1972 300sel 4.5

[Clunker]

I have a 1972 300sel 4.5 that, when you step on the brake while the car is moving in reverse, lets out the most god awful sound and vibration I have ever heard or felt. It is coming from the rear tire(s) area. Any Ideas to what it is?

[cth350]

welcome to air suspension.

When going in reverse and hitting the brake, the axle is has to react in a very different way than going forward and braking.

I can't tell you how to fix it, since I haven't been under an air suspension car for some years (though I have one sitting in the driveway, waiting for repair).

But I can tell you to check 3 things...

1. Forward from the axle by the wheels are two spurs that connect to the chassis under the rear seat. The spurs connect via stablizer-like links. Make sure that these links are OK.

2. On the axle tube are two replaceable bearings and seals ("brake holddowns"). You'll have to check the service manual (108-109-111-113 book) for instructions on checking and lubricating these. I would presume that this is where your problem lies.

3. Rear shocks.

Nice thing about reverse, is that you don't do it at 60mph, so this (hopefully) isn't life threatening. Try not to reverse near any big cliffs until you understand what's up.

I would strongly recommend that you get the factory service manual, if you haven't done so already, and then read sections 32 & 35 VERY carefully and do so over and over again until they make sense.

Once you know what's wrong, let us know, and don't forget to make us jealous that you have one of these great beauties. The 109 cars are very sexy cars.

-CTH

i have a '72 300 SEL 4.5 too and it has started doing the same thing. If you find a solution could you pass it on to climbingjon@hotmail.com thanks alot,
jon

[cth350]

Check out the rear, there should be a pair of grease nipples for that bearing. Give them a shot of grease and see if it helps.

-CTH

[stevebfl]

Greasing the nipples on the caliper carrier will often stop the problem.

The real problem is that the carrier is loose on the differential housing. The carrier is a large cast iron fixture made of two halves that surround the axle tube. They go together almost like a large connecting rod. Inside are bearings similar to engine main bearings only made from some white plastic/nylon.

I have redone 20 or more of these over the years and have found that even with new bearing shells they are still loose do to wear on the axle tube itself.

The solution is similar to resizing a rod. I usually take heavy sand paper on a piece of glass and take each half and sand away. After removing .005-.010 (I imagine)I reassemble and check for play. I keep doing this untill the assembly is fairly hard to turn (by hand) fully torqued and greased.

[cth350]

You are referring to sanding each half of the plastic bearing shell? The idea being a smaller circumference on the bearing gives you a small diameter and thus a tighter fit on the axle?

-CTH

[stevebfl]

No leave the new bearings alone.

Its the two halves of the caliper carrier that must be reduced with the idea being to reduce the diameter of the carrier. Remember the idea is to stiffen the ***** out of the final assembly.

ALSO - What I am describing is a way to repair an assembly where the replacement of the plastic pieces isn't enough. The axle tube gets pitted from corrosion and reduced in diameter from that and natural wear. Its a big job to rebush the carrier. One should try to get it as stiff as possible or you will be greasing it every week to keep the shudder out.