I own a 1977 300D which is giving me problems with the Glow Plug Circuit. About a month ago, I changed all 5 plugs along with the 50 amp in-line fuse and the ground wire. That started the car back up. However a month later the idicator lamp went off again and the car will not start. All of the plugs but one had power so I changed the plug - nothing. I changed the other four plugs (again) and the fuse - still nothing. One may say "check the timer relay" however everything in the engine is getting power. If the relay was bad, wouldn't that cut the power to the plugs? If it is the timer, where is it? I checked under the dash, but there's numerous relays and I'm not a mechanic. I need help!!

I have read that these cars should start in warm weather without the need of the glow plugs. Is this true?? It's a little over 80 degrees here the past few days (hot for Buffalo). Does this car always require the pre-heating? Or am I receiving flase info?

This was discussed in the following thread:

Glow Plug Basic Questions

Best Regards,
Jim

I have a 1977 240D, and have had MANY similar problems. The circuit (on my car) was not in parallel, which meant that one bad plug (or any single interruption) prevented the system from working. At the same time, there was a batch of Bosch Glow Plugs apparently made in India, which just didn't last. Anyway, my INDY (God bless his soul) installed a kit some years ago, creating a system in parallel; have not had a problem since, and my "fishing and hunting" car is wonderfully noisy and reliable at 386K miles. Hope this helps.

Thanks Toni - I've heard that the conversion from the series to parallel plugs works pretty good. For now, I'll have to try to find where the problem is. Can you tell me where the Timer Relay for the plugs is located in your '77? I'm gonna have to check this out next. Thanks again.