I have been losing glowplugs on my 84 300sd. Here is the story:

Engine is impossible to start. Ohm out all the plugs and find one plug at 35 ohms. Swap it out with one measuring 1 ohm. Car runs great for two days then impossible to start. Check plugs and two more fail. replace the plugs and verify that relay turns on and off. 10 volts at the plugs, car starts. couple days later and repeat. Change the plugs and exchange the relay with my spare. Confirm relay operation. Good for two days and last night, nothing (no relay light).

My guess is the relay is leaving the glow plugs on too long and burning out the plugs. There must be another component external to the relay that sets the relay on time.

This seemed to start when I replaced my battery. All of the plugs I have been replacing are used. When a plug goes bad the Glow light stops coming on the dash.