As part of preventative maintenance, all 5 glow plugs in my 91 300D were replaced (at the same time) in 2012 with new Bosch glow plugs.

55,023 miles, 5 years & 4 months later - 3 of the 5 glow plugs are shorted to ground .

Is there an average life expectancy for glow plugs?

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You got good service out of them. they do last years though, depending on your mileage and starting vs driving habits.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

That is highly unusual for one glow plug to be shorted to ground, but all 3?

How many fuses did you blow to find out that they were shorted?

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Happy to say that I didn't need to replace a single fuse .

Removed the glow plug harness off the relay, all 5 pins of the relay show 12v for 30 seconds. Next I tested each of the 5 harness feeds by checking resistance between the (female) pins and ground, 2 have resistance, 3 have no resistance (ie: shorted). To determine if the issue is the harness or glow plugs I removed the wire from one of the glow plugs with no resistance. The wire tested fine - no short. Tested the glow plug (still in the engine) and its shorted.

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No resistance = open, NOT shorted. If you take a wire and connect it across a glow plug when it is powered, you will have shorted it and it would quickly blow the fuse.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

funola I understand what your saying and I understand the difference between a open circuit (no continuity), a short (resistance value of 0) and expected resistance of the heating module in the glow plug.

As I mentioned in my previous posting, after the harness was removed from one of the suspect glow plugs (A) there is no connection to ground in the harness for that lead and (B) testing the glow plug between the tip where the wiring harness was removed and the engine block and it's shorted to ground.

Reviewing the MB service manual it references the relay having a built in short detection circuit that automatically disables +12v when a short is detected. Based on that it doesn't seem unreasonable that a fuse was not blown.

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So 3 glow plugs were shorted? Amazing! It would be interesting to apply power to one of the shorted glow plugs with a 30 amp fuse in series and see what happens. If it is shorted, fuse should blow.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I share your amazement and that was the reason for my initial question about life expectancy.

After receipt of the new part's and the repairs are complete I'll test the old glow plugs and share the results. This is my primary driver and right now it's cold in the northeast US - so the Merc is in the garage with block heater connected .

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The life of Glow Plugs is determined by the timing of combustion and if your Injectors are spraying as they should.
If the Injector is spraying onto the Glow Plug it can cut it in two.

Next is carbon build up around the base of the tip and the Cylinder head.

On a test I did years back when you first apply voltage to a glow plug the amperage spikes to over 60 amps (60 amps was as high as my gauge would go) then as it gets hot the plug drew about 16 amps.

From this I believe that carbon around the Glow Plug conducts the heat partly into the Cylinder Head/Coolant keeping the plug colder and making it draw higher amperage longer then it normally would. I believe pulling that higher amperage for a longer time also shortens the life of the Glow Plugs.

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84 300D, 82 Volvo 244Gl Diesel

You didn't replace a fuse because the glow plugs are OPEN CIRCUIT. A short will read 0Ω resistance, not "no resistance". A single shorted glow plug in your system will blow the breaker or fuse in the glow plug relay IMMEDIATELY when energized.

There's no magic here, just a misuse of terminology.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

That word you keep using doesn't mean what you think it means.

But I would say the life expectancy you've experienced isn't out of the ordinary. Sounds like something fishy going on though.

-Rog

What happens when you start your car does the GP lamp illuminate?
With the GP relay you have the lamp should not come on when you position your key to run rather after 2 minutes the lamp should turn on indicating you have at least one GP that has a dead short.

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92 e300d2.5t
01 e320
05 cdi
85 chev c10

As soon as you start cranking the Starter your Glow Plug Relay is also on and your Glow Plugs are heating.

Here in mild weather when the Golw Plug Light goes out and the key is kept in the glow plug pisiton the timer is stll on and the relay is still on until the timer shuts it off. There is a toltal relay timer time of about 27-30 seconds available.

I don't know what it does in cold weather.

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84 300D, 82 Volvo 244Gl Diesel

Diesel911 there are 3 modes of operation for that relay
1 if GPs are all good when you turn key to position 2 GP lamp comes on after the engine starts the after glow remains on as you mentioned.
2 if any GP is high resistance or open cct then the GP lamp flashes when the key is turned to position 2 indicating a problem however the GP relay preforms as described in 1 above.
3 if any GP is shorted to ground the lamp does not come on and no GPs are powered. The lamp will come on after a 2 minute or so delay indicating a dead short.

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92 e300d2.5t
01 e320
05 cdi
85 chev c10

There is no stock relay with the afterglow feature on my year and model.

What I am saying is that here in S CA when you pre-glow normally the glow plug light goes out before the timer has run the full amount of time available. That means you need to keep the key in the pre-glow position if you want the plugs to glow the full time available with the timer.

When you start cranking the Starter there is a separate circuit that turns the glow plug relay on during cranking.

The trouble codes are based on a comparison with some of the other glow plugs and will not pick up any difference if all of your Glow Plugs are surrounded by carbon until one of the plugs actually fails.

I don't use the glow plug light code do determine which plug might be bad. If I see the light acting abnormally I simply check all of the Glow Plugs at the Relay Plug with an ohm meter.

My Glow Plug light had been out for over 1 year and a few months ago it came back on. I have no reason why. But, I had no symptoms of any bad glow plugs so other then checking with the ohm meter when the light first went off I just drove the car.

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84 300D, 82 Volvo 244Gl Diesel