Thank you for the thoughtful response- I certainly wouldn't have made the drive if it persisted.

To your second point- it was just so strange that the second I touched the key in the ignition (at 15 mph?) the car just...powered down. It really didn't feel like a fuel delivery issue. Now with the battery NOT discharging dramatically it's behaving fine, starts right back up, etc. That whole scenario just didn't seem to make any sense to me- I could understand a short in the ignition switch etc, but why the engine would die too, I do not understand.

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1985 300D Surfblau "Blueberry" 250K R.I.P
1984 300CD Manila Beige "The Banana" 238K R.I.P
1984 300TD Cypress Green "Olive" 390K (M.I.A.)
1982 300D Orient Red "Steak" 195K
1985 Euro 300TD Lapis Blue “Pancake” 200k KM
1982 300D Light Ivory “Butter” 183k
1984 300TD Black “Pepper” 55k

Thank you all SO much for your VERY quick responses. It seems the main culprit has been uncovered with the glow plug relay, but theres still that lingering dimming/brightening battery light. This is what the search function is for, I've seen so many threads on this, my 'powering down' issue was just totally inexplicable to me-

many thanks for the help getting home safely

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1985 300D Surfblau "Blueberry" 250K R.I.P
1984 300CD Manila Beige "The Banana" 238K R.I.P
1984 300TD Cypress Green "Olive" 390K (M.I.A.)
1982 300D Orient Red "Steak" 195K
1985 Euro 300TD Lapis Blue “Pancake” 200k KM
1982 300D Light Ivory “Butter” 183k
1984 300TD Black “Pepper” 55k

Thanks for posting what resolved your immediate issue...many don't.

Flickering battery warning light may be due to poor connections/corrosion at the battery terminals, alternator harness plug, or the chassis grounds.
Also, low idle, or loose belt tension might be the culprits.
Check these first before condemning the alternator or the voltage regulator.

Edit: If the keeper spring for the alternator harness plug is AWOL, the plug can vibrate lose and prevent the battery from geting recharged. BTDT

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

Dimming and brightening of the battery light is caused by the brushes ( voltage regulator ) inside the alternator. You can replace it or replace the whole alternator. Good luck

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Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

Good point, I had forgotten they were not electric fuel control.

Steve

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Steve A
92 W140 OM603
97 VW Jetta TDI
90 Passat variant TDI 6 speed MT
94 Chevy K1500 6.5TD
05 E320 CDI
+ others

Just sharing my weird experiences with my 617 engined cars.
I too have had glow plug relays stick and drain batteries at idle but hold ok-ish voltage when revved up and driving. I did a half brightness mod so the light comes on full brightness like normal, and then then goes to half brightness when it is ok to start, then goes out entirely once the after-glow goes off, so if it stays half brightness, you've got a stuck relay.

I'd say that mod has saved my glow plugs in at least one car.

I've replaced the 55A stock alternator with AL129X (115A) on both my current cars, which has kept me from having to replace a battery in YEARS, the higher output alt seems to keep the batteries healthier, Not sure if there is any scientific reasoning behind that or just a new alternator being better at regulating voltage than the nearly 40 year old ones.

I've driven both of my Mercedes as well as both of my Cummins with severe electrical issues where batteries were basically non functional. the mercedes act and operate as normal, the cummins automatic transmissions are a bit quirky, and default to 3rd gear in D, so you start in L, shift th S and then into D on those trucks and get home a-ok.

The vacuum shutoff on the mercedes wouldn't/shouldn't turn the engine off, unless perhaps your vacuum valve on your keyswitch is a little weird and wiggling the key was enough to let some vacuum to the shutdown pod.

Hopefully you can figure out the culprit there!

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1982 300D (w123, "Grey Car")
1982 300D (w123, "Blue Car")
2001 Ford F150 "Clifford" (The Big Red Truck)
1997 Dodge Ram 2500 12V Cummins
1996 Dodge Ram 2500 12V Cummins
Previous Vehicles:
1995 E300D, 1980 300SD, 1992 Buick Century, 2005 Saturn Ion

Lack of electricity will not stop a 617. Lack of fuel will. Lack of fuel can be caused by vacuum fuel shut off. Do some diagnosing. At least measure the voltage output of the alternator with the glow plugs disconnected. It either charges or it doesn't.

Autozoo tried to sell me an alternator when the plugs were glowing and voltage was down so be sure they're out of the equation.

