Weird main pulley failure. HELP....
 

1980 300D. No turbo. 280K+

I’m driving down the road today. All’s well. Then all of the sudden, I loose power steering. Then about a quarter of a mile down the road I notice the temp gauge screaming for the red-line, and the battery light comes on. Engine at idle, I coast into a parking lot and shut it down. Water boiling out of the overflow. Open the hood, thinking I’m going to see some missing belts, but no. All the belts are fine, but the main pulley is no longer connected to the crankshaft. The harmonic balancer is fine, and still attached to the crank but the main pulley assembly will slide right out the front of it. I can see the bolt holes in the back of the pulley assembly. They look fine. I can’t get the assembly out, do to the A/C hose in the way, and will have to tow the car home to pull the fan, etc to get it out through the top. I haven’t reached into the harmonic balancer to see if the (6 ??) bolts have sheared off, or just backed out.

Has anyone else experienced this? Is there a trick to pulling the balancer off if I need to? Special puller?
What are the odds of those (6 ??) bolts coming out? I’ve never touched them in 26 years. I don’t mind a failure with a 26 year old car now and then, but this one seems rather odd.

These crank pulleys sometimes want to take a walk for unknown reasons. From what I've read it because the pulley bolts shear. Kind of has me worried myself. Doesn't seem to be any way to prevent it. Hasn't happened to me yet, but it has to a lot of others. Fun part about it is getting the balancer/pulley assembly aligned properly when you put it back together. There are round metal dowels on either side of the crank, make sure you still have yours. They're easily damaged when the whole thing is reassembled. Hopefully the crank nose is not damaged.

Up-Date. And thanks.
 

Thanks for the reply.

Further investigation with a flash-light and a mirror, reveals 6 bolts sheared off in the harmonic balancer.
The balancer, the big bolt and the crank (which I’m assuming is covered/protected by the balancer and the bolt.) seem to be fine. If the pins you speak of are between the balancer and the crankshaft, they are probably still good for now.

So, I guess the balancer will need to be pulled. Any tricks to getting it off? What’s a good way to hold the crankshaft while I loosen that big bolt holding the balancer on?
I hate broken bolt extractions!! Especially when they’re recessed down inside something. Should I just replace the balancer at this point and age/mileage?

I put a socket and bar on the crank bolt and bump it over. It works every time and has done so for years. Just make sure the bar is solit against the frame or floor just a little bump of the starter should do it.

Go to the nearest machine shop
 

It is a difficult job to get those six broken studs out of there. But there are any number of small shops that can do it with the HB on the crank. I wouldn't take that off unless there is no other way. Getting it off is difficult, getting it back on right I understand is near impossible for a novice mechanic. So find a mechanic who will come out, or one that has a shop, pull the radiator and do it. From reading past threads, I think that over tightening of the belts can really do a number on those six small studs. Don't make those belts too tight!

The only trick to getting the balancer off is loosening the big bolt. The balancer should have a couple of cutouts. One option (per FSM) is to jam the balancer with a chisel or 1/2" extension bar through one of the cutouts. Use a wrench with a long handle, 24" at least. Once the bolt is out, just pull the balancer off with a generic balancer puller. Autozone will lend you one for free.

The trick is getting the balancer back on. Getting it lined up properly is just about impossible. So don't take it off if you don't have to. It's entirely possible that the bolt ends will come out easily since they are under no torque. If you do remove the balancer, match mark it to the crankshaft so you don't put it back on upside down.

WVOtoGO, did you experience any load vibrations/noise at a particular rpm band leading up to this? I ask because I'm having really annoying vibrations between 2800-3300 rpm and its a devil to track down. Thanks.

Let the fun begin !!
 

Thanks for the help guys.

I got the car home tonight, and will dig into it tomorrow or it will have to be next week.

I figured I’d better go with OEM bolts and not something out of the NAPA bins. I talked with the parts guy at the dealer. He did say they were hardened. Duh, Like I didn’t figure that much out already.:rolleyes: I have a guy that runs his own "MB Only" shop that I’ve dealt with for years. He’s got his OEM parts supplier delivering a set Tuesday. He’ll cut the dealers price in half for sure. $4.50 each is a bit much for that small of a bolt, regardless of its hardness.

