 Volvo 244Gl Diesel

You can probably determine when it is time to crank by the increased brightness of the interior ceiling light .

People like myself Don't want the interior lights on when I open the door. I don't want some criminal knowing I am entering my Vehicle and am going to be sitting there with my limited view of things till I get my Vehicle moving.

It is not hard to figure out how many seconds it takes to preglow long enough for you to start and repeat that the next time.

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84 300D, 82 Volvo 244Gl Diesel

Thanks everyone for the help.