Glow Plug Questions

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 the plugs?
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?
Thanks
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The remaining GPs are working because the engine starts. Replacing GPs is much like replacing spark plugs except one ought to use a 1/4" metal drill to cut the accumulated carbon from the hole before installing the new GP. Just twirl the drill with your fingers - it's very easy.
Buy only Bosch or Beru replacements - they last about 100K miles.
I replace all to avoid having to come back to it again. It's not a physically hard job but getting to some of them is tough on one's hands - and #4 one's patience.
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T.G. Lambach wrote:

Thanks for the information.
I take it from your reply that the Mercedes diesels won't start at all without glow plugs, right?
I notice the Beru glow plugs cost a little more than the Bosch plugs. Are they better quality,or just more expensive?
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Welllll, not that simple. Bosch and Beru last a decent amount of time, and Monarchs last not as long. But, they "burn" hotter. So in a cold climate I'd recommend the Monarchs.
I've had better luck with Beru than Bosch; but I use Monarchs now; 3 years and counting.
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False. I started mine with only 2 (out of 5) working glo plugs in -44 degree weather. It, um, took some doing though.
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Glow plugs are cheap. Draining your battery trying to start your car is nuts. Over cranking the starter will shorten the service life of the starter by a significant degree. The same is true for your battery. The more deep draws on the battery the shorter time it will last.
Consider the option of replacing your glow plug relay with the new afterglow model also.
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Roger Shoaf
83 300SD
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I just finished fixing this very problem on my '83 300SD. The #5 GP had been out for some time and I had no idea other than the car cranked a lot more to start. I lost the #1 GP and the car would hardly start without running it up on the starter and lots of black smoke from unburned fuel after it started. After changing the #1 and #5 GP's the car starts and runs like new. The GP's appear to be independent from each other with no current comparing relay, although I didn't look for that. Basically the way I saw it was a relay box with an 80 amp fuse and a wire harness that went directly to each GP independently. The connector from the GP relay box pops off and you can measure each GP to ground while it is still installed, and any GP that indicates near zero resistence (just over 1 ohm) is fine, the burned out one(s) will show as an open. To me, GP's are not wear items like light bulbs and need to be changed as they burn out not as a set. The theory is if I burn out a tail light bulb, I don't change every bulb in the car because they're all the same age.
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BUT..... make sure you replace them to match. They come as 11v and 11.5v. Do not mis-match them!

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Karl I replaced my 2 with the MonarkDiesel brand (11 volt) that I bought for $7.68 each from BenzBin. I was unaware that they came in 2 different voltages. As I see it, the GP's are wired in a parallel configuration and to me it would be irrelevent whether they were 11.5 or 11 volts, the 11 volt model may have a shorter lifespan and the heat would be higher if subjected to 12 volts.
Roger What is the afterglow relay you espoused? What is the difference between that and the OEM one?
The OEM GP system in my '83 300SD cycles for 15 +/- seconds with the GP indicator light on and then continues for an additional 25 +/-seconds with the GP light off and then times out.
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=========================P/N F1101-60420
Note: 4 Glow Plugs & Relay Bosch has superseded the glow control relay to a new glow plug kit which includes a new controller and 4 glow plugs. This new controller allows the glow plugs to continue functioning up to 3 minutes after the vehicle has been started, providing a smoother engine start up and less exhaust smoke. It is important to keep this in mind when testing the glow circuit func- tionality. For vehicles with 5 cylinders, 1 additional glow plug is required (0 250 201 039). =============================== This is the description from Rusty's catalog. If you have never done business with Rusty you are in for a treat. http://parts.mbz.org /
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Roger Shoaf

About the time I had mastered getting the toothpaste back in the tube, then
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I've done business with Rusty and you are right... you are in for a treat! A bad one.
I never got my parts any faster than 10 days (2 months in one case). When I would call they never really seemed to know where my stuff was or if it had really been ordered. Only once did I receive a packing list. Parts were always missing from the orders and when calling all I would get is "maybe it was backordered". Never could they tell me for sure if it was backordered.
I've heard a lot of people talk about how great it was to do business with Rusty and I did not come to my decision to stop lightly. I gave them more than one chance and each time I was disappointed. I finally started dealing with BenzBin and have not had a single bad experience. My parts are always delivered within 3 days from the day I order and most of the time in two days. Only once have I received the wrong part and it was replaced promptly.
The only thing I can say about Rusty is that his folks do have more knowledge of Benz parts than those I've spoken with at BenzBin, but if you know what you need I think you'll get better service from BenzBin.

required
then
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Rusty's dad died and he was a bit of a basket case for a while. Next time ask for Gary. Or order directly from the website to avoid the human elemant.
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I
had
it
backordered.
dealing
always
I can only speak from my personal experiences, and all have been good.
--

Roger Shoaf

About the time I had mastered getting the toothpaste back in the tube, then
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Thanks for the update Roger, didn't know this existed. It would make the diesel rattle a lot less on start up. Possible shortened GP life to me is irrelevent, the first ones lasted 170,000 mi (21 years). Doug Roger Shoaf wrote:

the GP

+/-seconds
which
allows the

has
smoke.
func-
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tube, then

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