Mercedes Coupes/Sedans/Wagons 1974-1984 Repair Guide

1977 Vehicles

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The following tests should be performed in the order given with the engine at normal operating temperature.

230 MODELS Testing the System



See Figure 1

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Fig. Fig. 1: Emission control component locations1977-79 230 models

EGR VALVE
  1. Run the engine at idle speed.
  2.  
  3. Remove the brown vacuum line at the carburetor and the gray vacuum line at the intake manifold. Connect the brown line at the intake manifold.
  4.  
  5. The engine should run roughly or stall. If the rpm does not change, check the vacuum line connections. Check for leaks and blow through the vacuum connection on the carburetor.
  6.  
  7. Check the thermo-vacuum valve (blue plastic portion and "50AB5" stamped in metal housing). Remove the brown/purple vacuum line and race the engine. Vacuum must be present at the open connection when accelerating.
  8.  
  9. Check the EGR valve. Remove the EGR valve. Connect the brown/purple vacuum line to the EGR valve and slowly increase engine rpm. Cover the bores in the intake manifold. The valve stem should lift from its seat; if not replace the EGR valve with a new one.
  10.  

AIR INJECTION
  1. Connect a CO tester to the test connection and remove the vacuum hose from the vertical connection of the thermo-vacuum valve. The air injection thermo-vacuum valve has "50AA13" stamped in the metal housing. Plug the connection on the valve. The CO reading should drop noticeably.
  2.  
  3. If the CO does not drop, and vacuum IS indicated, replace the thermo-vacuum valve. If the CO does not drop and vacuum is NOT indicated, clean the vacuum line to the intake manifold with compressed air.
  4.  
  5. Check the connection of the vacuum lines.
  6.  
  7. Check the vertical connection of the thermal vacuum valve for vacuum. If vacuum is present, replace the diverter valve. If no vacuum is present, remove the blue vacuum line from the thermo-vacuum valve and check for vacuum at the line.
  8.  
  9. If the CO still does not decrease, check that the thermo-vacuum valve is open; if not, replace the valve.
  10.  

Below approximately 120-F, the valve should be open; above that, it should be closed.

THROTTLE VALVE LIFT
  1. Remove the vacuum hose from the vacuum governor on the carburetor. The idle speed should increase. Reconnect the hose. The idle speed should decrease.
  2.  
  3. If the rpm does not increase, check the connection of the hose. Check the hose for leaks.
  4.  
  5. Run the engine at idle. Remove the vacuum line on the carburetor; the idle speed should increase. If not, replace the vacuum governor.
  6.  

FUEL EVAPORATION CONTROL SYSTEM
  1. Remove the solenoid plug from the float chamber vent valve. Reconnect the solenoid; it should click audibly.
  2.  
  3. If the solenoid does not click, turn on the ignition and connect a test lamp to the plug. The test lamp should light with the ignition ON. If not, check the proper fuse. If the lamp still does not light, replace the solenoid.
  4.  
  5. Remove the middle purge hose to the carburetor from the charcoal canister and cover the hose opening with your finger. Slowly increase engine speed to over 2000 rpm. At idle no vacuum should be present. As engine speed increases, vacuum should increase. If no vacuum is present as the engine speed increases, check the connection of the purge hose. Check the hose for leaks and clean it out with compressed air.
  6.  
  7. On 1978-79 models remove the hose from the purge valve and repeat the test. If vacuum is present, replace the purge valve.
  8.  

280E & 280SE MODELS (FEDERAL)Testing the System



See Figure 2

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Fig. Fig. 2: Emission control component locations1977-79 280E, CE and SE models (Federal)

The following tests should be performed with the engine at idle speed, at operating temperature and in the order listed.

EGR VALVE
  1. Remove the brown vacuum line from the EGR valve and slowly increase engine rpm. At about 1200 rpm, the engine should run roughly or stall.
  2.  
  3. If it does not run roughly or stall, check the vacuum line connections. The connections at the exhaust pressure transducer are marked with colored rings and must be connected to the same color code.
  4.  
  5. Disconnect the vacuum line from the vertical connection of the thermo-vacuum valve marked "50AA4" in the metal housing. Run the engine and increase rpm. Vacuum should be present at the connection.
  6.  
  7. Run the engine at idle and disconnect the brown line between the EGR valve and exhaust pressure transducer. Cover the line with your finger; vacuum must be present at idle. If not, replace the exhaust pressure transducer.
  8.  
  9. Run the engine at idle and remove both vacuum lines from the EGR valve. Connect the brown line to the connection on the red/purple line on the EGR valve. The engine should run roughly or stall. If not, replace the EGR valve.
  10.  

