Mercedes Coupes/Sedans/Wagons 1974-1984 Repair Guide

1974 Vehicles

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Design and function of the system remains basically unchanged from 1973, except for the addition of an air pump on 280, 280C and 450 California models.

It is impossible to list test procedures for all the various switches and valves in this guide, so only basic tests and results appear here.

The following tests should be performed on a warm engine at normal operating temperature, and should be performed in the sequence listed. Be sure to check the fuses if a malfunction is suspected.

230 MODELS



See Figure 1

Click image to see an enlarged view

Fig. Fig. 1: Emission control component locations1974 230 models

Ignition changeover is accomplished through vacuum and oil temperature. Vacuum retard is only activated during acceleration, while vacuum advance is activated under the following conditions:



Oil temperature below 77-F.
 
Oil temperature above 77-F and engine speed above 2000 rpm.
 

The throttle valve is also opened slightly during coasting, through a vacuum governor on the carburetor.

Exhaust gas is being recycled by the EGR valve under the following condition:



Oil temperature above 77-F up to 3600 rpm.
 

Exhaust gas recirculation is not effective under the following conditions:



Oil temperature above 77-F up to 3600.
 
Speed above 3600 rpm.
 

Testing the System

77-F TEMPERATURE SWITCH
  1. Disconnect the plug to the temperature switch in the oil filter housing and ground it. The engine rpm should increase, indicating that vacuum advance is present.
  2.  
  3. Increase the engine speed to about 2500 rpm and remove the red vacuum line at the distributor. The engine rpm should drop slightly, indicating that vacuum advance is no longer present.
  4.  

If the results are not as specified, check the connections of the vacuum lines. The blue line from the carburetor should go to the center port of the red switchover valve and the red line should go from the outer port of the switchover valve to the distributor.

EGR SWITCHOVER VALVE
  1. Place your hand over the brown EGR switchover valve. It may be necessary to remove the valve to isolate its operation. Increase the engine speed. The valve should be felt to switch.
  2.  
  3. If it does not function, check the voltage at the plug. If no voltage is measured at the plug, below 3600 rpm, replace the rpm relay.
  4.  

EGR VALVE
  1. Connect the EGR valve directly to intake manifold vacuum with the blue vacuum line. The engine should run poorly or stall, indicating that the valve is open.
  2.  
  3. If the speed does not change, remove the valve and connect it to vacuum. The valve stem should lift from its seat. Remove exhaust deposits from the valve with a 10mm drill and blow it clean with compressed air.
  4.  

VACUUM GOVERNOR
  1. Connect a tachometer and increase engine speed to about 2500 rpm and release the throttle slowly. The vacuum governor should pop out above 2000 rpm and retract below 1800 rpm.
  2.  

If not, check the vacuum lines. The blue line should connect the center port of the gray switchover valve. The gray line should connect the vacuum governor and the outer port of the switchover valve.

  1. If the vacuum lines are connected properly, remove the relay box plug and connect terminals 2 and 8. With the ignition ON, the switchover valve should click. If the switchover valve functions properly, the relay box is defective.
  2.  

280 & 280C MODELS (FEDERAL)



See Figure 2

Click image to see an enlarged view

Fig. Fig. 2: Emission control component locations1974 280 and 280C models (Federal)

An ignition changeover is installed to retard or advance the ignition. Ignition is retarded under the following conditions:

  1. When the oil temperature is above 62-F and coolant temperature is below 212-F.
  2.  
  3. Engine speed is below 3200 rpm.
  4.  

Ignition retard is negated under the following conditions:



Oil temperature below 62-F.
 
Coolant temperature above 212-F.
 
Engine speed above 3200 rpm and oil temperature above 62-F, and coolant temperature below 212-F.
 
When shifting into fourth gear.
 
When switching on the air conditioner.
 
With vacuum between 0 and 2.8 psi.
 