Sudden shut down is a separate issue from battery charging. Try disconnecting the vacuum line from the switch then see whether you can replicate the sudden shut down. Probably not.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

I did a half brightness mod so the light comes on full brightness like normal, and then then goes to half brightness when it is ok to start, then goes out entirely once the after-glow goes off, so if it stays half brightness, you've got a stuck relay.

How?

Inquiring minds want to know! I would love to do this as well.

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1985 300D Surfblau "Blueberry" 250K R.I.P
1984 300CD Manila Beige "The Banana" 238K R.I.P
1984 300TD Cypress Green "Olive" 390K (M.I.A.)
1982 300D Orient Red "Steak" 195K
1985 Euro 300TD Lapis Blue “Pancake” 200k KM
1982 300D Light Ivory “Butter” 183k
1984 300TD Black “Pepper” 55k

So not being well-versed in plate fuses, I’d reckon a hairline crack such as this could be enough to cause my relay issues?



About a second after I took this photo it completely cracked in half.

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1985 300D Surfblau "Blueberry" 250K R.I.P
1984 300CD Manila Beige "The Banana" 238K R.I.P
1984 300TD Cypress Green "Olive" 390K (M.I.A.)
1982 300D Orient Red "Steak" 195K
1985 Euro 300TD Lapis Blue “Pancake” 200k KM
1982 300D Light Ivory “Butter” 183k
1984 300TD Black “Pepper” 55k

That will keep the relay from functioning.

Same happen to me. When I looked at the strip fuse I did not notice the crack and sometimes the relay worked and sometimes not.
Later in the dark of the evening when I activated the relay I saw some small sparks and found the crack.

In the attached pic is what I did to mine till I got a new strip fuse. Notice that with the Alligator Clip while it connects both sides of the Fuse you still have the protection of the Fuse.

Attached Thumbnails

 

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84 300D, 82 Volvo 244Gl Diesel

Subscribed to learn what the real problem was in the end .
For now, replace the alternator's regulator & brush holder, if you take the time to source the correct BOSCH branded one you'll find a better charge rate out of the stock 55 ampere alternator .

I'd also fix that wonky ignition switch .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

I'll have to find my spare relay to take a picture, essentially you solder a 68 ohm resistor between two contacts inside the relay and reassemble the relay.

Super easy if you know how to solder, I read about it on here, figured out how to do it, and have been modifying every relay in every car I've owned since then.

I feel like I made a youtube video about it some time ago, I'll look for that too

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1982 300D (w123, "Grey Car")
1982 300D (w123, "Blue Car")
2001 Ford F150 "Clifford" (The Big Red Truck)
1997 Dodge Ram 2500 12V Cummins
1996 Dodge Ram 2500 12V Cummins
Previous Vehicles:
1995 E300D, 1980 300SD, 1992 Buick Century, 2005 Saturn Ion

I drove a bug once, all night thru Utah, and half of Colorado, by turning off, everything, and following a tractor trailer. VW's if your points were bad, it would not charge for some reason.

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1999 w140, quit voting to old, and to old to fight, a god damned veteran

I thought I had seen that someone posted suggesting I got the idea from this thread: Easy Glow Plug Relay mod: better dash indicator

But their post appears to have vanished. I do indeed believe it was that thread, or some other thread on here, and I have done the mod to every single glow relay I've had. 95 om606 was the same exact locations as my om617 engined cars.

My relays did not have the unpopulated hole, so I've attached a photo of where I soldered the resistor on a relay that ended up somehow filling with water years after I did the mod and holding the glow plugs on draining the battery and causing charging issues.

I wasn't happy with the 47 ohm resistor suggested in that thread, the light was too bright on afterglow for my taste, so I used a 68 ohm resistor to dim it down a little more.
I also put a little kapton tape under the resistor to ensure nothing could ever short out.

I'm not sure if anyone would think posting a link to the youtube video I made about doing it would be considered uncouth or not, here is a link to the video, if anyone deems it unacceptable, I will gladly remove it, its honestly pretty awful, I made it many years ago: https://youtu.be/LXb7KJoaK8s

Attached Thumbnails

 

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1982 300D (w123, "Grey Car")
1982 300D (w123, "Blue Car")
2001 Ford F150 "Clifford" (The Big Red Truck)
1997 Dodge Ram 2500 12V Cummins
1996 Dodge Ram 2500 12V Cummins
Previous Vehicles:
1995 E300D, 1980 300SD, 1992 Buick Century, 2005 Saturn Ion