I have a 1984 300 SD. It will no longer start. Glow plug light not going on. on 1990 Mercedes-Benz 300D

Would like to know how to check the current and working condition of the glow plug relay before buying a new relay or new glow plugs. Plugs are 3 years old.

Asked by for the 1990 Mercedes-Benz 300D
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
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