A throttle positioner is installed which will open the throttle slightly when the oil temperature is above 62-F, when the coolant temperature is below 212-F and when engine speed exceeds 200 rpm.

Exhaust gases are recycled when engine oil temperature is above 62-F, when coolant temperature is below 212-F and when manifold vacuum is between 0 and 2.8 psi, up to 3200 rpm.

Testing the System

IGNITION TIMING
  1. Check the ignition timing. It should be as specified.
  2.  
  3. If not, check all vacuum connections and the temperature switches before adjusting the timing.
  4.  

62-F TEMPERATURE SWITCH
  1. Disconnect the plug of the relay box.
  2.  
  3. Connect a voltmeter to terminals 5 and 8.
  4.  
  5. The voltmeter should indicate 0 volts when the oil temperature is above 62-F.
  6.  

212-F TEMPERATURE SWITCH
  1. Disconnect the plug from the relay box.
  2.  
  3. Connect a test lamp to terminals 4 and 8.
  4.  
  5. Switch on the ignition.
  6.  
  7. The test lamp should light when coolant temperature is above 212-F.
  8.  

THROTTLE POSITIONER
  1. Connect a tachometer to the engine.
  2.  
  3. Start the engine and increase the speed to approximately 2500 rpm.
  4.  
  5. Release the accelerator linkage and observe the tachometer. At speeds above 1800 rpm, the adjusting screw should rest against the actuating lever. At speeds below 1800 rpm, the adjusting screw should be off the actuating lever.
  6.  

RPM SWITCH

Use only a voltmeter to test the rpm switch.

  1. Disconnect the plug of the switch valve and connect a voltmeter.
  2.  
  3. Start the engine and increase speed.
  4.  
  5. The voltmeter should indicate about 13 volts, above 2000 rpm.
  6.  
  7. Decrease speed below about 1800 rpm and the voltmeter should read approximately 0 volts.
  8.  

EGR SWITCH VALVE
  1. Disconnect the plug from the switch valve and connect a tachometer.
  2.  
  3. Connect a voltmeter and increase rpm.
  4.  
  5. The voltmeter should read about 13 volts up to 3200 rpm.
  6.  

149-F TEMPERATURE SWITCH
  1. Disconnect the plug from the relay box and connect a voltmeter to terminals 6 and 8.
  2.  
  3. The voltmeter should indicate approximately 13 volts above 149-F.
  4.  

VACUUM SWITCH
  1. Disconnect the plug from the relay box and connect a voltmeter to terminals 7 and 8.
  2.  
  3. Idle the engine.
  4.  
  5. The voltmeter should indicate 0 volts.
  6.  
  7. Disconnect the vacuum line from the switch. The voltmeter should now indicate about 13 volts.
  8.  

EGR RPM SWITCH
  1. Disconnect the plug from the relay box.
  2.  
  3. Connect a voltmeter to terminals 1 and 3.
  4.  
  5. Start the engine and increase speed.
  6.  
  7. The voltmeter should indicate 0 volts up to approximately 3200 rpm. Beyond that, voltage should be about 13 volts.
  8.  
  9. When rpm decreases, the voltmeter should return to 0 volts at about 2800 rpm.
  10.  

EGR VALVE
  1. Start the engine and run it at idle.
  2.  
  3. Remove the lower, brown vacuum line from the EGR switchover valve and connect it to the carburetor in place of the blue vacuum line.
  4.  
  5. If the EGR valve is working, the engine will idle roughly or stop running. If the engine does not do one or the other, replace the EGR valve.
  6.  
  7. Do not forget to replace the vacuum lines.
  8.  

280 & 280C MODELS (CALIFORNIA)



See Figure 3

Click image to see an enlarged view

Fig. Fig. 3: Emission control component locations1974 280 and 280C models (California)

The California system for these cars is a modification of the Federal system. A Saginaw air pump is used and a reactor with injection tubes is used in place of an exhaust manifold.

