joe prosser
Member
Before Christmas an intermittent whine started at rear of the car. Reaching under the offside rear wheel I could tap the rear fuel pump and the sound would respond accordingly. The weather being atrocious put me off fixing things immediately. The whine got worse and eventually the engine had issues starting.
The fuel supply system on the 93 - 96 W124 estates consists of two identical pumps (Bosch 058 025 4950, mercedes part 0020915901) fitted in tandem. The pump that failed was the one immediately next to the fuel tank that hangs down on rubber collars. I replaced the pump with a new Bosch unit; an expensive choice but I figure if I am going to do a job I want some reassurance that I am fitting the best parts. I also decided to check on the causes of failure in the replaced pump.
I cut open the aluminium pump casing with a Dremel tool to reveal a fairly standard electric motor arrangement of armature and brushes. Fitted to one end is a precision engineered pump assembly consisting of an outer circle of hardened steel and an inner solid circle with 5 evenly spaced cut-outs in its periphery. This inner circle is offset so that one side almost touches the outer ring and on the opposite side there is gap about 3mm wide. Within each of the cut-outs is a small cylinder of steel (roller bearing). As the inner circle rotates, driven by the motor, these rollers are flung out to roll around the inner face of the outer ring. A slot in the back face of the assembly allows fuel to drawn in as the gap widens with rotation and as the gap narrows again the fuel is pushed through a hole in the front face. Having not seen this before I was impressed by the very elegant design and beautiful engineering. My pump failed as dirt or rust from the tank had caused some damage to occur to the inner face of the outer hardened steel ring. As time went on this damaged area was eroded further by the passing of each roller. Eventually the recess in the outer ring was so deep that the rollers had difficulty getting passed it hindering the pump motor from turning.
The lesson is that when you hear a fuel pump complaining replace it sooner rather than later other wise it will just pack up, often at the most inconvenient time.
Pump specifications:-
Inlet 15mm (rubber fuel pipe from tank held in place by jubilee clip)
Outlet 12mm banjo union 1.5 mm thread
3 bar normal pressure (rated 6 bar relief valve...spring loaded ball bearing)
2.7 litres per minute
53mm Body diameter
198mm pump overall length.
The fuel supply system on the 93 - 96 W124 estates consists of two identical pumps (Bosch 058 025 4950, mercedes part 0020915901) fitted in tandem. The pump that failed was the one immediately next to the fuel tank that hangs down on rubber collars. I replaced the pump with a new Bosch unit; an expensive choice but I figure if I am going to do a job I want some reassurance that I am fitting the best parts. I also decided to check on the causes of failure in the replaced pump.
I cut open the aluminium pump casing with a Dremel tool to reveal a fairly standard electric motor arrangement of armature and brushes. Fitted to one end is a precision engineered pump assembly consisting of an outer circle of hardened steel and an inner solid circle with 5 evenly spaced cut-outs in its periphery. This inner circle is offset so that one side almost touches the outer ring and on the opposite side there is gap about 3mm wide. Within each of the cut-outs is a small cylinder of steel (roller bearing). As the inner circle rotates, driven by the motor, these rollers are flung out to roll around the inner face of the outer ring. A slot in the back face of the assembly allows fuel to drawn in as the gap widens with rotation and as the gap narrows again the fuel is pushed through a hole in the front face. Having not seen this before I was impressed by the very elegant design and beautiful engineering. My pump failed as dirt or rust from the tank had caused some damage to occur to the inner face of the outer hardened steel ring. As time went on this damaged area was eroded further by the passing of each roller. Eventually the recess in the outer ring was so deep that the rollers had difficulty getting passed it hindering the pump motor from turning.
The lesson is that when you hear a fuel pump complaining replace it sooner rather than later other wise it will just pack up, often at the most inconvenient time.
Pump specifications:-
Inlet 15mm (rubber fuel pipe from tank held in place by jubilee clip)
Outlet 12mm banjo union 1.5 mm thread
3 bar normal pressure (rated 6 bar relief valve...spring loaded ball bearing)
2.7 litres per minute
53mm Body diameter
198mm pump overall length.
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