Preglow time relay test

[Wheatstein]

I have a 1982 300SD, engine 617.95. The car is relatively new to me but has 236,000 miles. Had been starting well, but now preglow lamp does not light and car will finally start after several attempts. When key is turned to position 2, I hear a humming from the engine and after about 20 seconds I hear a click form the relay and the humming changes pitch slightly.

I have read thru several posts to gather info but am admittedly still confused - cars and electricity are not my strengths. I am following a procedure in the Mercedes service manual. I have pulled the 4-point coupler in the relay and bridged jacks 1 & 3 to get the light. Next step: "If glow bulb is in order, test black line from coupler jack 3 of preglow time relay to preglow indicator lamp for interruption, remove interruption."

I have no idea what that means, but if the light comes on, the relay is supposedly defective. I know the glowplugs were installed within the past year and the car has not been driven a whole lot, but I have not done any test on the plugs themselves. I was just now researching that on DIJ.

Thanks for any help, Scott