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