Originally Posted by LaughingGravy

I changed out the battery in my dad's 85 300D.

Once done, the car started fine.
The next time we started it, no glow plug light and a constant buzz coming from the relay box at the front left fender when the key is turned to the glowplug position. Car won't start. Metal fuse strip is intact.
So, we replaced the box with a new one from Fastlane; not OEM, but I've heard no complaints about Phil's boxes.
We turn the key and get no glowplug light, but do get a short buzz from the box at the end of what would normally be the relay timing, compared to the constant buzz of the original bad box. Car still won't start.

We think then, that maybe one or more of the glow plugs is bad, so we use a jumper wire to test each glow plug directly to the battery from the larger 5-female end plug that goes to the box. We get spark on all 5, so plugs appear OK.

We needed to start the car, so we use the jumper wire from the battery to the first glowplug, heating up the fuel enough to start the car. We bypassed the relay box entirely.

Does anyone here think there could be any other cause other than the box from Fastlane happened to be bad?