Mercedes Coupes/Sedans/Wagons 1974-1984 Repair Guide

1981-84 Vehicles

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6-CYLINDER ENGINES



See Figure 1

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Fig. Fig. 1: Vacuum line color coding chart

The emission control system is the same as 1980, with two exceptions. The air pump intake is connected to the clean side of the air cleaner. A rubber scoop inside the air cleaner facilitates air intake. The second change is that the fuel evaporation control purge system is controlled by a thermo valve and is effective only at temperatures above 122 degrees.

Color coding of the vacuum lines is as indicated in the accompanying chart.

Lines originating at a vacuum source have only one color stripe; lines terminating at a vacuum operated device have 2 color stripes and purple is always the second color.

Testing the System

Refer to the testing procedures for 1980 models.

V8 ENGINES



Color coding of the vacuum lines is identical to the 6-cylinder engines. Operation of the Lambda oxygen sensor control system is the same as 1980. Only the oxygen sensor itself has been modified for production reasons. All models in 1983 are equipped with a standardized O$SUB2 sensor, with a new plug connection. The plug is no longer below the vehicle, but inside the passenger compartment, accessible after removing the control unit cover plate. The air pump is a maintenance-free vane type pump, similar in operation to the 1980 system. Three-way-catalysts have been slightly modified dimensionally, but operate the same as those in 1980. The fuel evaporation control system is the same as 1980, except that the purge system is controlled by a thermo valve that allows purge only at coolant temperatures below approximately 122

Testing the System

Refer to the testing procedures for 1980 models.

ELECTRONIC IDLE SPEED CONTROL1981-82 Models



See Figures 2, 3, 4 and 5

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Fig. Fig. 2: Electronic idle speed control systemV8 engines



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Fig. Fig. 3: Wiring schematic for the electronic idle speed control systemV8 engines



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Fig. Fig. 4: Wiring schematic for the Lambda control, air injection and electronic idle speed control



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Fig. Fig. 5: Location of the time delay relay (28)380SL models

  1. The engine should be at operating temperature with the ignition ON.
  2.  
  3. Pull the plug from the idle speed adjuster and check the voltage. If 12 volts are present, go to Step 4.
  4.  
  5. If there is no voltage, pull the plug from the control unit and check the voltage between terminals 2 and 4 (arrows). There should be approximately 12 volts. If there is no voltage, check the voltage supply and replace any defective parts. If 12 volts are present, check the wires from the plug of the idle speed adjuster and the control unit plug (black/yellow wire to terminal 5 of the control unit and the black wire to terminal 1 of the control unit). If there are infinite ohms, replace the wire. If there are 0 ohms, connect the coupling to the control unit and measure the voltage at the idle speed adjuster. If there are 0 volts, replace the control unit.
  6.  
  7. Check the control unit. With the engine idling at operating temperature, connect the plug to the idle speed adjuster so that you can check voltage at the plug. If there are 4-6 volts, go to Step 5. If there are no volts, replace the control unit.
  8.  
  9. Check the idle speed adjuster. Idle the engine and pull the plug from the coolant temperature switch. Bridge the terminals. The idle speed should increase. If not, apply battery voltage to the idle speed adjuster (for no more than 5 seconds). If the speed drops or the engine stops, replace the control unit. If the idle speed does not drop, replace the idle speed adjuster.
  10.  

1983-84 Models

The electronic idle speed control system has been slightly modified. A new control unit on V8's, processes the following inputs:



Engine speed (rpm)
 
Idle and partial load
 
Engine oil temperature
 
Transmission shift lever position
 
Engagement of A/C compressor
 

The signal for engine rpm is controlled by the oil temperature switch, which also sends a signal to the Lambda system control unit at the same time.

The A/C compressor has a time delay relay in place of the relay used in '82. Through the relay, voltage is supplied to the idle speed control unit prior to engagement of the compressor clutch. The idle speed solenoid will already be maintaining sufficient rpm when the clutch is engaged.

When the automatic transmission shift lever is in P or N, the starter lockout is closed and the solenoid is ground. When the shift lever is in a driving position, the starter lockout switch is open and the idle speed is lowered.

This new version of the electronic control unit cannot be used on older model vehicles.

Troubleshooting

ENGINE STALLS OR WILL NOT START
  1. Turn the ignition on and off. The idle speed control valve switch on and off noticeably (audibly).
  2.  
  3. If not, remove the valve and see if the aperture is open. If not, follow the Idle Speed Control Test.
  4.  

ENGINE SURGES OR SHAKES AT IDLE

Perform the Idle Speed Control Test.

IDLE SPEED TOO HIGH OR LOW, ENGINE STALLS

See Figure 6

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Fig. Fig. 6: Disengage the wiring harness from the idle control solenoid (23)

  1. Disconnect the plug from the idle speed control valve and reconnect it.
  2.  
  3. The idle speed should increase to about 1500 rpm.
  4.  
  5. If no change in idle speed occurs, replace the control valve.
  6.  
  7. Put the transmission in a driving gear. The idle speed should drop to about 500 rpm. If not, perform the Idle Speed Control Test.
  8.  

IDLE SPEED TO HIGH, ENGINE AT NORMAL TEMPERATURE
  1. Disconnect the plug from the temperature switch.
  2.  
  3. If the engine rpm drops with the automatic transmission in P or N, replace the temperature switch. If the engine rpm does not drop, perform the Idle Speed Control Test.
  4.  

