I have one bad glow plug (#1 cylinder) on my 83' 300SD. I have a couple
of questions about how the glow system works.
1) Apparently, the glow system relay has an internal reed relay which
compares the current of #1 glow plug against the current of the the
other 4 glow plugs combined. I guess the idea is that if #1 fails, or
if any of the other 4 fail, the imbalance in current will trip the
relay. My question is, if this relay is tripped because #1 fails, will
the other four glow plugs still operate? I know the reed relay keeps
the indicator light from working, but will it also shut down the rest of
2) Is there any reason why a glow plug shoould be changed before it
fails? Should I change all five now, or just keep a spare handy and
change them as they fail?