Mercedes Coupes/Sedans/Wagons 1974-1984 Repair Guide

Electric Fuel Pump

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Do not confuse the electric fuel pump with the injection pump.

All Mercedes-Benz fuel injected engines are equipped with electric fuel pumps. The electric fuel pump is located underneath the rear floor panel. The fuel return line was also eliminated and a check ball installed in its place.

Two types of fuel pumps have been used. One, the large pump, has been replaced with a new small design which has a bypass system to prevent vapor lock.

REMOVAL & INSTALLATION



  1. Jack the left rear of the car and support it on jackstands. This will provide sufficient working clearance.
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  3. Remove and plug the intake, outlet, and bypass lines from the pump.
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  5. Disconnect the electric leads.
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  7. Unbolt and remove the fuel pump and vibration pads.
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  9. Install the fuel pump in the reverse order of removal. Be sure that the electric leads are connected to the proper terminals. The negative wire (brown) is connected to the negative terminal (brown plastic plate) and the positive wire (black/red) is connected to the positive terminal (red plastic plate). If the terminals are reversed, the pump will operate in the reverse direction of normal rotation and will deliver no fuel.
  10.  

TESTING Fuel Pump Pressure



1974-75 MODELS

Temporarily reduce the pressure in the ring line by unplugging the connection at the starting valve. Connect the terminals of the starting valve to the battery for approximately 20 seconds. Reconnect the starting valve.

  1. Remove the air filter and connect a pressure gauge at the branch connection at the ring line.
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  3. Run the engine and measure the pressure. It should be 26.5-29.5 psi.
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  5. Stop the engine. The fuel pressure may drop to 21 psi, after approximately 5 minutes. If the fuel pressure drops uniformly to 0, check the following points for leaks.
  6.  

Starting valve Switch on ignition and disconnect hose at starting valve. If there is no drop in pressure, the valve leaks.

Pressure regulatorSwitch on the ignition and disconnect the fuel return hose, as soon as the fuel pump stops. If there is no drop in pressure, the regulator leaks.

Ball valve in delivery connection of fuel pumpSwitch on the ignition and disconnect the fuel hose in front of the ring line the moment the fuel pump stops. If there is no drop in pressure, replace the fuel pump.

Injection Valvesremove the kick panel and bridge terminals 1 and 3 on the relay shown. This will energize the fuel pump with the engine stopped, and ignition ON. Check the valves for leaks.

  1. Before removing the gauge, reduce the pressure in the ring line.
  2.  

The fuel pressure can be adjusted on the regulator with the adjusting screw. Adjust to 28 psi. If a slight turn of the screw shows no change of pressure replace the regulator.

1976 - 84 MODELS

Remove the fuel return hose from the fuel distributor. Connect a fuel line and hold the end in a measuring cup. Disconnect the plug for the safety switch on the mixture regulator and turn on the ignition for 30 seconds. If the delivery rate is less than 1 liter in 30 seconds, check the voltage at the fuel pump (11.5) and the fuel lines for kinks.

Disconnect the leak off line between the fuel accumulator and the suction damper. Check the delivery rate again. If it is low replace the accumulator.

Replace the fuel filter and test again. If still low, replace the fuel pump.

 
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