Today on the first start of the day the glow plug light was normal and the engine started instantly as it always does.Then drove about 20 minutes and stopped to go in the hardware store,probably in there 15 minutes at the most and when i went to start the car the glow light did not come on until after the engine started, the start was normal (instant). The light stayed on for i guess 30 to 45 seconds and then went out.I let the engine cool down for about 4 hours and then restarted it holding the key in the glow plug position for the normal amount of time and the engine started normally, so it seems the glow plugs are functioning and the glow light is ok as it does come on after the engine has started. Visually the large strip fuse seems ok,i havnt had a chance to actually check it for continuity yet.1991 300d 2.5 All suggestions and opinions are welcome TIA Don

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Red Green "This is only temporary,Unless it works!"

97 E300D 157000 miles
87 300TD ?141k? miles

I have seen this when I loose the #1 glow plug in my 603. If you have concerns, you could check glow plugs by measuring each ones resistance. They should all measure about 1 ohm. You can check them by removing the glow plug harness connector at your relay and check resistance of each terminal to chassis ground.

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Dean

Deanot
Thanks for the help,i checked my glow plugs today and as you suggested i found 1 bad.I hope thats the answer,i wont know for a while as i have several things i need to do before ill get a chance to replace it.The bad news is that it is the #3 and i can just barely see it so it looks like the intake manifold will have to come off to gain access.
Thanks again Don

__________________
Red Green "This is only temporary,Unless it works!"

97 E300D 157000 miles
87 300TD ?141k? miles

yep...#1 went bad on me and the glow plug light did exactly what you are describing.

Supposed to do that when #1 is bad, or two of #2-#5 (or #6).

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Gone to the dark side

- Jeff

Mine is strictly #3,the other 4 check out fine and all the same resistance within .1 ohm of each other.Seems to be another undocumented diagonistic feature. I am glad there are so many experienced people here,i cant imagine how one could own/operate one of these cars without this forum or a VERY FAT wallet. Thanks Don

__________________
Red Green "This is only temporary,Unless it works!"

97 E300D 157000 miles
87 300TD ?141k? miles

The light should fail to illuminate after any single plug fails. However, because the current for plug #1 is balanced against the remaining plugs, a single failure of plugs 2-5(6) can fail to trip the sensor.

According to the owner's manual in my '87 602 and '87 603: #1 or two of #2-#5 (or #6).

The OP might have a different engine, I'm not sure, it wasn't idetified in his positng. I'm not familiar with the iron-head engines.

__________________

Gone to the dark side

- Jeff

That's also how the 616/617 works.

That depends on the year model and the particular glow plug relay that is used. Some models used a relay with one Reed relay per glow plug; the indicator lamp flashes to indicate a failure. On later models, only one Reed relay was utilized; the lamp was supposed to fail to illuminate if a glow plug failed. However, the service manual states that it is possible for the light to continue to work if only one plug fails, due to "unfavorable tolerances" on the part of the Reed relay. In other words, except for plug #1, it might take two bad plugs to trip the sensor.

You may have read my other post about changing glow plugs without intake removal. There was a MB diesel specialist who I cannot find now that states the "impossible looking to access" glowplugs on the 300 and 350 series can be changed without intake removal. I would love to see how. Just getting the wiring harness off without removing the intake looks tricky as hell. I don't see how you would ever get to the couple plugs most hidden by other stuff even if you managed to get the harness off. I've also heard that the glow plugs can get impossibly stuck after being in for a long time. Breakage trying to get super old plugs removed can happen as well. Is there a good way to avoid damaging old "tough to remove" plugs? Are there special tools to pull this off?