So, I figure I’ll pull the fan out. Then the radiator out. Hopefully that will give me enough room to center punch and drill the studs. A soak of some penetrating oil, and if I’m lucky, they should unscrew with an easy-out. They are all broken off flush with the balancer, so no chance of grabbing them with small Vice-Grips.

If that doesn’t get it, I’ll look into pulling the balancer. I have a box full (tractor owner thing) of various size/length breaker bars to get after the bolt with. I have a few different pullers, but most would need to incorporate two of the bolt holes. (Unless there’s two other holes in it for pulling. Fat chance, huh?) So, I guess if I get one stud out, I’ll shoot for the one 180 out from it next. I’ll be sure to mark the alignment well before I pull.

Thanks again for all the help. Comforting for sure. Beats the heck out of diving in blind. I'll keep ya posted.

Funny you should ask...
 

Seth;
Funny you should ask.

Not really any odd/new vibrations, but a couple of days ago, I had it idling cold in the hangar. Man, was it making a weird knocking noise from the front of the motor! Too metallic to be a rod but seemed to be inside the front of the motor. One of those funky detonation dirty injector type of knocks. I had just (within 20 miles) run a can of Purge through the injectors, I figured maybe it was an injector thing. I took my stethoscope to it but couldn’t find it. Soon as I touched the throttle it stopped. After warming up it seemed to go away. Yesterday, it didn’t do it at all when cold. But had a weird knock after sitting at idle for 5 minutes or so. Stopped when I touched the throttle. This morning it did it, but not for long. Numerous stops and starts today before the failure. I’m actually hoping this was the cause of the knock.

If I were you, I’d get a mirror and a flash-light and crawl under. Count the small bolt heads. Grab the main pulley assembly and hope it doesn’t wiggle. If it does, you may have found your problem.

BTW - Do you sail (or is that a Navy thing)? Texoma? Always looking for a good tach/trap-tician for the Laser-II regattas !!

I just had my balancer off ('82 300D) for a front crankshaft seal replacement. The original bolts were grade 8.8. My local Ace hardware store has 12.9 grade for $1.00 each. Just make sure they are the correct length. If too long, they will damage the front seal.
And if you wind up having to pull the balancer, you might consider changing the front seal while the balancer is off.

Great thread.
So, it is recommended that we check our pulley bolts, yes? Also be sure the belts aren't too tight, and wiggle the pulley to check for play which would indicate one or more sheared bolts.
I must admit this is one failure that scares me, because it can damage the nose of the crankshaft.

It happened on my "79" 240D. The fan pulley on that car had 3 bolt holes and the bolts had sheered off. There were 6 bolt holes in the balancer so I just rotated the pulley to "new" holes using new OEM bolts and lok tite.

So far. So good. But not really into it yet.
 

Well it only took about 15-20 minutes to pull the fan, radiator and shroud out. No problems there. The main pulley assembly lifted right out the top, no problem. The crank and balancer look fine as well. But, the busted bolts don’t look like they’re going to be such a walk in the park to remove. Not quite as recessed as I had envisioned though. Just not a lot of room to work down there, even with the radiator out. I’m still set to go at it with the balancer installed. Think I might try to track down some reverse drill bits, too.

My chiropractor is going to love me for this one. :(

Here is something of importance that I’d like to pass on to those now worried about such a fate. When inspecting your pulley assembly for those 6 bolts (Allen head, by the way, not hex.), don’t be fooled by their presence alone. Just because you see the 6 heads doesn’t mean they aren’t broken. I pulled my pulley assembly out, and all 6 bolt heads are still stuck in the assembly looking like they are at proper torque and doing just fine. I think only one was backed out about a 16th of an inch, and that is seen only when looking at the correct angle. So, upon inspection, don’t just look. Get in there with a wrench and give them a check for “snugness“. (Neat new word there. You pros can stick with “Proper torque“.:) )

Well - it’s back to the back breaking trials of bolt removal for me. I’ll keep you posted on the job as things progress. Thanks again for the helpful tips.

It might be easier if you do it on your back, from underneath.

My theory is that the bolts shear pretty much simultaneously, probably after loosing torque and allowing the pulley and vibration damper to rotate relative to the balancer. I would think that the best preventive action action would be to loosen and retorque all six bolts using thread locker. (Replacing all six bolts with a higher strength bolt would be the next level of prevention.)

As far as the crankshaft being damaged, it is not likely with this failure mode. The crankshaft snout is completely "enclosed" by the balancer and the washers on the crank bolt.