Mercedes Coupes/Sedans/Wagons 1974-1984 Repair Guide

1980 Vehicles

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6-CYLINDER ENGINES



The base color of the emission control vacuum lines is white. Colored stripes identify various functions:



Advanced timing = red
 
Retarded timing = yellow/purple
 
Air injection = blue
 

A Lambda oxygen sensor control system ensures a constant air fuel ratio of approximately 14.5:1. The oxygen sensor is screwed into the front part of the exhaust pipe to constantly monitor the oxygen content of the exhaust gases. An electronic control unit, located behind the kick panel, receives input from a throttle valve switch and oil temperature switch to maintain an ideal fuel mixture in conjunction with the 3-way catalyst. The oxygen sensor must be replaced every 30,000 miles (light on the dash warns driver).

An air injection system is used, the components of which are very similar to those used in 1979.

Testing the System

A special test adaptor that connects to the electronic control unit plug and a special test meter that connects to the adaptor are necessary to properly test the emission system. Do not attempt to try and test electrical components of the system with an ordinary volt-ohmmeter.

The only tests that can be performed without special equipment are as follows:

AIR INJECTION
  1. Pull the Y-fitting from the angled connections at the thermo-vacuum valves and check for vacuum at the Y-fitting. If no vacuum is present, clean the connection at the intake manifold with compressed air.
  2.  
  3. Be sure the fitting and connecting lines are not plugged. If vacuum is present, the thermo-vacuum valves should be open. If the valves are open, replace the diverter valve.
  4.  

FUEL EVAPORATION CONTROL SYSTEM
  1. Remove the purge line from the charcoal canister (connected to the throttle valve housing) and block it with your finger. Slowly increase the engine speed to more than 2000 rpm.
  2.  
  3. There should be no vacuum at idle, but vacuum should increase with rpm.
  4.  
  5. If vacuum does not increase with rpm, test the purge line connection and purge valve. The purge line must be connected to the throttle valve housing, and must not leak. Clean out the throttle valve housing with compressed air. If vacuum is still not present, remove the purge line from the front of the purge valve. If vacuum is present, replace the purge valve.
  6.  

V8 ENGINES



See Figures 1 and 2

Click image to see an enlarged view

Fig. Fig. 1: Emission control component locations1977-80 4.5L V8 models (Federal)



Click image to see an enlarged view

Fig. Fig. 2: Emission control component locations1977-80 4.5L V8 models (California)

Color coding of the vacuum lines is identical to the 6-cylinder engines and operation of the Lambda oxygen sensor control system is the same as 6-cylinder engines.

Air injection uses a shut-off valve in a special shaped hose between the air filter and the aspirator valve, which is in the air injection line leading to the cylinder head.

Primary, underfloor and catalyst/muffler combination catalytic converters are used, depending on application.

The fuel evaporation control system is identical to the 1979 system.

Testing the System

A special test adapter that connects to the electronic control unit plug and a special test meter that connects to the adapter are necessary to properly test the emission control system. Do not attempt to try and test electrical components of the system with an ordinary volt-ohmmeter.

The only tests that can be performed without special equipment are as follows:

FREQUENCY VALVE
  1. With the engine idling at normal operating temperature, place your hand on the frequency valve.
  2.  
  3. Operation of the frequency valve can be felt. If not replace the valve.
  4.  

AIR INJECTION
  1. Idle the engine and remove the specially shaped hose from the air shut-off valve. A suction sound should be audible.
  2.  
  3. If there is no suction sound, check the vacuum lines and vacuum supply. Check the blue line connected to the air shut-off valve. Disconnect the line at the air shut-off valve. If vacuum is not present, clean out the vacuum connection at the throttle valve housing.
  4.  
  5. If vacuum is present at the air shut-off valve, remove the valve. If the suction sound is still audible, replace the air shutoff valve. If no suction sound could be heard at the air shut-off valve, replace the aspirator valve.
  6.  

 
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