AIR INJECTION
  1. Connect a CO tester to the exhaust back pressure line. Remove and plug the vacuum hose from the vertical connection at the thermo-vacuum valve. The CO should increase.
  2.  
  3. If not, check the vacuum line connections. The (large) cap end connection of the check valve must face the intake manifold (Federal only).
  4.  
  5. Remove the blue vacuum line from the thermo-vacuum valve and cover it with your finger. Vacuum must be present at idle; if not, check for leaks and clean the line with compressed air.
  6.  
  7. If vacuum is present, check the thermo-vacuum valve and replace if necessary.
  8.  
  9. Disconnect the blue/purple line from the thermo-vacuum valve marked "50AB5" on the metal housing. Run the engine. Vacuum should be present at the vertical connection. If not, replace the diverter valve.
  10.  

FUEL EVAPORATION CONTROL SYSTEM
  1. Remove the black purge line from the charcoal canister. Cover the opening with your finger and increase rpm to over 2000 rpm. No vacuum should be present at idle and vacuum should increase with engine speed. If not, check the vacuum line connections, check for leaks and clean the line with compressed air.
  2.  

280E & 280SE MODELS (CALIFORNIA)Testing the System



See Figure 3

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Fig. Fig. 3: Emission control component locations1977-79 280E, CE and SE models (California)

EGR VALVE

Refer to the 280E and 280SE Federal models procedure.

AIR INJECTION
  1. Connect a CO tester to the exhaust pressure transducer. Remove the vacuum line from the vertical connection of the 122-F thermo-vacuum valve and connect it to the vertical connection of the 62-F thermo-vacuum valve. The CO should drop.
  2.  
  3. If not, check the vacuum line connections.
  4.  
  5. Run the engine at idle and check for vacuum at the vertical connection of the 62-F thermo-vacuum valve. If vacuum is present, replace the diverter valve.
  6.  

If no vacuum is present, remove the blue line from the 62-F thermo-vacuum valve and check for vacuum at the valve. If vacuum is present, replace the thermo-vacuum valve. If no vacuum is present, remove the line from the intake manifold and clean it with compressed air.

  1. If the CO still does not drop, check the 122-F thermo-vacuum valve. It will have "50AA13" stamped in the metal housing. Above 122-F, the valve is closed and no vacuum should be present at the vertical connection. Below 122-F, vacuum should be present at the vertical connection. If these conditions are not met, replace the valve.
  2.  

FUEL EVAPORATION CONTROL SYSTEM

Refer to the 280E and 280SE Federal models procedure.

450SEL, 450SL & 450SLC MODELS (FEDERAL)Testing the System



See Figure 4

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Fig. Fig. 4: Emission control component locations1977-80 4.5L V8 models (Federal)

The following tests should be performed with the engine running at normal operating temperature in the order given.

EGR VALVE
  1. Remove the brown line at the EGR valve and slowly increase engine speed. Above 1200 rpm, the engine should run roughly or stall. If not check the vacuum line connections.
  2.  
  3. Check the thermo vacuum valve with "50AA4" stamped in the metal housing (104-F valve). Remove the red/purple line from the vertical connection, run the engine and accelerate briefly. Vacuum should be present at the vertical connection.
  4.  
  5. Run the engine at idle and remove the brown vacuum line at the EGR valve. If no vacuum is present at idle, replace the exhaust pressure transducer.
  6.  
  7. Run the engine at idle and remove both lines from the EGR valve. Connect the brown line to the connection for the red/purple line. The engine should run roughly or stall. If not, replace the EGR valve.
  8.  

AIR INJECTION
  1. Remove the suction hose from the aspirator/check valve in the air cleaner and cover with your finger. Vacuum should be present and a suction noise should be audible. If no vacuum is present, replace the aspirator/check valve.
  2.  

FUEL EVAPORATION CONTROL SYSTEM

Refer to the 280E and 280SE Federal models procedure.

450SEL, 450SL, 450SLC (CALIFORNIA) & 6.9 MODELS Testing the System



See Figures 5 and 6

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Fig. Fig. 5: Emission control component locations1977-80 4.5L V8 models (California)



Click image to see an enlarged view

Fig. Fig. 6: Emission control component locations1978 6.9 models

EGR VALVE

Refer to the 450SEL, 450SL and 450 SLC Federal models procedure.

AIR INJECTION
  1. Connect a CO tester to the exhaust gas back pressure line and remove the blue/purple vacuum line from the vertical connection of the 62-F thermo-vacuum valve. Plug the connection. The CO should increase.
  2.  
  3. If not, check the vacuum line connections. The check valve must be installed with the larger (cap) end toward the intake manifold.
  4.  
  5. Remove the vacuum line from the angular connection of the 62-F thermo-vacuum valve and cover the valve with your finger. If no vacuum is present at idle, check the lines for leaks and clean the vacuum pick-up bore with compressed air.
  6.  
  7. If vacuum is present, check the thermo-vacuum valve and replace if necessary.
  8.  
  9. The thermo-vacuum valve can be identified by the "50AB5" stamped in the metal body. Remove the blue/purple vacuum line, run the engine at idle and accelerate briefly. Vacuum should be present at the vertical connection; if not, replace the diverter valve.
  10.  

FUEL EVAPORATION CONTROL SYSTEM

Refer to this test under 280E and 280SE (Federal).

 
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