SLS - Self Leveling Sag
 

System has been loosing about 1 litre of hydraulic fluid per month although there is no visible leakage in the rear near the accumulators or the shocks. There appears to be some spotting on the driveway where the front of the car is usually parked.
Recently the rear drivers side has been riding lower than the passenger side and when the car is loaded and turning right it sounds as if there is something grazing the tire. This situation appears to be deteriorating as the car requires less loading to make the same sound when turning or going over a speed hump.
Where would be the logical place to start looking without throwing parts at it?

Site search did turn up a few possible culprits, from differential mounts to bad accumulator, but i'm not certain that they may apply in this case. Suspension does not feel unduly harsh, nor bouncy. The fluid loss has been an issue since i purchased the car 2 years ago, albeit the rate of loss has slowly crept up over time from ½ liter to about a full litre per month.

Any suggestions as to what else can be checked to narrow down the troubleshooting process would be appreciated.

TIA
Patrick

U/V dye ? If so, clean your engine bay and undercarriage first, before adding the dye.

:-) neil

Just curious, what kind of car are you asking about?

Peter

Sls
 

Hi Peter

Car is a 1986 560SEL

OOOKKK. Now we know we are talking about a 126 Mercedes. Hopefully you never let the fluid go so low that the pump ran dry, because the hydraulic fluid is all that lubes the pump. The leak is probaly in the low pressure return hose from the pump to the reservoir. If you think that one side is lower than the other, you could have a failed hydraulic cyl on that side of the car. If the car is sagging evenly, it could be the pump or the control valve on the rear of the car. Real for sure diagnostics on this system requires a high pressure gauge with all the special fittings to test all the components. If the pump has been destroyed or the leveling valve has failed, you will need to get replacement parts from another 560SEL. Although the system functions exactly like the 123 wagon, the parts are totally different.

Peter

Sls
 

Thanks again Peter :cool:

The return line from the suspension is low pressure return. With the engine running the pump supplies fluid to the suspension contstantly. With no load the pressure holds up the suspension, and the rest of the fluid returns to the reservoir. You can take off the cap and watch a stream of oil returning to the tank. When you add weight to the back, the leveling valve moves and ports more fluid to the hyd cyls to raise the car. When the car is level, the valve zeroes out and all the fluid is again put into a recycling stage. I don't suspect a sphere, because they are just shock absorbers since there are no shocks. I'll look for a diagram.

Peter

I'm not familiar with the undercarriage of the SLS, but here's a thought. On my W140, the levelling valve and hoses for the rear suspension are directly over the exhaust pipes. I have a very small leak in one of the return lines. The fluid either drips directly onto the exhaust, or the drops run along a line until they get to a low point, and then drip onto the exhaust. The fluid burns off when the car is running, leaving no trace of the oil. Wipe your lines so that they're clean and dry. Monitor them closely for a while, and see if they are wet.

Hi Peter

Thanks again for the reply. was pondering some more...

I appreciate your help. :)

Patrick

When the valve is moved off center it ports fluid to or away from both hyd cyls. It doesn't play favorites. Since there are no rear shocks the sphere has a very thick diaphragm at the diameter with nitrogen on one side and hyd fluid on the other. When you hit a bump the fluid compresses the gas on the other side. When the diaphragm springs a leak, the whole sphere fills with fluid and makes for a very bouncy ride.

Peter