rough gear shifting & sagging rear suspension
 

Hello everyone,

I am an absolute newbie on this forum. Prior to registering, I have read some of your threads and I was truly impressed with everyone's knowledge. I am hoping that I will expand mine during my membership.

So, this is something that I have been dealing with for the last 4 months now. I am suspecting that some of these problems have been widely covered on this forum, however, all these issues started happening pretty much at the same time. And I think it is worth mentioning.

My MB: 1985 300SD - 170K miles, mostly city driving

- suspension (?) - just recently, I noticed that my rear end is sagging a bit. 90% of the time I drive by myself and hardly have any passengers. So, when I sit 2 adults in the back, the rear end goes down so much that my exhaust pipes grind against the ground when driving on uneven surface.

- transmission - gear shifting has been inconsistently rough lately (changing the fluid and the filter did not change anything). Going from 1st to 2nd has been rough to the point that I can feel a significant jerk. Going from the 2nd to 3rd gear is much better, however, I can hear a slight thud.

- during quick acceleration, then letting go off the pedal and then breaking to a complete stop, a loud thud in the back is noticeable. The same thing happens when I accelerate and quickly let go off the pedal during city driving.

- when putting into Reverse, I can hear a thud in the back plus the whole rear end 'jumps up.'

I am thinking that there could be different things that are happening at the same time. Has anyone had similar symptoms with their cars? What possibly can be causing these things all of sudden. Any feedback will be greatly appreciated. TIA.

Firstly, it would be good to get a measurement from the pavement to the inside of the wheel wells. We could then determine how bad the problem is. Do it with no weight in the vehicle and with little fuel. Do it on both sides.

If the car is original and nobody has done any work, the following items are the culprits:

1) Differential mount
2) Shocks
3) Subframe bushings
4) Springs

However, you may be able to avoid new springs if the remaining items are changed and you obtain a thicker spring pad for the top of the existing spring. It all depends on how severe the problem is.

On my SD, I cured the problem by going to the thinnest possible pads above the front springs. The front end came down and the back end rose slight. Coupled with new shocks and a new diff mount, the SD sits perfect.

The rear end sag is probably due to tired springs. They don't hold the rear up as high as they should when they get old. The differential mount could also be tired.

The hard shifting is most likely vacuum related. Danged near everything, or so it seems, is run off vacuum in these cars. Door locks, engine shut off, EGR, transmission shifting quality, climate control, and various other things depending on the year and model. Do some searching on vacuum leaks. When the system starts leaking the transmission doesn't receive the proper amount of vacuum which causes hard shifting.

You clunking noise under changes in acceleration sounds like a bad flex disc on the drive shaft. Or the center bearing on the drive shaft. The flex discs are what hold the drive shaft to the transmission and differential. The center bearing is what supports the center of the drive shaft. This is an important one to not let go. If one of the flex discs give out your drive shaft will flop around under the car. And at highway speeds you can imagine the damage it will do. Climb under the car and inspect both flex disc. Your looking for cracks or miss-shapen rubber. For the center bearing wiggle the center of the drive shaft back and forth some, it should move like 1/2" to an 1" to one side or the other, but no more than that. Also look at the rubber.

Putting the car into reverse often lifts rear end because of the torque of the engine and transmission wanting to move in the car in that direction. I notice on my wagon when shifting from reverse back into drive the rear will drop back down for the same reason. The clunking isn't a great think, but again pretty normal on these old cars. But I'd have a look at the flex discs and center bearing as well as the differential mount. All of those things could be contributing to the clunking noise.

Hey,

thanks for the comment. Greatly appreciated. I have been struggling with this for a while and now I am tired of it. Yes, it is perfectly reasonable (and the cheapest) to start with the spring pads. I am guessing it would not hurt to do all of them at the same time if everything in there is still original. Is changing the pads a DIY job, are there any pictorials that you could provide? Thank you.

I think I am already determined to replace the springs.

