Main.TrailIndexPage | Fuel System | Fuel Pump Rebuild | Early (tall) electric fuel pump rebuild

Early (Tall) Electric Fuel Pump

The W113 series of cars uses one of two types of fuel pump:

(Cautionary note and disclaimer - The reader is advised that this discussion represents the thoughts and ideas of the author only and not repair or servicing procedures authorized or approved by the pump manufacturer. The reader is further advised that the manufacturer of these pumps strongly advises the user against performing any repairs or servicing on the motor portion of the pump, including replacement of the electrical brushes. Any such repairs or servicing should only be performed by an authorized manufacturer agent.)

Contents

Pump Parts Numbers with Drawing


Early (Tall) Electric Fuel Pump

Author: - Bob Smith (aka hands_aus)

Many thanks to George Des and Naj for their guidance and contributions.

The discussion described here applies to how the author has restored to function several long body, electric Fuel Pumps used on Mercedes Benz Mechanical Fuel Injected models of the 1960s and 1970s such as the 230/250/280/300 Sl/SE.

Background Information

The pumps used on these models have several different configurations and identification numbers, but for all intents and purposes, they are basically the same in their construction and manner of operation.

The pumps are all electrically driven, 12 volt DC (direct current), low pressure pumps of the rotary vane type. They are of the Dry motor variety in that the motor portion of the pump is sealed from the vane portion of the pump and fuel does not flow through the armature/brush areas.

Sealing action is provided by a metal bellows type sliding ring (mechanical) seal. A “leak off” pipe provides an outlet for any seepage of fuel past the ring seal. The armature rides in two deep-groove ball bearings- sizes 626 and 627- and power is provided to the 12 segment brass commutator via 2 metallic graphite brushes.

Some earlier models include provisions for periodic lubrication of the top bearing via a screw type access point in the top cover of the pump. On some models a .47MFD capacitor is connected in parallel to the brushes to dampen arcing of the brushes and lessen any possible radio frequency interference. The outlet side of the pump is provided with a mechanical check valve to prevent back flow of fuel into the pump and to maintain pressure in the fuel delivery lines when the pump is not operating.

In addition, later models of the pump include a fine filter element on the inlet side to ensure a flow of clean fuel into the vane portion of the pump. In so far as the author can determine, most, if not all of the parts are fully interchangeable amongst the various model numbers.

Many of these pumps have been in continuous service for well over 35 years and this is a true testament to their fine German engineering and robust design. While mechanical wear will eventually take its toll on these pumps, perhaps larger sources of pump failures are the use of dirty fuel or prolonged periods of sitting idle while filled with fuel.

Dirty fuel has the effect of either interfering with the rotary action of the vane i.e. causing a jammed vane or preventing the main bellows seal from properly sealing the pump body from the motor section of the pump. A jammed vane will usually result in a burned out fuse, but if efforts continue to operate the pump by bypassing the fuse, the motor will eventually “burn out” through destruction of the armature or field windings.

Prolonged periods of idleness can have a similar effect on the vane if the fuel is allowed to congeal over time in the pump body. If the sealing action of the bellows seal is adversely affected by either of these situations, fuel can seep into the motor portion of the pump where it combines with the carbon brush dust to form a thick paste that eventually chokes off and destroys the bearings.

An often overlooked source of fuel seepage past the bellows seal is a small rubber o-ring that seals the top portion of the bellows against the armature shaft. The sealing action of this o-ring is most often destroyed when a user attempts to free up a jammed vane. There is a small tab washer on the top of the bellows seal that keys (locks) the bellows to the shaft and prevents the seal from spinning independently of the armature shaft. It is the spinning action of the seal against its mating portion in the pump body that provides for the primary source of sealing. If this tab is broken, the bellows is allowed to float against the small o-ring eventually causing it to wear out with attendant loss of sealing.

Removal of hoses, fuel pump protection canister (if applicable) and fuel pump

Safety: disconnect the power at the battery or remove fuse no. 4.

Make sure you pinch off the fuel line from the tank to the pump before you remove the 12mm (ID) fuel line from the Tank, or you will get a very unpleasant "shower"!

Note: The PUMP-END is concealed in the Protection Canister and is not accessible.

Remove the 9.5mm (ID) EXIT hose from the pump.

The Protection Canister is held in place by 2 nuts with washers and 1 bolt with washers.

Note: The bolt fastens into the chassis. The nuts unscrew from bolts welded to the boot floor.

