My 83 240D starts normally after what I think is a normal preheat interval but also often comes on after warm up. How is this controled? time , temperature or what and is it adjustable. This car has been in storage since at least 2001 and shows 118K on odometer. I have no other history.

There is a temp sensor in the glow plug relay that controls the glow time. The glwo light is not directly connected to the glow plugs. While running, if the relay gets a trigger signal from the ignition switch somehow, it could turn the relay on. Easy way to verify if that is actually happening is wire up a light bulb across a glow plug, run it out the hood and watch if the bulb comes on when the glow light comes on.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I am having the same issue, along with some other odd stuff I suspect to be related.

This is the electrical switch you mean, Funola, not the keyswitch?

I have verified that the glowplugs are being activated when this occurs.

Thanks

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Mac
2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22

I am talking about the ignition switch, specifically the rd/bk wire at the glow relay that is powered from the ignition switch. If that wire gets reset while running, it may trigger the glow plugs again.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I did some research on the glow plug relay when I had the same issue about a year and a half ago on my 83 300D. The relay turns the light on when it is supplying voltage to the glow plugs. My problem was cured by a new glow plug relay....

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2012 Mercedes ML350 Bluetec 102K (hers)
2005 Corvette 55K (fun car)
2002 VW Jetta TDI 238K (mine)
1998 Volvo S70 T5 Turbo 211K (kids)
1994 Ford F150 4WD 246K (firewood hauler)
1983 Mercedes 300D 384K (diesel commuter)

Could very well be just a flaky glow plug relay, or corrosion on the rely terminals, or flaky ign switch. Hey these parts are 30 years old.

__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

By reset, I meant power momentarily removed then back on. This can happen if the ign sw. is flaky, the small connector corroded causing intermittent connections.

I currently have a glow plug relay opened up on the bench. By resetting the rd/bk wire, or violet wire, or the brown (ground) wire, I can make the glow relay start a new glow cycle.

__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I had the same issue, glow light coming on while driving; turned out to be water in the gp module box on the inner fender, shorting out the board. Got a good used one and made a manual box out of the other one for emergency use.

The glow plug relay is waterproofed. If it is water logged, someone was in there and did not install the seal correctly.

__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

The rubber seals were in place but broken in three spots. Must have dried out or a PO put them back that way. With 271k on the clock gaskets and seals can dry and crack. $15.00 at the junkyard is not too bad.