glow plug controlers
 

any out there that knows how the controlers work on a 81 300 SD

what do you mean? what do you exactly want to know and what is the
problem that makes you ask?

george

and
 

In addition, is it a CA car or a fed car don’t you know us Californians make every component conform to our rules and dang that effects how glow plug relays work.
Not trying to be difficult but if, you are trying something custom there is a major difference
Dave S

Operation of Preglow System
 

Take a look at this document for an explanation of preglow:

http://ken.kdharris.net/kdhbenz/15-705.pdf

Also, Bosch has a new and improved glow plug control relay as a drop in replacement for the one that came with your car...

-Ken