Disconnect the wires and note the correct electrical connections for re-installation.

Note: If you reverse the connections the pump will run backwards.

Remove the pump from the 3 Pump-to-Chassis rubber mounts. Replace mounts if needed. MB Part # A116 988 05 11

Drain the pump and clean the exterior.

Dismantling the pump unit…..Pump end

There are 6 M4 Cheese head screws holding on the cover. They can be rusty so it is a good idea to soak them with penetrating oil of some sort and leave for a few days.

To remove the screws……

Find an un-broken flat blade screwdriver with a large handle that exactly fits the screw heads.

Note: If the screwdriver is old or worn it will round off the screw head slot.

Insert the screwdriver in the slot and GENTLY hit it 3 or 4 times. (This will break any metal-to-metal growth). Then lean on and turn the screwdriver at the same time. The screws should undo.

There is an O-Ring Seal (# BS 039.N70) under the pump end cover. MB Part # A004 997 04 45 up to 113044 no. 010241

Of course if they break then the job becomes more involved, see below

Possible problem area

This is usually the first place where difficulty is encountered in the disassembly since the screws often times become “weld rusted” to the body and the heads torque off when an attempt is made to remove them. If one or two screws torque off, it usually presents no great problem since the plate can often times be carefully levered off using a disposable blade from a utility knife. This will expose the studs which can then be removed by judicious application of heat from a small oxy-propane torch and use of a set of vise grips.

Note: This is extremely dangerous work and suitable eye protection must be worn while using the blades.

Note: It is essential that the pump be purged of all gasoline and fumes before any open flame source is brought near the pump.

The Bottom cover can still be removed using this method even if all of the heads are torqued off—it just takes a little more time, ingenuity and work. Use a small hammer to force wedge a blade between the plates and the pump body and it sometimes takes “stacking” two blades side by side to provide sufficient levering action. This is done after the first blade provides a crevice which can be further widened by the second blade.

Note: Great care is required to ensure that no damage is caused to the pump body or parts being removed.

Inside the Pump Housing…..

Have a look at the bronze fan-like Impeller and notice a small square section cut out near the centre of the shaft. Impellers may also be made of steel.

Notice the scoring on the impeller and pump end cover. Shallow scoring can be removed by using fine to medium wet and dry paper on a flat glass plate. Result:

When you remove the Impeller there will be a very small woodruff key (made of hardened steel 2 x 1.5 x 1.5mm) in that square cut out. Keep that woodruff key very safe. There is a small flat area ground on the shaft that locates the Woodruff Key and when assembled it stops the impeller from spinning on the shaft.

As you remove the impeller, make a note of its orientation on the shaft. Mark it somehow. If you put it back the wrong way the pump will not work correctly.

Pump housing internal seal operation

To seal the shaft to the pump housing there is a black plastic-looking insert with what looks to be a neoprene collar that fits into the opening of the housing.

It has a flat surface that matches a flat surface on the Bellows seal. Description below

The Bellows Seal is called a “Mechanical Seal” or “Slide Ring Seal” (from MB Service Manual).

The Impeller conceals the Bellows Mechanical Seal …See pictures.

The Bellows Mechanical Seal is held on the shaft by a small C-Clip that is very hard to remove. I used lots of patience and an O-Ring hook.

Under the C-Clip is a black steel Locating Washer with 3 locating tabs on it. 2 are external and the third tab is on the inner hole circumference. The shaft has a groove machined in it with a tapered end. The internal Tab of the Locating Washer has the same taper and hence it fits on the shaft “only one way”. Be careful to use straight pulling action, otherwise, the small tab on the Locating washer will be broken off by any twisting action.

Note: record the orientation of this washer as you remove it.

Remove the washer. Next remove the Bellows Seal by pulling it. Take great care to ensure no damage occurs to the bellows itself since it must be re-used.

Note: This is a critical part and the only known source of a replacement is a donor pump!

A small O-Ring seals the Bellows onto the shaft. It can be replaced with one sized 5mm ID x 1mm.

Alternately a slightly larger O-Ring 5mm x 1.5mm can be fitted to the shaft and then 'sized' to allow the bellows seal to just fit over it.

Note: If this O-Ring is old or broken your pump will leak fuel out the overflow spout on the back of the pump.

Description of Operation

The Bellows Seal has what looks like a brass spring on the outside and on the inside it is impervious/sealed.

