A few months ago I purchased a 1983 300SD Turbo as my first non gasoline powered car and could not be happier with my choice. The old girl has over 350,000 miles on her and has been running like a dream. Recently my glow plug light stopped coming on and it is obvious that the glow plugs are not heating (takes forever to crank her up and I get a plume of raw diesel smoke when it does fire). I have searched here and on a couple of other UK mercedes forums for testing procedures for the GPs and the GP control relay. After performing the tests that I've found, here are the results:

1) Tested glowplugs by unplugging connector on the control relay and ohming them out. All plugs were between 0.7 and 0.8 ohms. I stuck one lead of my meter in the 1 - 5 positions of the connector and the other to the engine block and took the reading through the wires leading to the glowplugs.
2) Checked voltage on the 4 pin connector on the control relay and all seems to be up to snuff.
3) 80 amp strip fuse in the control relay is good according to my meter.

I can hear the control relay energize when I turn the switch on but it sounds like it is cutting off within a second or so of energizing and as far as I can tell I'm getting no voltage to the glowplugs. I also hear some sort of relay energizing inside the dash behind the temp control panel just under the radio and then cutting off within 2 - 3 seconds. I've got no idea what this second relay is. I've heard talk of a temp sensor of some kind that has something to do with this circuit. Is there such a beast? If so, where is it located and is there a way to test it? Could this temp sensor be bad and telling the GP control relay that it's too warm and not to energize the glowplugs? Although I am adept at working on gas engines I've got to admit to my ignorance of diesels and would greatly appreciate any suggestions.