This 300 is going to drive me crazier..... When I bought the car I felt the vibration but thought it was the tires or rims. Nope that wasn't it, I swapped out the known good rims and tires off of the 240.
The vibration is noticable at around 40 mph and you can feel it all the way up through highway speeds 70mph. The vibration never really gets worse at any speed but appears to be constant. It feels like it coming from the back somewhere but I really can't place it. The best way I could describe it is like being on a ship and feeling the constant steady drone of the engines through the deck.
If you guys could give me some things to check out I'd appreciate it.

Thanks,
Scott

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82 240D - 433,000
WVO kit installed 5000 miles ago.
83 300D - 188,000
Just aquired.

Pine Mountain, Georgia

Wheel bearings check out ok?

check engine & transmission mounts...

Look so see if the drivshaft support looks fine...or if its new....some idiot may have taken it apart without indexing it and cause a vibration issue by putting it back together incorrectly (driveshaft halves).

__________________
Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
---------------------
Section 609 MVAC Certified
---------------------
"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

Vibrations are REALLY DIFFICULT to troubleshoot over the internet, but I fully expect that it is either a motor mount or driveshaft related.

Good luck,

also could be failing cv joints in half shafts. the driveshaft thing though possible seems less likely. the diffs are very durable and seldom cause trouble, ditto wheel bearings.

crawl under car with half shafts not stressed... meaning not being held by being in park or with parking brake or in gear on a slope... chock wheels if on slope and disengage all mechanical devices, then grab half shafts and see if they wiggle. there should be NO play at all. if they wiggle there is a problem. also if they are going bad they sometimes vibe when going over dips in road... more or less.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

I went through this with my '85 300D too.
After replacing all the driveline rubber, driveshaft carrier mount and bearing, motor, tranny and diff mounts, flex discs, the vibration remained unchanged.

I put the front wheels on ramps (for more normal suspension), and put jack stands under the bottom of the rear shocks.

I then had a helper "drive" it at the speeds where the vibration was prominent. No signs of wobbling parts. The vibration was still apparent on the ramps/stands.

We put a stainless pipe clamp on the driveshaft, then tested it with the car still on the ramps/jacks. Then moved the clamp 45 degrees, and tried again. With the clamp in one position, the vibration was better. We tried more weight and and less to find the best weight. We then moved the clamp aft (on the forward section of the driveshaft as my vibration seemed to come from between the seats) and wow, the vibration was gone. The whole process took us about an hour.

On the road, the vibration is gone, my ride is sweet at all speeds now.

Since your vibration is in the rear, I'd look at the rear flex disc, driveshaft carrier mount/bearing, and differential mount. There are factory alignment marks on the drive shaft- check your shop manual for their location and see if somebody did you a dirty deed.

I've also had a rear half axle go bad and start to wobble- this made a serious vibration but it was noticably rear axle and one sided. This half axle failure was plainly visible with the rear wheels jacked up and spinning.

Bruce McCreary
'85 300D
Snowflake, AZ

Thanks guys for the information. I got my big arse under the car and looked at some of the suggestions. The most obvious to me were the motor mounts (which I could see from the top). The flex disk look OK, but then again I'm not 100% sure what I'm looking at.
I'll bring it to my tire guy and pretend to know what I'm talking about when I ask him to check the drive shaft and read axles.

Thanks again, it's nice to ask a question without getting a bill.

__________________
82 240D - 433,000
WVO kit installed 5000 miles ago.
83 300D - 188,000
Just aquired.

Pine Mountain, Georgia

You are looking for cracks in the rubber with the flex joints...you need ot drop a shield to check for the center drive shaft support (rubber) is not torn.

If Flex jointys have cracks its time to replace them. But I bet you problem is not with the Flex joints...they will work fine right up to the point of failure when bad things will happen.

__________________
Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
---------------------
Section 609 MVAC Certified
---------------------
"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche