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Discussion Starter · #1 · (Edited)
Well, here's hoping you guys can help me! The car is actually a 2005 Magnum with the 3.5L Automatic but the drivetrain is the same and so far I've recieved no help from their forum so hoping the good people here have some insight.


Here is the rough story


Bought this '05 Magnum SXT with a blown motor with only 65,000 miles on the car! It's nothing super fancy but for the price I won't ever complain.

So I bought a replacement motor, swapped it, it runs super nice so I took it for a quick tour down some back roads near my farm and noticed above 50MPH it starts shaking, all the way up to 70MPH it's there. It seems to be very fast vibrations so doesn't "feel" like tires out of balance. I did take the rims off and cleaned all the dirt etc out of them just to check but it still vibrated. RPM doesn't seem to cause it, so don't think it's engine/torque converter so I'm thinking it has to be something downstream of the transmission

When I came back from my initial tour I noticed also a little puddle of tranny fluid under the car.


My thoughts now are this vibration is diveshaft related, which explains why the vibrations feel fast... explains the tranny fluid as it may have wrecked the output seal on the transmission causing it to leak... it makes sense.


But I don't know why/how it could be damaged? We disconnected the driveshaft at the coupler on the rear end (those 3 torque head bolts) and then simply tied the driveshaft forward so it wouldn't touch the rear end, so the driveshaft was disconnected when we towed it... I don't think the driveshaft is key'ed or anything to the rear end coupler is it? I was going to try lifting the car up and rotating the driveshaft to the next set of holes... then the next if need be... just to prove it's not the coupling. When we swapped the motor we didn't remove the transmission, it remained in the vehicle. So the driveshaft itself basically was disconnected... towed 12+ miles... then after engine swapped put the 3 bolts back in and that's it.


Another quirk to this story which I'm also thinking maybe related, I found these two bolts in the back of the car under the seat. I didn't take them out... they are beefy... they look just like the driveshaft bolts on the coupler (these 2 are bigger) so I'm wondering what they are for? They don't look like they are on a rotating piece as the paint is on one side and not on the other...

I'm wondering, if these are maybe the bolts that hold the driveshaft coupler on??? He took this car into a mechanic shop before selling it to me, soooo maybe they removed the driveshaft or something?? I have no idea I'm at a complete loss where these bolts came from



So looking for help. Not sure what to look for, what the cause is, maybe there is something common on these Magnums I should focus on? Maybe someone recognizes those bolts and can tell me where they came from? Dunno. Any info is appreciated

I was reading online and saw one comment someone had about possibly over-torquing the bolts on the driveshaft can cause a vibration, guess the rubber mount can deform and cause the shake? Anyone have the proper torque specs for those bolts? I'm pretty sure when I installed them I HE-MAN'ed them a little bit so maybe I did over do it?
 

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I'm dealing with the same issue. No-one seems to be of any help with the diagnosing. My vibration occurs, starting at about 50mph, and is a rhythmic oscillating vibration. Kind of on and off, but in a rhythm.

Here's what I've done.

In order to try to isolate the problem, and make sure it wasn't another component, I took the car to the top of a very large, long hill. Got it up to where the noise/vibration starts, and put the car in neutral. This takes the transmission out of the equation. I then shut the car off, and just let it continue to roll. The noise/vibration persisted.

Then, I replaced the front and rear rubber driveshaft couplers. The problem remained.

Then, I replaced the driveshaft center bearing (on driveshaft loop). The problem remained, even though the bearing was bad.

While I had the driveshaft off for the center bearing replacement, I noticed that the U-Joint has 2 tights spots in it, about 20 degrees off center in each direction.

So, a replacement rear section of driveshaft from the dealer is about $800. A Driveshaft Shop driveshaft is ~$1200 all in.

I found a thread on LXforums of a guy selling a 300c driveshaft with 50k on it. He wanted $130. So, I asked him to check the u-joint, and he said it was perfect. Got it, and the damn thing has a tight spot right at the center of the U-Joint. Lying bastard.

I've still got that driveshaft in the trunk. I haven't gotten a chance to throw it in yet, but I'm hoping this fixes it. If not, it could still be the U-Joint, since I don't like this one 100%. However, I'm hoping that at the very least, the problem is better.

Other info - I do have a little bit of play in the rear axles. However, it's not excessive, and due to the cost of the axles, if they need to be replaced, I'll be looking at an entire Getrag swap.

Best of luck to you, and if you figure it out first, let me know.
 

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Discussion Starter · #3 ·
So you've done the driveshaft coupler!

Do those bolts look anything like the ones you have to remove? They are obviously from this vehicle... obviously something stationary... and obiously something fairly important since they are quite beefy. I was raising a serious eyebrow when I found them...

The guy I bought it from hasn't returned my email either asking if this was an issue before or not, but he seemed like quite the honest guy and sortof local so I wouldn't expect him to lie about it he just said it ran great until one day engine started ticking, took it in they did a bunch of "tests" and said your engine is pooched. I pulled the dipstick when I got there and no oil on it, opened up the pan when I got the old one out and connecting rods loose, bearing matieral all over... so she was pooched!


Anyway, needless to say I don't think this was an issue prior. I'm wondering if somehow the driveshaft needs to go on the same way as it came off? I didn't mark it unfortunately I just put it back in whichever hole it lined up with.

But yeah keep me posted! Nice to get some replies and possibly save myself some troubleshooting.
 

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Those are the standard bolts. I replaced my bolts when I did the couplings, and that didn't help. Also, I didn't pay attention to the orientation of my driveshaft when I first starting taking things apart, but the problem persisted, so I put it on the lift, unhooked everything, rotated 90 degrees, bolted it back up... problem persisted... did it again... problem persisted. .. went through the whole 360 degrees... didn't change.
 

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Tom,

Just an update for you, the other day while picking up the kid from daycare, pulling out of the parking lot, I got a nasty clunk clunk clunk. Up to speed, the vibration that was persistent before at 50mph and above became constant. Same rhythmic vibration though, but at all speeds. Took it down and put it on the lift, and I've got a crack all of the way up the rear diff cover.

Whatever it was that was vibrating before went to absolute crap and fully broke.

The replacement diff will be here tomorrow, at which point I'll drop the old diff and see exactly what went to hell inside of it. I'll report back after doing so. My mechanic buddy seems to think that maybe a pin fell out and destroyed the unit.
 

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Discussion Starter · #6 ·
Thanks for keeping me up to date! Sorry to hear about your rear end... :D

I got mine figured out the other day, finally back at the farm so put it back on the hoist and took a look, the rubber coupling between the driveshaft and rear end had a tear in it (likely from me over-torquing the bolts) So $200+ for a new rubber "flex plate" from the dealership which is way to much IMHO, so went to a wrecker and found a Charger which had the same one, took and look and it still had it and also noticed it used 6 bolts not 3! So took it apart, $10 for the part and bolts, swapped the one from the car with this one and got it up to about 80MPH on the highway and nice and smooth.

My theory is still someone before me towed/worked on this vehicle and those bolts I found in the back were the missing 3 from the coupling. I re-assembled it as I found it and it was simply put together wrong from when I bought it. So mystery solved

Now I just need to figure out why the very small puddle of transmission fluid is on the ground... I can't seem to find any information about an output shaft seal so maybe these transmissions have something else?
 

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