300TD Rear End

[DFisher]

Several years ago I purchased a 1987 300TD wagon. This is a great car, and I use as my primary vehicle.

One problem -- It took the car about a year to rip up a set of rear tires (worn out the insides of the tires). I took it to dealer, who told me to replace the rear springs and the self-leveling mechanism. The cost quoted was about $1600.

Due to the high cost, I took it to a local independent Mercedes mechanic for another opinion. He agreed that the rear springs were worn out and that the rear self-leveling mechanism was totally shot. He also said that he thought that the rear wheels needed a little aligning (by a frame shop). He suggested that (in light of the fact that I do not use the car for heavy loads) I take a "wait and see" approach with the rear self-leveling mechanism, thinking that was not contributing to the tire problem.

So, at a much lower price (like $500), he replaced the rear springs and had a frame shop adjust the alignment. Since then, I have driven the car for about a year and put maybe 10k miles on it. The repair seems to have done the trick and fixed the tire-wear problem. However, it seems to have affected the handling of the car. That is, sometimes when I hit bumps in the road, the rear end of the car feels like it pulls to one side or the other, like I was towing a trailer that was starting to fishtail.

The issue does not seem bad enough to cause a loss of control, but feels just a little squirrelly and I would like it fixed. Before I throw $600 or $700 into fixing the rear self-leveling mechanism, I would appreciate opinions as to whether the broken self-leveler could cause or contribute to the handling problem? Any other ideas of how to address it?

[psfred]

Probably dead accumulators on the self leveling and bad links in the rear suspension allowing unwanted camber/caster changes on the rear wheels.

Have it inspected again to check for bad links -- probably need all of them on both sides plus an alignment.

Check for bad accumulators by bouncing the rear suspension by putting you knee or foot on the rear bumper. If the tires bulge, but the rear suspension doesn't move, replace the accumulators pronto -- if you don't, the strut seals will fail from the excess pressure, and they cost about $600 each. Accumulators are about $100 each, and the ride will be MUCH better with new ones.

Note that the self-leveling will work just fine with the accumultaors bad, but it will ride VERY hard, to the point the rear wheels bounce off the ground.

Peter

[DFisher]

Thanks for the helpful information about the leveling problems with my 87 300TD. With that new information, I was able to read the numerous posts regarding self leveling troubles and get much more up to speed.

When I do the bounce test described above, there is give in the springs and struts, leading me to believe that the accumulators are not the chief culprit (though I may replace them anyway). Is that a valid conclusion?

I added weight to the rear end. The self-leveling did not work, and the rear end just sagged. This leads me to believe that the issue is either the tandem pump, the level controller (including the linkage, as mentioned above) or the (ouch $$) struts.

I visually inspected the components. Everything looks okay, except that the hydraulic struts look worn, mainly due to the fact that the rubber bellow around each is dry-rotted and broken up. Does that necessarily mean they are dead? If not, can/should the rubber bellow be replaced?

I released the pressure in the system via the nipple on the top of the controller. Surprisingly, the rear end dropped, suggesting that the system maintains pressure.

Next, I plan to test the oil pump and controller. Does anyone know what threading the nipple is on top of the controller (it looks like 3/16 brake line fitting, but I suspect it is M10 1.0 or something close to that). I understand that the pressure at the bleed fitting on the controller should be at least 133 bar when the car is on and the level controller is in the "filling" position (up). What should the pressure be right after the controller is switched to the "emptying" position?

If pressure checks out okay, I will bleed and re-pressurize the system, check the controller orientation and linkage, as suggested above, top up the oil and visually inspect the hydraulic struts for leakage. Beyond that, are there any other suggestions for confirming whether or not the hydraulic struts need to be replaced (at a cost of $1100 plus labor)?

Thanks

[DFisher]

I am still trying to figure this problem out... However, it really has me stumped. I tested the hydraulic pump and level controller valve on the 124TD per the service manual (by hooking hyrdaulic gauge to place where bleeder screw goes in level controller valve). The pressure was only getting up to 30 bar with the arm in "Fill" postion (it is supposed to get up to at least 133 bar). There were no leaks in the system, at least at that pressure, as it maintained 30 bar for days. That made me believe (WRONGLY) that the pressure relief valve in the level controller was shot, and I installed a new level controller valve (and accumulators, since I was under there anyway). It did not work. After all that, the pressure would still not exceed 30 bar, no matter what I did to the level controller arm.

As I see it, that leaves only 2 possible causes -- the hyrdaulic struts or the tandem pump. I think (maybe wrong) that the only way the struts could be causing the low pressure is if they were leaking. They are not leaking hydraulic fluid.

That leaves the pump. Is it possble for the tandem pump (which doubles as a stearing pump on this vehicle) to fail such that it would deliver low hydraulic pressure? The pump does not appear to have a pressure relief valve in it. It does have a shear pin, but I would think that if that failed, the pump would not pump at all.

Please, please provide thoughts on this. Both the struts and the pump are expensive to replace, and I would like to figure this out before throwing more money at it. Thanks much for any help.