On my '85 300d, I have had this occur now with three different preglow relays.... When the temperatures drops to about -20 deg C (- 4 deg F) or below, and the car is parked outside all day where I work (no plug-in), the relay won't engage by itself.

That is to say: I have to open the hood and tap or press on the side of the relay for it to engage. Then it works fine. Sometimes, the action of springing open the hood is sufficient.

I always blamed this on the relays being old. However, this winter I have one in there that has seen maybe 9 months of use - albeit it's a Huoco. The others were Bosch and Hella and of unknown age.

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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