Driving on the interstate at approximately 75 mph, I felt a jolt, heard a loud pop followed by a whoosh and the battery light and I think the antilock and SRS light come on.

I immediately thought that I lost either just the alternator or the serpentine belt. Luckily I was right by an exit, so I gunned the car and cut between teo semi trucks and off the freeway.

I shut off the engine on the off-ramp, but the temperature still got up to nearly the 120C mark (!!!). I parked on the offramp, popped the hood and the belt was in 2 pieces. I pulled it out of the pulleys, let it sit for a couple minutes and let the aux pump drop the temperature about 5 degrees before starting the engine for another 5 seconds to bring me to the shoulder of a country road. I called someone to bring the trailer down. I left the key on, pulled the temp down to 40C before resigning myself to waiting for the trailer to arrive.

The alternator pulley is totally loose...it will wobble about 20 degrees off its axis. I think the shaft broke, bearing may have siezed.

Anyway, here I sit...let down by my always faithful 300D. Back to driving the Saturn, I guess!

Only time I was ever stranded by an MB diesel was an alternator going out...on a 92 300D.

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14 E250 Bluetec "Sinclair", Palladium Silver on Black, 153k miles
06 E320 CDI "Rutherford", Black on Tan, 171k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver, 142k mi, wastegate conversion

19 Honda CR-V EX 61k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

This is your opportunity to upgrade to a modern high output alternator.

Man, that stinks! Sorry that happened. Luckily nothing super major to fix if it's the alternator, right?

I had a very similar thing happen on my Mazda years ago; alternator seized. Pretty much without any warning. It snapped the serpentine belt, left me stranded without a working water pump, much like yours. New belt and alternator, was up and running soon after.

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"The MB W123 is so bulletproof, you can drive them forever. Which is a good thing as it takes that long to get anywhere."
Betsie: 1984 W123 300D (hobby, 280k miles)
Myrla: 2001 Mazda Protege 2.0 ES 5spd (daily driver, 130k miles)
The Turd: 2007 Toyota Camry (wife's car, 118k miles)

I see this mentioned constantly on this forum. Unless you're running zillions of lights or an 800w stereo, what the heck do you need more output from the alternator for?

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

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

Realistically, it's just brownie points.

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Allen Kroliczek
Oak Grove Autosport | Oak Grove Autosport
01 G500, 82 300TD, quite a few more.....

If you do short trips and you have afterglow yes it makes a noticible difference. Later alternator designs are small, lighter for the same output. Then you can get into really modern designs with anti stall, load response and over running pulley for that smooth as **** from a duck's ass belt drive. A larger alternator will also extend the life of your battery by quickly rechaginging it after each start.

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CENSORED due to not family friendly words

Put on a 115a alternator from an om606. Less stress on your battery and charging system. Those old 70a alternator are dinky. If you find an e420, or e430 at the yard they also have 115a alternator.

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Only diesels in this driveway.
2005 E320 CDI 243k Black/Black
2008 Chevy 3500HD Duramax 340k
2004 Chevy 2500HD Duramax 220k

Unfortunately it is not unknown for the pulley nuts to come loose even on the older models.

Since 1968 I have only had that happen one time on US made Vehicles and have never hand anyone tell me that same happend to them. Don't know if it is a Bosch issue or not.

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84 300D, 82 Volvo 244Gl Diesel

I always carry some 1/4" nylon rope. One use is to tie a loop to turn just the water-pump pulley. I did that when my alternator bearing seized, squealing the belts. Trick is to tie a barrel knot (like fishing) and get the rope taught. If you get the diesel started, you won't need any more battery until you get home, unless driving at night. Hitting 120 C for a short time will not hurt the engine. Indeed, the temp sensor in my 1984 & 85 300D's will show excessive temp from just an air pocket by the sensor.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

I wouldn't mind upgrading slightly to a newer style internal fan alternator, but I'm not too worried about output, as the car is all stock -- are there any newer style alternators that are plug and play? I will probably add the INA clutched alternator pulley as well to smooth out the belt a bit.

If I recall correctly, I will not be able to put the replacement belt on to the pulleys without removing all the mechanical cooling fan bits (I hope I'm wrong?) so I will probably replace the water pump and radiator as preventative maintenance.

I'd also probably like to upgrade the water pump pulley to the smaller one from the OM606 for increased cooling capacity -- summer is coming. Any tips on any of the above? I replaced the tensioner, idler, fan and clutch (with the plastic version from the later diesels) in 2015.

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RenaissanceMan Labs: where the future is being made today.

Garage:

2017 Chevy Colorado Diesel (nanny state emissions)
2005 Volvo S40 T5 AWD, 77k
1987 Mercedes-Benz 300D turbodiesel, 4 sp auto, 156k - 28.7 mpg
1996 Tracker 4x4, 2 door, 16v, 3 sp auto. 113k - 28.6 mpg

WARNING: this post may contain dangerous free thinking.

Good idea. I did that to my 124.

Just be sure to get the wiring harness and the distribution block from the donor too. The existing harness in the 87 has a plug in connector (that can go weak) while the larger alternators have a bolted connection which is much more reliable.

The extra capacity is helpful when running with heavy load, like the lights, wipers, blower, rear defrost all going at once, and you're still trying to top up the battery from the glow and start cycle.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

Think those 70 amp alternators are wimpy? Try the old 35 ampers in the 220 D's!

Any MB alternator up

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CENSORED due to not family friendly words

Any MB alternator up around the year 2000 is a a bolt on affair.

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CENSORED due to not family friendly words