Refurbishing the SLS Hydropneumatic Pump
 
by George Murphy – Diagrams from Mercedes-Benz Workshop Manuals
 
Pierre Hedary discussed in the January-February 2014 issue general care of the self-leveling hydraulic system found in several Mercedes-Benz models such as the 450SEL and others from 1977 well into the 1990s. The SLS hydraulic pump is a critical part of the system because it provides the hydraulic pressure needed to operate the system, but like any mechanical pump, eventually it stops operating.

The good news is that any reasonably handy and careful mechanic can refurbish the hydraulic pump by dismantling it and cleaning the components, then replacing all the seals. We’ll show you how to do that in this article.

Required Parts and Tools

More good news is that Mercedes-Benz offers the shaft seal, pump cover O-ring, and paper gasket for the pump mounting as standard parts. The bad news is that two critical parts, the internal O-rings, are not readily available. As the years add up, these O-rings become hard and flattened, allowing fluid pressure to decrease, with consequent failure of the system to level the car, so it’s critical to replace them at the same time as you replace the other parts.


Figure 1: The parts required to refurbish the SLS hydropneumatic pump.

The Mercedes-Benz parts (shown in Figure 1) include the shaft seal (Mercedes-Benz part no. 004-997-01-47, #11) the cover O-ring (M-B part no. 007603-012111 #12, 12X16X1.50) and the paper gasket (M-B part no. 114-236-00-80, #1d) for the pump mounting.

In addition, you’ll need two O-rings for the internal seals. The challenge is that you can’t just walk into an auto parts store and order these off the shelf, and the material and dimensions are critical to assuring good seals that will last as long as the rest of the repairs you’ll be making to your hydraulic system.

After some research, I determined that the correct material is Duroblack Buna N and the right dimensions are 1/16 c/s x 2 3/8 ID x 2 1/2 OD and 3/32 c/s x 2 3/8 ID x 2 9/16 OD. They have to be made up by a good O-ring supplier, and I’ve only been able to find them in quantities of 100. After figuring out the dimensions, I had a batch made up, which will probably last for a very long time, and I make them available to other owners through my company, Performance Analysis Co. For convenience I supply them as a kit with the required Mercedes-Benz parts as shown in Figure 1.

To carry out the pump rebuild, you’ll need the following tools:

• 5mm hex drive
• Quarter-inch wood dowel 1.5 inches long with rounded ends
• M-B  hydraulic fluid part #000 989 91 03 – 3 liters
• Lint-free rags or paper towels

Repair Procedure

These expensive precision-made pumps used on certain models are generally trouble-free as long as no metal particles enter the pump and the right fluid is used. The hydraulic oil pump is a radial piston-type pump with four pistons arranged radially in relation to the eccentric shaft, as shown in figure 2.


Figure 2: A radial piston pump with four pistons (1, 2, 3, & 4) arranged around the central eccentric shaft A.

To begin the repair, the pump must be dismounted. Loosen the hydraulic oil lines and remove them from the pump body. Cap or cover the ends of the lines to prevent dirt from getting in. Loosen, but do not remove, the two short 3mm Allen hex screws in the cover.



Figure 3: Section of Hydraulic Oil Pump. Detail showing long metric allen head (1F) screws used to attach pump to engine.

1. Hydraulic oil pump
1b  Driver
1c  Intermediate flange
1d  Gasket
1e  Allen screw with special head
1f   Allen screw
1h  Gasket
1i   Idler sprocket (oil pump drive)
1k  Shaft

To dismount the pump from the engine, use a 3mm hex drive to remove the four long metric Allen-head screws (Figure 3, part 1f) that secure the pump to the engine.
The pump is flanged at the front and driven by an extension of a camshaft gear or oil pump drive as shown in figure 4, a cross-section of the pump with the various internal parts identified.



