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.
  2.  
  3. Remove and plug the intake, outlet, and bypass lines from the pump.
  4.  
  5. Disconnect the electric leads.
  6.  
  7. Unbolt and remove the fuel pump and vibration pads.
  8.  
  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



See Figure 1

Click image to see an enlarged view

Fig. Fig. 1: View of the regulating linkage used on electronic fuel injected 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.

Cold Start Valve

  1. Disconnect the plugs from the safety switch and mixture control regulator.
  2.  
  3. Remove the cold start valve with fuel line connected.
  4.  
  5. Hold the cold start valve in a container.
  6.  
  7. Turn on the ignition. Connect the valve to battery voltage. It should emit a come shaped spray.
  8.  
  9. Dry the nozzle off. No fuel should leak out.
  10.  

Hot Start System See Figure 2

Click image to see an enlarged view

Fig. Fig. 2: Wiring schematic for the hot start system

Perform the test at coolant temperature 104--122-F.

  1. Remove the coil wire.
  2.  
  3. Connect a voltmeter to hot-start terminal 3 and ground.
  4.  
  5. Actuate the starter. In approximately 3-4 seconds, the voltmeter should read about 11 volts for 3-4 seconds.
  6.  
  7. If 11 volts are not indicated, check fuse 10. Connect the plug of the 104-F temperature switch and ground and repeat the test. If 11 volts are now indicated, replace the temperature switch. If 11 volts are not indicated, or if the time periods are wrong, replace the hot start relay.
  8.  

Fuel Pump Safety Circuit See Figure 3

Click image to see an enlarged view

Fig. Fig. 3: Wiring schematic for the fuel pump safety circuit

The pump will only run if the starter motor is actuated or if the engine is running.

  1. Remove the air filter.
  2.  
  3. Turn on the ignition and briefly depress the sensor plate.
  4.  
  5. Remove the coil wire from the distributor.
  6.  
  7. Connect a voltmeter to the positive fuel pump terminal and ground.
  8.  
  9. Actuate the starter. Voltmeter should indicate 11 volts.
  10.  
  11. If the fuel pump runs only when the sensor plate is depressed or only when the engine is cranked, replace the fuel pump relay. If the pump is already running when the ignition is turned ON, replace the safety switch.
  12.  

ADJUSTMENTS Control Linkage



1976-79 MODELS
  1. Check the control linkage for ease of operation.
  2.  
  3. Disconnect the control rod. The throttle valve should rest against the idle stop. Reconnect the control rod.
  4.  
  5. Adjust the control rod so that the roller rests tension free in the gate lever slot.
  6.  

Full Throttle Stop

1976-69 MODELS
  1. With the engine stopped, press the accelerator pedal until it rests against the kickdown switch.
  2.  
  3. The throttle valve lever should rest against the full throttle stop. If necessary, adjust the throttle valve lever.
  4.  
  5. If the full throttle stop is not reached, adjust the control rod (bell crank lever to accelerator pedal) to 4.8 in. (from center to center of ball sockets).
  6.  
  7. Adjust the accelerator pedal linkage if necessary with the fastening screw.
  8.  
  9. Adjust the control pressure rod (at idle) by compressing the adjusting clip, and moving the rod completely to the rear against the stop.
  10.  

Throttle Valve Switch

1980-84 MODELS
  1. Set an ohmmeter to 0-infinity.
  2.  
  3. Check the idle speed stop. Push the throttle valve against the idle speed stop. Connect the ohmmeter across terminals 1 and 2. Rotate the throttle valve switch until the ohmmeter reads 0.
  4.  
  5. Advance the throttle valve slightly. The ohmmeter should read 0-infinite ohms.
  6.  
  7. Check the full throttle stop. Push the throttle valve against the full throttle stop and connect an ohmmeter across terminals 2 and 3. The reading should be 0 ohms.
  8.  
  9. Turn the throttle valve back slightly. A reading of infinite ohms should result.
  10.  

 
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