I suspected it would be vacuum related and I am glad that someone validated that for me. However, everything everything else seems to work properly that depends on the vacuum system. I remember, a while back, one of the mechanics was playing around with the throttle linkage. I don't know if it may be backfiring on me now.

This is definetely something to look at. I will make sure that this is checked soon. I will post my findings as soon as I get that taken look at.

Thank you all for your comments. This has been very helpful.

The pads is a DIY job if you have the spring compressor, although some will struggle to get the job done on the rears without it. The pads sit on the tops of the springs and come out with the springs.

I'm not aware of any pictorials for that job, but there are some writeups about it if you do a search.

Unfortunately, I don't have the spring compressor but it does not mean that I cannot get one :) Anyway, I have gone through some writeups already and there are tons of info available. Again, thanks for your time.

Hi,
I guess the more stuff I read, it all brings next questions. So, if I decide to to replace most of the rear suspension (I just would feel more comfortable) including subframe mount, subframe bushings, shocks and the springs what is the recommended thickness of the spring pads? One would say that these are the least important in the overall scheme of things but I want to stick to all the original specs. Should I even worry about the spring pads if I get the most important components out of the way?

The spring pads are used to tune the ride height because the springs are not manufactured all that precisely. There are recommended pads based upon the specific equipment on the vehicle, but in the end, the ride height that you get determines the pads that you desire.

If you're replacing the springs, I'd stay with the existing pad thickness.

But, as mentioned, I would not spend the money to replace the springs unless I confirmed that the pads could not fix the problem. You might be spending good money for nothing.

Plenty of folks condemn the springs on here without the slightest hint of evidence that the springs are NG. It's just the first and easiest thing to do.

I'm about to replace my diff mount and subframe bushings in the next couple weeks. The sagging rear didn't bother me until I installed my trailer hitch.. Scraping exhaust.. no big, but you can FEEL it when the receiver tube scrapes!

Today, I decided to take my initial measurements of the clearance between wheel well and the level ground. I have the original 14 inch wheels with original size tires of 205/14/70 with a decent air pressure. The fuel tank is pretty empty with about 2 gallons left in it. I have some clutter in the trunk like tools, accessories and some other stuff weighing about 60 pounds. Here are the results:

- front driver side - 27"
- front pass side - 27.5"
- rear driver side - 25.25" , 24"
- rear pass side - 25.5" , 24.5"

Please note that I put two different values for the rear suspension clearance. The second one represents a measurement with me sitting in the back weighing about 230lbs.

Do you still recommend replacing the front spring pads for thinner ones or I should keep them as they are? I think I had seen one of your posts saying that 27" clearance is about right. Looks like the rear ones would need to be replaced for thicker ones, possibly the thickest available. What do you think?

Firstly, at 27", the front is too high. That's common on the SD and I found the original pads to be the thickest available............without any good reason. I replaced those pads with the thinnest available and the front end came down to 26".

The 25.5" dimension for the rear is very close to the dimension that you seek and new springs are definitely unwarranted. A thicker spring pad, new differential mount, and new shocks will easily gain another inch in the rear. 26.5" is about the limit............it starts to look ridiculous at 27".

Since it is a conventional suspension, you must suffer with decreasing ride height in the rear when weight is added. It all depends on how much you typically carry. You can set it up a bit high (27") with no weight and it will still do a good job with 400 lb. of payload.

I guess it would make sense to replace the front pads with the thinnest available to see how much clearance gain I would get in the back. The other thing is that I have been looking into replacing the subframe mount anyway. To summarize, thinner pads in the front, new subframe mount and thicker pads in the rear would do the trick. That, after all, would bring me to the 27" clearance in the rear without any weight.

It's the "differential mount" that you're referring to.

If you want 27" in the rear, you'll need to thin the front pads, increase the thickness on the rear pads and change the differential mount. If the shocks are original, they should be replaced and you'll gain another 1/2" or so.

Yes, the differential mount is what I had in mind. I have seen people call it a subframe mount as well. Or these two are two totally different mounts? I shall get the rear end suspension done next week or so. Will send a follow up post.