1980 300SD The glow plug light on dash lights for a few seconds or doesn't light at all. Hard to start and when starts lots of smoke from exhaust. Changed glow plugs.

What is the little wire coil on the firewall near battery do? it is open with
a metal shield around it and has 3 connectors on it?

 

if the glow pluggs are replaced the only thing it could be is wiring or the glow plug relay there is a drawn out electrical check out procedure but its easier to just replace buy the glow plug module it located behind the driverside headlight on frame rail itll be a box with 2 or 3 connectors that controls the amprage to the glow plugs and the preglow light in the kombi