Glow plug indicator (and more) -- 1987 300D - Mercedes-Benz Forum

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post #1 of 6 (permalink) Old 04-27-2008, 05:00 PM Thread Starter
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Vehicle: 1987 Mercedes 300D, 1983 Mercedes 240D
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Glow plug indicator (and more) -- 1987 300D

The last advice I got regarding the start-up smoke was helpful, and I discovered it smokes much less if I keep the glow plugs on past the time when the indicator light goes out. Thanks!

Now another related (?) issue has arisen: the glow plug indicator light stopped working this afternoon. I did a search of the problem here and got to dieselgiant. Tested the plugs' resistance at the socket; 4 out of 6 were below 1 ohm and 2 at 10 ohms. Got good voltage going to the relay and at either end of the fuse strip. Also got good voltage coming out of each pin going to the plugs. For good measure we tested voltage where the wires screw to the top of the plugs -- still good. And although I haven't tried to start it from an overnight cold temperature, it sure seems like it starts just like before after sitting several hours. In conclusion, the system seems to be operating normally, just the light doesn't work.

So given all this, would it be safe to assume that the plug indicator light simply burned out? Or is there some other small relay, fuse, etc. that would cause the light to not work? And if it is just the light, how much of a hassle is it to replace an instrument light in the dashboard? (In my old Saab 900, you can reach most indicator lights by going down through the dash speaker hole.)

Additionally, the cruise control began operating erratically this afternoon -- it would accelerate, but not hold speed, until after running at speed a little while. Maybe that was because of a problem with the brake lights, though, as the computer was changed out last week, and I discovered a brake light bulb loose in the socket (while changing out a burned out marker light). Still was erratic after reseating the buld, though. Does the computer need time to reset itself after resolving the problem?

Thanks for any input anyone can share.
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post #2 of 6 (permalink) Old 04-28-2008, 09:22 AM
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Sure sounds to me like you've got two bad glow plugs. The bulb itself on the indicator is the very last thing you should suspect. Good plugs wil measure about 0.6 ohms cold.

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post #3 of 6 (permalink) Old 04-29-2008, 08:22 PM Thread Starter
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Had my mechanic change out the two bad plugs (2 & 6), and the indicator light works again. Funny thing is, on our 1983 240D, the indicator light continued to work even when it was reduced to only one good glow plug (#1). Go figure.

As for the cruise control, they replaced the amplifier they installed a week and a half ago, and it works fine. For now.

For all the goofy little things I'm getting cleaned up after buying this car last month, I really enjoy it! It feels solid, like a comfortable tank, and is a real joy to drive.

-- Kris
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post #4 of 6 (permalink) Old 02-23-2011, 03:52 AM
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Post Glow plug indicator


Having trouble on w124 e200 1988.

Glow plug indicator don't turns on. The glow plugs seems ok, because car starts normally. in the present i turn ignition on, wait for 5 or 6 seconds and start the engine. the engine starts good and with no black smoke.

The glow plog module makes the normal noise when i turn the ignition key. So everything seems to work, except for the indicator!

Any idea?
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post #5 of 6 (permalink) Old 02-23-2011, 07:49 AM
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In this case it sounds like burned light bulb.
From the experience above, on W124 the light might not come on when you have a burned plug. With 1 or even 2 burned plugs the engine will still start normally, although some extra smoke for few seconds is expected.
I would start with whatever is easier. In this case testing the plugs at the relay seems to be easier than pulling the cluster for bulb replacement.
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post #6 of 6 (permalink) Old 02-23-2011, 09:34 AM
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Glow plug indicator

Thanks for the answer

Is there any easy way to test the plugs at the relay?
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