Testing the System

62-F TEMPERATURE SWITCH
  1. Disconnect and ground the plug to the switch in the oil filter housing. The engine rpm should increase. If not disconnect the relay box plug and connect terminals 2 and 10. With the ignition ON the valve should click. If the valve clicks, replace the relay box.
  2.  

212-F TEMPERATURE SWITCH
  1. Unplug and ground the temperature switch. The engine should increase and the auxiliary should run. If not, replace the relay box.
  2.  

RELAY BOX VOLTAGE
  1. Turn the air conditioner ON. The engine speed should not drop. If the engine speed decreases, remove the relay box plug and connect a voltmeter to terminals 3 and 8 of the plug. If less than 13 volts is present, replace the relay box, or, if no voltage is present, check the air conditioner circuit.
  2.  

VACUUM SWITCHOVER VALVE
  1. Remove the vacuum line from the top of the switchover valve and remove the blue line from the vacuum switch. The engine speed should increase. A voltmeter should read 13 volts between terminals 11 and 2 of the relay box plug with the engine running. Remove the vacuum line from the vacuum switch. No voltage should be present. If both readings are correct, replace the relay box.
  2.  

EGR VALVE AND VACUUM SWITCHOVER VALVE
  1. Remove the blue vacuum line from the vacuum switch. The engine should run poorly or stall.
  2.  
  3. Unplug the brown vacuum line at the connection on the firewall and connect it to the vacuum line for air conditioning. The engine should run poorly or stall. If the switchover valve and EGR valve are functioning, replace the relay box.
  4.  

ANTI-BACKFIRE VALVE
  1. Disconnect the center hose on the air filter. There should be no air flow. If the switchover valve clicks by bridging terminals 1 and 2 of the relay box plug, the anti-backfire valve must be replaced.
  2.  

AIR INJECTION SWITCHOVER VALVE
  1. Increase engine speed slowly to above 3450 rpm. The air flow should stop in the injection line at about 3450 rpm.
  2.  

AUTOMATIC CHOKE RELAY
  1. Disconnect the plug to the 62-F temperature switch in the oil filter housing and ground the switch. The automatic choke resistor relay should click. Voltage at the switch should be about 13 volts. If none is present, replace the relay box.
  2.  

FLOAT CHAMBER VENT VALVE
  1. Shut the engine OFF. Disconnect the gray vacuum line at the float chamber vent valve on the carburetor. No vacuum should be present. Reconnect the line.
  2.  
  3. Start the engine and remove the line again. A hissing sound should indicate the presence of vacuum. If no vacuum is present, remove the float chamber vent valve. With the vacuum lines connected, turn the ignition ON and OFF. The valve stem should move in and out. If not, replace the vent valve diaphragm.
  4.  

CHARCOAL CANISTER PURGE VALVE
  1. Remove the thin center hose from the charcoal canister and close the end of the hose with your finger. Increase engine speed to more than 2000 rpm. At idle, slight vacuum should be felt, increasing with engine speed. If not, the purge valve should be replaced, or there is a restriction in the line.
  2.  

450SL, 450SLC, 450SE & 450SEL MODELS (FEDERAL)



See Figure 4

Click image to see an enlarged view

Fig. Fig. 4: Emission control component locations1974 V8 engines (Federal)

A two-way valve is installed in the vacuum line between the venturi control unit and the distributor. Ignition timing is retarded when the two-way valve is not energized, and advanced when the valve is energized (circuit completed to ground). The valve is controlled by a 212-F temperature switch in the thermostat housing, which activates the valve above coolant temperatures of 212-F.

A fuel shut-off solenoid cuts off the delivery of fuel under the following conditions:

  1. Accelerator pedal is in the idling position.
  2.  
  3. Engine speed is above 1500 rpm, determined by an electronic control unit. There is no fuel shut-off when coolant temperature is below -4-F.
  4.  

