It's capable of "Sensing" shorts and other Electrical anomalies that might damage
the Wiring and the Chassis. No Glow Plug Relay 80 amp Fuse.
It would be well worth your time and trouble to carefully disassemble and repair it.
IF after physically checking all connections (+Etc.) WITH A MULTIMETER it still malfunctions.
I believe that what's the matter is "Connection" (Wiring loose or shorted) OR
External Component (The Temp sensor that determines "After Glow" timing).
[B11/7]
The Pre-Glow ReLay does have it's Own separate dedicated GROUND.AND
if the Ground's not "Clean,Bright and Tight" the ReLay will mal-function.[W9]

The Link that Old Wolf posted in reply #2 of this Post will greatly assist.

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'84 300SD sold
124.128