I suspect one is giving me fits.

Thanks

Don[/QUOTE]

One failure mode of a really bad GP is shorting, when the wire element inside touches the case where it shouldn't. I have seen this, it blows fuses. If you have blown more than one fuse you might have one shorting plug, and testing with an Ohmmeter may not be useful in identifying the bad one. Change em all! The adage "leave well enough alone" may not be applicable here, and can invite more problems, if one is bad then others are going to fail and in my experience its a good plan to replace them all when one or two have failed, and if you expect cold weather where you live, Fall is the right time to check for any bad ones. With as many daily drivers using GP's as you have, GP problems are bound to surface!

Be warned however, if the GP's are old, I mean over 3 to 5 years old and have seen lots of use then you might find that the pencil type GPs tend to get gunked up with carbon and they won't come out easily. Do not yank on one if it is stuck, it may come apart on you (not a fun thing!). Some strong solvent like Lubro Moly is useful here with a twisting motion, unscrewing the plug from the gunk if possible (use the search button looking for glow plug) but I have heard of other recommendations).
In case you are interested, the resistance element in a glow plug is made from tungsten wire, the same material used in incandescent lamps. The Resistance ratio, hot to cold, is 14:1 (meaning 1 Ohm cold and the equivalent to 14 Ohms hot, hot being defined as around 1200 degrees). The elements in a glow plug never get as hot as the filament in a lamp, only dull red is necessary to help ignite the fuel, so a ratio around 3:1 is more likely, therefore 5 GP's would draw around 24 Amps with 14 V on them (engine running with alternator charging). Starting current is around 60 Amps for a few milliseconds anyway, dropping repidly as they heat up. This causes stress cracks on the fuse on the GP timer, so the first time one blows, look closely before you remove it, if it has a hairline crack, its a failure normally attributed to thermal cycling, a known failure mode seen in fuses.
Dieseldiehard
1971 220 (gas) 4-spd manual 106441
1979 300TD w/ ’85 turbo engine 295530
1983 300D 243280
1985 300TD 217300
1987 300D 258230

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While we are on the subject, more or less, how smart is the GP relay?
Is it temp/time sensitive? Mine has since I've had it only shown about 5 seconds of the GP light, though there on for whatever time after there suposed to be(I timed it once). Seem's like my VW's would hold the light longer when it was colder. Or I could be makeing things up again..

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1994 E320 195K

Good question.
Anyone?

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Mine is on longer in cold weather.
Perhaps you have bad glow plugs.
When I got the 1985 300SD, the light was on for five seconds, found three bad glow plugs, replaced them and all is normal.

The glow plug relays used up to about 1989 have a temp sensor inside the relay, that measures ambient temperature in the engine compartment. It will lengthen glow time in very cold weather. These have a maximum preglow time of 60 seconds and no afterglow. The relays from about 1990-up use an external coolant temp sensor in the cylinder head, which is much more accurate. These systems also have an afterglow function, which keeps the plugs activated for up to 180 seconds after startup (depending on temperature). This greatly increases cold-starting smoothness as well as emissions.

The inrush current is, IIRC, about 15 amps per plug. And yes, this decreases very quickly down to about 8 amps per plug. If you have a DC clamp meter you may be able to test current draw on each plug. The resistance check is very often accurate enough. Testing a loose plug will read 0.6 ohms with an accurate meter. If you test at the GP connector through the wires, the reading may be 1-2 ohms, which is still OK. If the reading is a few dozen ohms or higher, the plug is probably bad, or at least should be tested directly at the plug (with GP multi-pin connector unplugged!). Removing the plug and applying 12V would be the most definitive test, but that's a lot of work which shouldn't be needed.

FYI, for those of you with pencil-type plugs on the OM617.95x and vertical injected OM60x engines (tha's pretty much every MB diesel from 1980-1989), the correct plug is the Bosch Duraterm #80006, which also happens to be an afterglow-rated plug. Anyone interested in retrofitting a 'poor man's afterglow system' should read this thread thoroughly - preferably *before* asking questions that are already answered earlier in the thread.


HTH,

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eskimo is correct. an unenergized gp is less than 1 ohm. as soon as current is applied the element heats and resistance goes through the ceiling. a simple experiment: take a short piece of copper wire and measure it's resistance, which will be very very close to zero. then have a friend heat the middle of the wire with a cigarette lighter and you will see the resistance increase incredibly. in the gp electric current takes the place of the cigarette lighter.

Getting cold out there.
How well are your glow plugs working?

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Don't always trust your glow plug indicator light!
Don't always trust your glow plug indicator light!

Try this web site...It never fails!
http://dieselgiant.com/glowplugrepair.htm

That is a very good DIY; but there are glow plug failures where the meter gives false readings.

This is why I trust the simple bench glow test, it is never wrong.

So true!

After reading this post, I tested my glow plugs in hopes of solving the rough start issue and found that one of mine appears to be bad. Four of them measured right around 2 ohms while the other rang up at almost 35 ohms. Now that I have identified the problem, however, it looks to be nearly impossible to get to the glow plug in question (#3) on my 1987 190D with the 2.5L turbo. Has anybody had experience on this model to provide some advice before I go tearing everything apart??

You have two options:

1) You can replace the plug using 1/4-inch drive extensions, U-joints, and/or wobble joints. It's not easy but it is possible.

2) Remove the intake manifold, which provides easy access to all the plugs. Replace ALL of them with new Bosch or Beru plugs (not just one... do you really want to do this 5 times over the next year or two?). While you're in there, replace the plastic clips for the metal injection lines, which you can only fully access with the manifold removed:



If you pull the intake, I'd recommend an OE/dealer gasket... they are metal-reinforced, while most OEM/aftermarket gaskets are plain paper. Probably not a big deal but I prefer the reinforced gasket...

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that a good idea

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1996 E300 110,000
1985 300TD 212,000
1980 300D 238,000

In the snow belt...

Summer is the time to replace glow plugs, unless you enjoy freezing fingers.


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