When I turn the key to the On Position the glow plug light should come on for a bit, right? It doesn't come on till I turn it to the Start Position.
I'm using the Mercedes diagram at the link listed below. I have 12 volts to terminal number five all the time and when I turn the key to the On Position I get 12 volts at terminal 4. That should fire the preglow circuit, right? If I've got 12 volts and no juice to the glow plug light or the glow plugs then the relay needs replacing, right?

http://eva2.homeip.net/W115/program/ETM/75_300D.pdf

The aluminum box type relays are expensive. Some have replaced the relay with a pushbutton switch.

__________________
84 300D, 82 Volvo 244Gl Diesel

Hmmm. That sounds like a good idea. Just jump right past the box, like from terminal 5 to terminal 6?