To test it, put one end to your mouth and cover the other end with your finger, blow into it. No air is allowed to escape. If there is a leak in the Bellows Seal, you can re-seal it by covering the outside of it with POR-15 or soldering the hole closed, or using a flexible, fuel-resistive sealer.

Note: in the following description top or bottom refers to the orientation of the pump when mounted in the car. The pump is mounted vertically with the pump chamber at the bottom.

The bottom collar of the Bellows Seal (the end with the cut out for the locating washer) has a smaller diameter hole than the top collar with the flat surface. This difference in size allows the top collar to fit over the small O-Ring on the shaft. The inside of the bottom collar makes a seal with the O-Ring.

Note: The Bellows Seal sits on the O-Ring and rotates with the shaft held in position by the locating washer.

Once the O-Ring has sealed the shaft, the petrol enters the top because the Bellows Seal does not actually touch the black plastic fitted to the pump housing. The small gap allows enough fuel to enter the Bellows Seal around the shaft. When the space is full (as long as there are no leaks) no other fuel can enter and hence it forms a FLUID Seal. If that O-Ring or Bellows Seal is leaking, petrol will leak into the small chamber above the Bellows Seal and out the spout on the back of the pump.

Dismantling the pump, motor end

There is an End Cap held in place by two nuts with washers. Remove the nuts and washers and then the end cap can be loosened with a flat blade screwdriver. The End Cap is sealed to the body with an O-Ring (Part #: BS 034.N70, MB part # A 000 997 91 45).

When you remove the End Cap you will see the Brush Mount Pad

The Brush Mount Pad is held in place by two small springs that fit over the removable studs. There is a small O-Ring (# BS 007) on top of each spring. The End Cap seals on these O-Rings. Take off the O-Rings and Springs. Unscrew and remove the Threaded Metal Studs.

The Brushes can be inspected by lifting the Brush Mounting plate and if they need to be replaced, they can now be removed along with their springs with the help of a de-soldering iron. Brushes MB part # A000 548 02 42.

The springs are re-used unless damaged.

On some models a .47MFD capacitor is connected in parallel to the brushes to dampen arcing of the brushes and lessen any possible radio frequency interference. (Picture below)

Separating the motor body from the pump

Between the body of the pump and the motor you will see a fine line where they join. Put a locating mark on the pump housing and the motor body. This will help in correct re-assembly. Also you will see 3 flat screwdriver size marks. If you insert that good screwdriver in each of the marks and twist, the pump body and electric motor body will separate.

Note: they are sealed with another O-Ring (Part # BS 034.N70, MB part # A 000 997 91 45).

The Motor Body should lift off the Armature quite easily. The Armature shaft is attached to the Pump Housing by a plate with 2 M4x6mm screws with washers. Take out the screws and the Armature will come out.

The armature assembly parts consists of

  • armature/commutator section
  • two deep groove ball bearings
  • a small roller
  • an oblong mounting plate
  • a small ring shim

If the commutator is too worn or the armature windings are burned out, the armature is sent out for a re-wind and commutator replacement.

All parts with the exception of the bearings are used on re-assembly.

At this point you can decide if you need new bearings.

Bearing replacement

A suitable arbor press is used to press off the top bearing and companion roller/spacer. The same procedure is used to remove the larger bearing on the bottom portion of the armature.

The bearings are standard sizes, cheap and readily available.

  • 626 small, brush end
  • 627 larger, pump end

Note:

  • On the Brush End there is a Solid Steel Washer with 4 raised marks. These marks actually separate the washer and bearing by about the thickness of a piece of paper.
  • On the Pump End there is a Steel cup shaped Washer. This washer fits between the bearing and the pump housing.
  • Record the orientation of the Solid Steel Washer for re-installation.

Washer viewed from both sides, Note above.

Inlet and outlet connection, seals, filter screen… description and cleaning etc.

The inlet and outlet fittings are then removed using a suitably sized socket. The copper sealing rings under the fittings are disposed of since they will be replaced with new seal rings.

Alternatively

The inlet connection which has a fine mesh Filter Screen screwed to it can be cleaned without having to remove the housing from the Pump Body by using an artists brush dipped in Kerosene. Keep cleaning it until there are no particles coming out on the brush. Check the inside of the filter in the sunlight.

Note: This filter can become very badly clogged with particles from the tank and fuel line. The outlet connection has a non-return valve screwed to it. See picture below. You can test it by trying to blow into it. It should not allow any air to pass through.