Figure 4: Cross-section of hydraulic pump indicating parts of the pump

Using disk brake spray cleaner, thoroughly clean the exterior of the pump prior to disassembling it.  Do not direct spray cleaner into the pump openings. It is recommended that the pump be dis-assembled in a clean baking pan to preclude loss of internal parts.


Figure 5: the hydraulic oil pump with the cover removed.

Remove the previously loosened short cover screws. Note the orientation of the cover with respect to suction port A and discharge port B1. Carefully pry the cover (3) from the housing (1). Remove the O-ring (12) and clean the internal surfaces of the cover and set it aside.

Caution: From this point on, do not use metal tools on internal pump parts; any gouge or scratch can compromise the operation of the pump. Instead, use wood or plastic tools or your fingers to manipulate the internal parts. Bamboo chopsticks cut or carved to shape are also useful. All parts that are cleaned need to be oiled before installing them back into the pump.

Push the eccentric shaft (4) out of the center piece (2) from the rear of the pump. Remove the bronze inner race (8) and chrome outer race (7); wipe them clean and set aside. This will allow the four pistons (5) to protrude into the cavity left by removal of the races, as illustrated in Figure 6.


Figure 6: The internal components of the Pump disassembled for illustration.

Carefully remove two opposite pistons and their springs from the center piece.  Insert the wooden dowel into the two opposing piston bores as shown in Figure 7 so that it is centered in the eccentric shaft bore.  Use pliers or small vise grips to grasp the center of the dowel and lift the center piece from the housing. Watch that the valve band (9) retaining pins don’t get lost.  Make sure the flat bearing ring stays in place at the bottom of the housing bore.


Figure 7; Removing the center piece using a wooden dowel inserted into two opposing piston bores. Make sure the flat bearing ring stays in place.

Carefully pry out the shaft seal (11) using a wooden tool like a wood kebab skewer; do not scratch the soft aluminum seal bore – the pump can leak through the slightest flaw in the seal bore.


Figure 8: Housing with a new inner O-Ring in place.

Remove the inner O-ring (10a) from the housing. Clean the housing internal bore and oil it with clean hydraulic fluid. Insert the new inner O-ring (10a) into the groove in the housing, as shown in Figure 8. Replace the O-ring (10) in the groove on the center piece.


Figure 9: Center piece from the side, showing the o-ring (black on left) in the groove just under the outside edge.

Oil the center piece outer O-ring (10), insert it into the external groove of the center piece, as shown in Figure 9, and then replace the center piece into the housing. Make sure the valve band (9) and retainer pins are in place as the center piece is slipped into the housing bore.



Figure 10: The pump with the cover still off, but with center piece, and eccentric shaft inserted.

Replace the two pistons with their springs into the center piece. Replace the chrome outer race by depressing the pistons one at a time and working the ring into the center.  Clean and oil the eccentric shaft and insert it into the outer race. Make sure the flat bearing ring is present on the eccentric shaft, as shown in Figure 10.

Oil the surface of the center piece and install new cover O-ring (12).  Install the cover on the pump noting correct orientation noted above with respect to the housing. Install the two cover screws finger-tight.

Press the new shaft seal into the seal bore – a correct size socket can be used as a mandrel to carefully press the seal into place. Do not use a hammer to drive it in! Use a bench vise with the mandrel to gently press in the lip seal.

Caution: Press in the seal no more than the lower edge of the bore chamfer – the seal will leak if it is too far into the bore.

Pour a little hydraulic oil into suction port A and rotate the shaft a few turns to lubricate the pump internals. Mount the pump to the engine using the paper gasket provided and tighten all six screws to 9 Nm. Use the new seal O-rings for the pressure hose connection.

Now it’s time to remove the oil reservoir and clean it thoroughly. Flush with hydraulic fluid, mount it back in the car and reconnect the supply and return hoses. Replace the filter inside every 30,000 miles along with a fluid flush and refill. This will assure the system’s reliable operation. You’ll need about 3 liters of Mercedes-Benz hydraulic fluid to flush and fill the system.