Testing the System

IGNITION CHANGEOVER DEVICE
  1. Connect a timing light and check the timing at idle. It should be as specified.
  2.  
  3. Ground the connection of the 212-F temperature switch. The ignition timing should advance by 15- and engine speed should increase by about 300 rpm.
  4.  
  5. Check the oil pressure switch. This can only be done on the road or on a dynamometer. Connect a test lamp to the B+ terminal and terminal 87 of the relay. Disconnect the relay. Above a speed of 40 mph, the test lamp should light. Below approximately 30 mph, the light should go out.
  6.  
  7. Check the 212-F switch by connecting a test lamp to the B+ terminal and to the switch. At a coolant temperature above 212-F, the light should come on.
  8.  
  9. If there is no ignition changeover and the oil pressure switch is working, check the following:
    1. Fuse No. 6 in the main fuse box.
    2.  
    3. All vacuum and electrical connections on the two-way valve.
    4.  
    5. The two-way valve. Switch on the ignition and ground the oil pressure switch. This should energize the two-way valve.
    6.  
    7. The relay. Connect a test lamp to the plug of the two-way valve. Switch on the ignition and ground the 212-F temperature switch. The relay is working if the lamp lights.
    8.  

  10.  

450SE, SEL, SL & SLC MODELS (CALIFORNIA)



See Figures 5 and 6

Click image to see an enlarged view

Fig. Fig. 5: Emission control component locations1974 450SE and SEL models (California)



Click image to see an enlarged view

Fig. Fig. 6: Emission control component locations1974 450SL and SLC models (California)

The California system is a refinement of the Federal system and closely resembles it, with an added air pump.

Testing the System

212-F TEMPERATURE SWITCH
  1. Unplug the temperature switch and ground it. The engine rpm should increase and the auxiliary fan should run on the 450SE and 450SEL. If not, connect terminals 3 and 4 of the relay box plug. With the ignition ON, the switchover valve should click. If not, replace the relay.
  2.  
  3. Switch ON the air conditioning. The engine rpm should rise slightly. If the engine rpm does not increase, check the air conditioning. If the air conditioning works, replace the relay.
  4.  

EGR
  1. Remove the air filter top cover and check that exhaust gas is emitted from the recirculation line in the throttle valve housing. If no exhaust is emitted into the throttle valve housing, clean the throttle housing and EGR line.
  2.  

DIVERTER CONTROL VALVE
  1. Remove the air filter housing and lay it aside without unplugging the warm air sensor. Disconnect the brown vacuum line at the diverter control valve. Increase the rpm to over 2000 and release the throttle linkage. Vacuum should be present at the port of the diverter valve only when the throttle linkage is released. A hissing sound should be heard.
  2.  
  3. If no vacuum is present, replace the diverter valve.
  4.  

CO CONTENT
  1. Check the CO content of the exhaust gas. It should be a maximum of 1.0% WITH OR WITHOUT air injection. To check the CO, remove the air filter housing and lay it aside without unplugging the warm air sensor. Disconnect the brown vacuum line at the diverter valve and connect this to the vacuum supply line for the cruise control actuator.
  2.  
  3. If the CO content varies with or without air injection, check the brown vacuum line to the diverter valve for tightness. Also check the diverter valve.
  4.  

CHARCOAL CANISTER PURGE VALVE

See this test under "280 and 280C (California Only)." The 1975 emission control equipment closely resembles that for 1974. The 230 (California only) and all other gasoline engines are equipped with catalysts. These models must be operated only with unleaded gasoline.

All of the following tests should be made in the specified sequence with the engine at operating temperature.

As of 1975, the base color of vacuum lines for emission control is white. Lines originating at a vacuum source have only one color stripe. These lines are connected to the center connection of the switchover valve of the same color. Lines terminating at a vacuum operated device have 2 color stripes. Purple is always the second color. The lines are connected to the outer connection of the switchover valve of the same color.

Switchover valve filter caps are color coded as follows:



REDValve for ignition advance
 
GRAYValve for throttle lift
 
BROWNValve for EGR
 
BLUEValve for air injection
 

 
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