Check the heavy wire from the glow plug relay to the terminals on the opposite fenderwell, make sure all those terminals are clean and tight. Those terminals are from the alternator and the battery and they feed to GP relay. So to be clear, there is a direct connection from the battery to the terminal strip, and from the terminal strip to the gp relay. If the terminal is loose (just like the fuse) it will show up as a high resistance and cause your problem.

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mjk

'84 300SD 119KMi (Liesl der Diesel)
'84 300D 326KMi when the oil left (former parts car)
'82 300SD 253KMi (new parts car)