If you need to clean the inside of the pump even more, remove the covers but realize that you will break your existing non-leaking seals.

Each connection is held on by 4 M4 Cheese head screws and has a tapered shape that fits into a similarly tapered shape on the pump body. You can buy O-Rings 36 x 1.5 mm that will fit with a small stretch over the taper. I couldn’t find an MB part #. When re-installing the connections, the tapered shape will push the O-Ring into the pump body to form a good seal.

Possible problem area.

This is usually the first place where difficulty is encountered in the disassembly since the screws often times become “weld rusted” to the body and the heads torque off when an attempt is made to remove them. If one or two screws torque off, it usually presents no great problem since the plate can often times be carefully levered off using a disposable blade from a utility knife. This will expose the studs which can then be removed by judicious application of heat from a small oxy-propane torch and use of a set of vise grips.

Note: This is extremely dangerous work and suitable eye protection is worn while using the blades.

Note: It is essential that the pump be purged of all gasoline and fumes before any open flame source is brought near the pump.

The plates can still be removed using this method even if all of the heads are torqued off—it just takes a little more time, ingenuity and work. Use a small hammer to force wedge a blade between the plates and the pump body and it sometimes takes “stacking” two blades side by side to provide sufficient levering action. This is done after the first blade provides a crevice which can be further widened by the second blade.

Note: Great care is required to ensure that no damage is caused to the pump body or parts being removed.

Clean Up of pump body

This essentially completes the disassembly process and at this point, all parts are given a thorough cleaning. With the exception of the armature, the parts are dipped in carburetor cleaner to remove all fuel residue, grease, carbon dust, etc.

Care must be taken to carefully clean the synthetic seal face in the recess of the pump body without scoring it. This can usually be cleaned up sufficiently with a Q-tip dipped in a suitable solvent such as paint thinner.

If required, the metal parts can be carefully bead blasted (remembering to protect the seal face) to remove all rust and oxidation. The “can” portion can now be painted.

Re-assembly

Pump end

With both new bearings on the shaft, add the Cup Shaped Washer to the Pump end. On the Pump Housing, insert the shaft in the hole and screw on the cover. (two screws and washers)

Add in this order

  1. 5mm x 1.0mm O-Ring that fits on the shaft OR the alternate 'sized' 5mm x 1.5mm O-Ring
  2. Bellows seal
  3. Locating washer (note the correct orientation)
  4. C-clip
  5. Woodruff key on the flat surface of the shaft
  6. Impeller (note the correct orientation)
  7. O-ring for the end cover
  8. End cover and screws.

Motor end

Add in this order

  1. The new O-RING to the Pump Body
  2. Screw in the threaded shafts
  3. Fit the motor body over the Threaded Shafts

Note: Make sure it has the correct orientation for mounting under the car. Find the mark you made earlier.

Gently tap the motor body until the two parts come together. Make sure the brushes are sitting evenly on the armature. You may have to hold the brushes back against their springs to get them onto the armature.

Put the small springs on the threaded shafts. Then put the small O-Rings on the threaded shafts. Fit a new O-Ring to the end cap of the motor. Fit the end cap of the motor on the threaded shafts and bearing. Add the nuts and washers and gently give it a tap. Tighten the nuts.

Note: The whole motor body and pump body will be pulled together seating the bearings.

Make sure the shaft spins freely.

Inlet / Outlet Connection

Fit new O-Rings and screw (4 screws and washers each) the correct cover to the pump housing openings. The pressure of the outer housing should push the O-Ring into the correct shape and form the seal.

Note: The inlet connection (12mm) is on the right and the outlet connection (9.5 mm right angled pipe) is on the left.

Testing the unit

Connect Fuel hoses Insert the hoses into petrol or kerosene or diesel, not water. Connect 12V to the terminals in the correct order. Switch on.

Note: The pump is self-priming and should start to flow in a very short time.

Installation to the car is a reverse procedure to removal.

Specifications

This info is from "Service Manual - Maintenance, Tuning, and Unit Replacement - Passenger Cars starting August 1959".

The Delivery Pressure with the ignition on (minimum terminal voltage at the pump of 11 volts) at a point "behind the fuel filter" should be 0.8 to 1.1 Atm.

Discharge pressure (with a dummy plug on the fuel line) behind the fuel overflow valve on the injection pump should be a minimum of 1.3 Atm. The output rate at this same location should be 1 litre in a minimum of 15 seconds.


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