Back to this issue again. Car has been running fine for several months, with only a few times where light wouldn't come on, but usually if we just waited an hour or two, we would finally get the glow plug light and then car would fire up. But now I cannot get glow plug light on and engine will not start without it. Here is a tally of what I have investigated:

-glow plug resistance appears to be fine, all in the 0.6 to 0.8 ohm range
-glow plug relay seems to be fine, at least I see it providing 12V to output blades when I energize it, and you can hear relay click
-replaced the K40 relay
-problem appears to be worse when engine or ambient is hot

The problem appears to be that the signal to energize the relay is not working. I cannot consistently get 12v at the two input wires to the glow plug relay. My understanding is that this starts with the ignition switch, but it is also enabled by the temperature sensor located under the manifold. I wonder if this sensor has gone bad...??

Also, I need to start the car to move it. So I figured if I just apply 12V to the input side of the glow plug relay, then it will pass power to the glow plugs and I will be able to start it after 10-15 seconds - sort of bypassing the electronics that send the 12v. However, when I do this the relay only pops on for 1-2 seconds and turns right back off again, not nearly enough time to energize those plugs to allow me to start. Is that relay somehow super smart and know that my 12v is different than the one from the ECU?

If anyone out there has some input, or can even provide a wiring diagram for a 97 E210 E300D, that would really help.

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Treiberg's Collection: 1981 240D, 1985 300SD, 1992 300E, 1997 E300D, 2005 C230K