glow plug troubleshooting
 

This isn't a request for help, just a description of what I have done if others run into the same problem.
1977 300d Series glow plugs (The ones with the heavy wire running between each plug)
Glow plug light on dash not working.
Checked 80 amp fuse on firewall just behind glow plug #5--blown.
Removed ground strap connecting glow plug #1 to block. (one closest to the radiator.
Checked resistance between the threaded central part of the glow plugs (the part the wires attach to) and block. It should be open/infinite (I believe). Resistance 4.6 on digital ohmeter.
Disconnected the wires that join the plugs and checked each one separately.
First 4 about 160 ohms. #5--3.2 ohms. Found the culprit--basically a dead short between the block and the glow plug wiring. I also believe the rest of the glow plugs were going bad. The new ones showed infinite resistance. None of the old ones were better than 160. By the way, these numbers were the same once I removed the old ones from the block. Four at about 160 ohms and the last one around 3 ohms.
Replaced all the glow plugs.
Waiting for the 80 amp fuse. It's a Saturday. No Mercedes parts departments open. The generic parts stores, including the import parts stores have glow plugs but not the 80 amp fuse. I need to get a few fuses and keep them in the glove compartment.

I anticipate that when I put the new fuse in, everything should be back to normal or even better than normal since all the glow plugs seemed to be failing. I have 56k and 25 years on the car and I believe these plugs are the original.

TXBill:
Thanks for that very resourceful tip. Bought a pack today. 65 amp was the heaviest duty they had but it seems to be working fine. If it wasn't, a person could probably put two 45's in there.

Kerry