Engine running; no belts turning, no pulleys turning
 

Bought a '79 300SD a little over a month ago for a commuter car. been working great until today. Steering got difficult; battery, brake and steering lights came on. Temperature went up. I thought I had broken a belt but not the case.
All the belts were intact. The power steering belt was a little loose but not bad. I started her back up (started easily) and none of the pulleys were turning, not even the crankshaft pulley. There was no smoking or squealing.
Is there a clutch on the crank or something that's screwed up? I can't see how else the engine would run without the crank pulley turning.
Any help?

Broken crankshaft? Crank pulley coming loose and spinning on the crank nose? That is an odd one.

Hopefully the bolts that retain the crank pulley sheared. That is not a terribly uncommon occurence.

It will run just fine, 'til it gets hot.

Failure of the crankshaft pulley is not unknown on these engines. Don't run it anymore before diagnosis. Best case scenario would involve resecuring the pulley to the crank. Worst case scenario would necessitate crankshaft replacement which requires pulling the engine.

according to the manual there are 3 bolts holding the pully on and one center bolt holding the balancer. I dont know if the big bolt hits the pulley but my first guess is the three bolts holding the pulley vibrated out. might tell help to get a flashlight and an inspection mirror and have a look there in the well of the pulley to see whats up

Wow, that was quick. Such fast service! I hope the bolts just rattled out.
I wasn't able to mange a flashlight and mirror and my hands in the small space. I'll pull the radiator this weekend and see what I find. I'll probably replace the belts while I'm at it. Maybe the water pump also.
I assume a MB dealership would be the best place to buy the replacement bolts.

next time at the swap meet, buy a small mirror with a radio antenna stalk on the crap tool tables, the only things on those tables worth picking up are these type of things that dont require strength, accuracy or good steel

Or could be sheared keyway/ keys, I think.

Tom W

6 allen bolts, and one very hard, and very large bolt in the center, removes the pulley and balancer. The balancer is keyed with two steel pins, almost, but not quite 180 degrees opposite of each other. You need a gorilla sized torque wrench (and a gorilla) to get the big bolt to spec.
I hope for your sake, that this is just an anomoly, and not the beginning of a pattern.

More commonly, the bolts loose torque and shear off. It is also possible that the event was secondary to another failure, such as the alternator seizing, etc.

Can you just see if the pulley will pull away from the balancer?

Hardware store will work.

Thanks for the info. I did see at least one allen-head bolt. The pulley didn't wobble or anything and I could turn any of the pulley's by hand. I guess at this point, the worst case would be the grooves in the crank are stripped out and new keys wouldn't hold. I'll be back with updates.
One other think, from reading other posts, do I understand correctly, I could replace the front main seal fairly easily if I'm pulling the radiator, crank pulley and harmonic balancer?

Ok so I need some more help. I search the forum and got a lot of info on the problems I'm having but I'm more confused than before.
I pulled the radiator and was able access the allen-headed cap screws. All are in place and tight; I tried to unscrew them but the pulley kept turning. I put a 27mm socket on the main bolt and the crank seemed to turn and the thing with the timing marks on it (harmonic balancer?) turned.
So, I'm looking at doing something like sticking a screwdriver in the timing-marked cam thing to keep it from turning so I can get the cap screws and the 27mm nut off. I'm just concerned that I'll bend or otherwise screwup the timing-marked cam.
I'm thinking the dowels sheered and that's causing my problems. This is starting to sound expensive.
Any help is greatly appreciated.

I believe there is a special tool designed to lock on to the flywheel teeth to stop the crank from turning. Without the special tool, your best bet is probably a large screwdriver or small pry bar wedged into a flywheel tooth.

Do I recall correctly that there's some kind of rubber piece bonded to the harmonic balancer that can fail, causing this kind of problem. If so, it could be that and not a key/keyway problem.

If the front of the crankshaft is turning without the engine turning over, your crankshaft is broken, really bad news.

But the engine runs with normal amount or pickup and power and no clanking so i'm thinking the crank is intact.
I'll try the screwdriver tactic in the morning and see what happens. I'm more inclined to go with the rubber part failure. I've had an oil leak and I've been using engine degreaser to clean things up and track it down. I'm thinking the degreaser caused the rubber part to fail, or at least I'm hoping.

So if I get the cap screws out will I be able to pul that rubber part???