1993 300SD W140 GlowPlug light
 

I am currently experiencing a glowplug light issue. The glow plug light comes on after the car is started, but does not come on when the key is in the pre-glow position. Any advice?
Thanks

I forgot to ask if despite the Glow Plug Light issue if your Engine is starting OK?

You have been a member a long time. If you have read any of the threads or posts on this I believe the first thing folks would say to check as the Glow Plugs themselvers as the first plasce to start.

You can go to the Diesel Giant Site and follow his pictorial with the Ohm/Volt Meter (multimeter) or you can go to the DIY section and see if we have a proceedure. His pictorial I believe is for a 123 but the method is the same.

The good thing here is you can check the Glow Plugs without removing the Intake Manifold.

If it turns out any of them are no good I would read up on removing them before I tried to do that as the long Glow Plugs in the Aluminum Heads sometimes break off in the Head when you try to remove them.

DIY Repair Links
http://www.peachparts.com/shopforum/diy-links-parts-category/

http://www.peachparts.com/Wikka/DoItYourSelf

If the Glow Plugs appear to be OK and your system has a ExternalTemp Sensor connected to your Glow Plug Relay I would suspect that.

I do not have the correct manual for your car but these are about later model Glow Plug Systems.

Also on the 123 Glow Plug Systems when you are cranking your starter you Glow Plug Relay is avctivated bay way of another circuit and a wire coming up from Starter Solenoid.

In the comments on how the Relay worked normal it said that the duration of the preglow is decided by the Temp Sensor inside of the relay or a Coolant Temp Sensor.

So on most 123 the temp sensor is in the relay and would determine the preglow cycle; but when I turn the key and start Cranking the Starter the preglow relay is activeted again (manually).

On the 1988+ cars this means you have a bad glow plug or two. Your car must not have come with an owner's manual, that feature of the preglow system is covered :D

Get out the meter and check the plugs. Thankfully you can just unplug the connection at the glow plug relay, rather then using the terminals on the plugs themselves.

Note that your relay does not have a fuse, it has a self resetting circuit breaker.

-Jason