It is possible to use a Coolant Temp Sensor that is normally closed with a Relay between it and the Glow Plug Relay wired so it would only work in the specific Key position.

As the Coolant temp rises the Temp Sensor would rise and cut the Electricity off to the Glow Plug Relay.

You would also need to cut the Wire on the Glow Plug Relay that connects to the Starter Circuit if you do not want it to turn the Glow Plugs on during Starter Cranking.

However, I have a Manual Glow Plug Switch on My Volvo Diesel and I have had zero issues with it since 1992 and I have only had to replace 2 Glow Plugs (not cold where I am) up to now.
I have also never reamed out the Glow Plug Holes as I never knew they needed to be cleaned until I owned a Mercedes. After getting the Mercedes I bought and also made Glow Plug Reamers but in the 7 Years I have owned the Mercedes I have not had to replace any Glow Plugs on the Vlovo so I have never removed the Glow Plugs for any reason.

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84 300D, 82 Volvo 244Gl Diesel

Thanks for the info, especially the graph is very useful. I do not want a manual system, period. Basically, what I want is the intelligence to cut off the voltage to the GP if the engine is at THE pre-set temperature. THe GP light would turn on and then off as if it is glowing but it isn't. Then I spin the engine.

I think I can do it with a thermoswitch and a relay circuit. The temperature of the thermoswitch is what I am looking for. I may try 50C, may be a little higher. The idea is to cut the GP time to 0, if possible, to prolong its life. I have not done it yet but it is on the card. I will report back.

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Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

Quite the glowing reports.

Except in the depths of winter, I simply turn the ignition to glow, attach my seat belt, engage the starter, and drive off. I too see no point in skipping even the shortest glow.... BTW engine has well north of 400k

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1981 300TD
1985 300TD

To wire it the way you wish it to operate is no simple wiring task. The whole project may take longer than to replace the glow plugs. My glow plugs seem to last 7 years using the stock setup.

For a 617, 80 C.

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83 SD

84 CD