I have a 1984 300 SD. It will no longer start. Glow plug light not going on.

The glow plugs are easy to check disconnect the electrical connection and check the resistance is the same across each, the relay you need to see which terminal should have battery voltage, ground, voltage when cranking ect.... Have you looked for the heater glow plug relay on line, look at www.rockauto.com There is a company that independent auto repair shops get their information from, this same information is available inexpensively for people that work on their own cars.. The information is year make and model specific, covering repair procedures, torque specifications, fluid capacities and specifications, service bulletins, component locations, wiring diagrams ect.... Alldata is very easy to navigate http://bit.ly/AllData_Repair_Manuals_Online