Prop shaft flex plates on W123 failure exaggerated? - PeachParts Mercedes-Benz Forum

lietuviai · #1 12-08-2003, 09:41 PM

I was recently browsing parts on eBay and there was a seller who had prop shaft flex plates listed for W123's. In his listing he had written that if they should suddenly fail on you while driving that it can wipe out your transmission in the process. Is he just trying to exaggerate the failure to make you go out and buy a set? I have looked at the entire assembly and I don't really see how that could possibly happen. The way I see it is, that you'll get plenty of notice before they go to pieces on you.

rob_frick · #2 12-08-2003, 10:45 PM

I can speak from experience. On one of my 300D's, the front flex disk broke at 20mph. It sounded as if the transmission exploded! The front section of the propeller shaft grabbed the shift linkage, the speedo cable and the e-brake cable and wrapped them one complete turn around the transmission. That in turn caused the shifter to shift too far down, wrecking the transmission. The resulting flailing of the front section of the propeller shaft hammered a large hole through the passenger side of the propeller shaft tunnel.
Trust me, he is not exaggerating, and this is avoidable. The best way is to look for dryness and cracking at every oil change.

lietuviai · #3 12-08-2003, 10:57 PM

Oofah! What you're saying sounds a lot like what was in the listing. I'd check the disks everytime I change the oil but I'm usually in too big of a rush to jack the car up to do that. I couldn't get my fat head under that part of the car any other way. I'm sure I'm probably ready to change them since they are original (220K+ miles) and I get an occasional driveline shudder between 30 and 40 MPH.

79-240d-project · #4 12-08-2003, 11:06 PM

I read the same thing about the flex disks. Given the fact my car is 24(almost 25) years old I will be changing out all the rubber based components. It seems like the wise thing to do since I wish to drive my 79 240d as long as safely possible, cheap insurance as far as I can see.

On my list to do:
Flex disks
Motor mounts
Trans mount
Rear diff. mount

Anything I forgot?

BoostnBenz · #5 12-08-2003, 11:41 PM

I just had a new noise surface here not very long ago but I was pretty sure that when replacing the front suspension bushings that it'd go away but it didn't. I'm going to inspect these soon, if I see any cracking the car is parked. I don't know if it can take the whole tranny out but it is a mess when they go. Imagine how fast that thing is spinning and all of the things it could take out while slowing down! You are a very lucky man 300SDL.

BoostnBenz · #6 12-09-2003, 06:23 PM

Bad wording, sorry. I was replying to the original question whether a failed flex disc can lead to a transmission failure as well. I don't see how it could literally ruin the tranny but it will create havoc on all of the surrounding items. While the flex discs aren't fun to change you don't have to drop the tranny to get them out or anything that extreme.

lietuviai · #7 12-09-2003, 06:29 PM

Quote:

Originally posted by BoostnBenz
Bad wording, sorry. I was replying to the original question whether a failed flex disc can lead to a transmission failure as well. I don't see how it could literally ruin the tranny but it will create havoc on all of the surrounding items. While the flex discs aren't fun to change you don't have to drop the tranny to get them out or anything that extreme.

No problemo. Changing those flex discs is a job that I'm not looking forward to doing. Any ideas how involved it is to do and how much time should I put aside? The CD shop manual makes it look like its quite a PIA.

lietuviai · #8 12-09-2003, 06:43 PM

Quote:

Originally posted by BoostnBenz
I just had a new noise surface here not very long ago but I was pretty sure that when replacing the front suspension bushings that it'd go away but it didn't. I'm going to inspect these soon, if I see any cracking the car is parked. I don't know if it can take the whole tranny out but it is a mess when they go. Imagine how fast that thing is spinning and all of the things it could take out while slowing down! You are a very lucky man 300SDL.

I'm a bit confused, why would you need to remove your transmission? I thought that these discs are on the prop shaft. Is there something else that I need to worry about?

MikeTangas · #9 12-10-2003, 12:56 AM

Yes, failed flex disks can ruin a tranny.

I got the 560 after the front flex disk let go at speed and the PO's insurance totaled the car. The tailshaft housing was broken apart and the output yoke was gone. I got lucky and was able to just replace the tailshaft housing and output yoke (and tranny mount, exhaust bushings, crossmember and a few other parts). I have put over 20K miles on it since the repair and no problems yet.

Besides the tranny damage, the entire tranny tunnel was beat badly. I had to reskin the tunnel and replace the driveshaft. Then we move to the interior, where the work was centered around the damage to the console and the airbag system. The driveshaft hitting the tranny tunnel was of sufficient farce to blow the bag and retract the seatbelts. The fact the shaft yoke tore through the tunnel and made a direct hit on the crash sensor surely had nothing to do with the bag blowing

.

Many thousands of dollars worth of damage if a flex disk lets go at speed. Change the flex disks early.

Not a problem to change them. Just need a 19mm box wrench and socket you might need to open the shaft tunnel to loosen the centering nut or you may be able to slide the shaft halves together without opening the tunnel. The front shaft half needs to move back about 3/4" to make room for the swap. It sounds difficult but it isn't.

leathermang · #10 12-10-2003, 10:39 AM

I would like to mention here for future reference... that the flex discs come in AT LEAST TWO ' firmnesses"......
and in at least one situation the factory changed the specification for ONLY one of the two on a vehicle to correct some problem...(Ie, it had a car with two hard discs change to one soft and one hard.... or something to that effect )
So, with anyone thinking of replaceing them... I suggest close reading of the TSB's for your car and of the shop manual... not just go get any flex disc from a parts car or a parts place without knowing what your car needs...or what the updated recommendations are for your car....

lietuviai · #11 12-10-2003, 08:11 PM

It just looks like it's probably too involved to try it as a DIY project on a weekend. I've done U-joint jobs in less than an hour. Why couldn't MB just put in U-joints like the rest of the auto manufacturers?
OK, what should I expect a shop to charge me for the job?

wheelguru · #12 12-11-2003, 11:40 AM

It is not really too hard. six bolts on each end. If you loosen the bolts that snug the carrier bearing up to the chassis, you can slide the drive shaft back a little to get them in and out easily.

lietuviai · #13 12-11-2003, 04:05 PM

Quote:

Originally posted by wheelguru
It is not really too hard. six bolts on each end. If you loosen the bolts that snug the carrier bearing up to the chassis, you can slide the drive shaft back a little to get them in and out easily.

That does sound easy but what about the different types of flex disk materials and the need to center it correctly? Could I just buy whatever is listed for my year and model and just slap it together?

sohj · #14 12-11-2003, 04:17 PM

It is not so easy. I have been intending to do this for months, but haven't gotten the guts to just take a crow bar to the old disk to pry the shaft forwards. I've been told it telescopes, but so does the arm of a tower crane and I can't do that one by hand either.

wheelguru · #15 12-11-2003, 04:27 PM

I don't know about different disc materials. If I read LMGs post correctly, it sounded like a running change from stock that would make an improvement. As a running change, the old compostion flex discs should work fine, it is just the the newer composition might be even better. (if I read the post correctly)
As far as centering - not a worry. the discs have a metal insert that goes all the way through the rubber, and a little shoulder sticks out that nests into your bolt holes. It is self centering. You want to keep the shaft in the same position relative to the trans. for the sake of keeping everything balanced, but don't worry about centering.
I don't think you are looking at a weekend, more like a couple of hours, depending on how fast you can get the car jacked up high enough to slide under and still have some elbow room.