81 300sd injection pump question

on the forward engine side of the injection pump is an electrical unit of some kind it has an electrical lead with a 3 prong plug. it current is not plugged in anywhere as i can't find a recepticle on the
car for it. anyone know what this thing is and what it does? this car seem to run very well w/o it but would like to know... i believe this engine came out of an 84 -gary
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