Idle Speed Control Test

CHECK VOLTAGE TO CONTROL VALVE

See Figure 7

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Fig. Fig. 7: The control unit on the 380SL models is located under a cover (21) on the right side of the passenger compartment

  1. Turn the ignition on and off.
  2.  
  3. The idle speed control valve should switch on and off audibly.
  4.  
  5. If it does not, turn the ignition on and disconnect the plug at the idle speed control valve and see if voltage is present. If it is, replace the valve.
  6.  
  7. If no voltage is present, disconnect the plug on the control unit and check battery voltage between pins 2 (positive) and 4 (negative). If no voltage is present, check the power supply according to the wiring diagram.
  8.  
  9. If there is voltage at pins 2 and 4, bridge pins 1 and 2 and 4 and 5 simultaneously for a maximum of 5 seconds. The idle speed valve should switch audibly.
  10.  
  11. If the valve switches, replace the control unit.
  12.  
  13. If the valve does not switch, check the black and the black/yellow wires to the idle speed valve for continuity and repair if necessary.
  14.  

CHECK IDLE AND PART LOAD

See Figures 8, 9 and 10

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Fig. Fig. 8: The control unit is located under the cowl panel on 380SEC and SEL models



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Fig. Fig. 9: Turn the engine off before disengaging the control unit's electrical connection



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Fig. Fig. 10: A special test cable is required for testing the electronic idle control

A special M-B test cable is necessary for this test. The engine should be running at idle (with accessories turned off) at normal operating temperature. The transmission should be in P or N. Connect a multimeter to a test cable and connect the test cable between the idle speed valve and the wiring harness.

  1. The reading on the meter should be above 400 mA at about 650 rpm.
  2.  
  3. If the reading fluctuates and the engine surges, replace the idle speed valve or the control unit.
  4.  
  5. If the reading is 0 mA, perform the Idle Speed Control Test.
  6.  
  7. Disconnect the throttle valve switch connector and check the idle speed. The idle speed should increase to about 850 rpm.
  8.  
  9. If the idle speed does not increase, turn off the engine and disconnect the plug from the control unit. Set the multimeter on the 0 to infinite ohms scale and connect it to bushings 4 and 7. With the throttle valve against the stop, the meter should read 0 ohms. With the throttle valve slightly opened the meter should read infinite ohms. If the readings are not present, adjust the throttle valve switch, or if necessary replace it. Check the wires to the throttle valve switch.
  10.  
  11. Reconnect the plug to the control unit. Start the engine.
  12.  
  13. The idle speed should increase to approximately 850 rpm.
  14.  
  15. If not, replace the control unit.
  16.  

CHECK FAST IDLE

See Figure 11

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Fig. Fig. 11: The oil temperature switch helps control the engine RPM

The engine should be idling at normal operating temperature with the shift lever in P or N. Connect the plug of the temperature switch to ground to simulate an oil temperature of less than 16-C.

  1. If the idle speed increases to about 800-900 rpm, check the temperature switch and replace if necessary. Below an actual oil temperature of 16-C, the switch contacts should be closed. Above an actual oil temperature of 16-C, the switch contacts should be open.
  2.  
  3. If the idle speed does not increase, disconnect the plug from the control unit and check the wire from pin #6 to the temperature switch for continuity. If 0 ohms are present, replace the control unit. If infinite ohms are present, check the wire to the temperature switch.
  4.  

CHECK IDLE SPEED WITH AND WITHOUT THE TRANSMISSION ENGAGED

The engine should be running at idle, at normal operating temperature, and the wheels blocked.

  1. Put the car in gear. The idle speed should drop to about 500 rpm.
  2.  
  3. If the idle speed does not drop, disconnect the plug from the control unit. Turn the engine off, then on.
  4.  
  5. Connect a voltmeter between pins 8 (negative) and 2 (positive). Battery voltage should be present with the transmission in P or N.
  6.  
  7. If battery voltage is present, replace the control unit. If battery voltage is not present, and no voltage is present at pin #2, check the wiring according to the wiring diagram.
  8.  
  9. If no ground is made at pin #8, check the writing to the starter lockout/back-up light switch.
  10.  

CHECK IDLE SPEED WHEN A/C COMPRESSOR IS ENGAGED
  1. Turn the ignition on and engage the A/C compressor. Disconnect the plug from the control unit. Battery voltage should be present at pin #9.
  2.  
  3. If battery voltage is not present, check the voltage supply on the wiring diagram.
  4.  
  5. If battery voltage is present, but the idle speed drop is too much when the compressor is engaged, replace the ACC time delay relay or the control unit.
  6.  

IDLE SPEED TOO HIGH ON WARM ENGINE

The engine should be running at idle, at normal operating temperature. The transmission should be in P or N.

  1. Disconnect the plug on the temperature switch.
  2.  
  3. If the idle speed drops to about 500 rpm, replace the temperature switch.
  4.  
  5. If the idle speed does not drop, disconnect the plug from the control unit. Check the wire from pin #6 to the temperature switch for continuity.
  6.  
  7. If 0 ohms are present, replace the control unit.
  8.  
  9. If infinite ohms are present, check the wire to the temperature switch.
  10.  

 
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