Checking function of trip computer - RA54001070420X(54-420)

Operation No. of Job Texts and Work Units, Standard Texts and Flat Rates

Models 107, 124 and 126
1. Checking setting of clock
  After an interruption of the operating voltage, the clock indicates 12:00 and the display flashes. The clock must be set so that the trip computer is operational (refer to Function of trip computer ).
2. Checking tank capacity, fuel consumption and speed displays
  Connect the 1-pole test connection (X11/10) to ground. This causes the trip computer to operate in test mode and the displays in each function change as follows:
  - DISTANCE - UNTIL TANK EMPTY the tank capacity in "l" instead of "km".
  - CONSUMPTION - MOMENTARY the effective fuel consumption in "l/h" instead of "l/100 km".
  - SPEED - AVERAGE the momentary speed in "km/h" instead of the computed average speed.
    
    Fig 1: Identifying Test Connection
    Courtesy of MERCEDES-BENZ OF NORTH AMERICA.
Arrangement for model 107

Arrangement for model 124

Fig 2: Identifying Test Connection - Model 124
Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Arrangement for model 126

Fig 3: Identifying Test Connection - Model 126
Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Checking tank capacity display
1. For this test, the fuel tank must contain at least 15 l fuel. The float of the fuel gauge sensor must not touch the reserve contact otherwise the trip computer will operate with a model-specific fixed value from the coding plug.
  NOTE: If it is necessary to fill the fuel tank, wait a while until the float of the fuel gauge sensor has stabilized at the new fuel level.
2. Turn key in steering lock to position 2. Use keyboard to set display to DISTANCE - UNTIL TANK EMPTY.
  
  Fig 4: Identifying Keyboard Key
  Courtesy of MERCEDES-BENZ OF NORTH AMERICA.
  
  Display:
  
  Tank capacity in "l" according to the fuel quantity contained.
  
  Fig 5: Identifying Tank Capacity Display
  Courtesy of MERCEDES-BENZ OF NORTH AMERICA.
3. If the fuel quantity is only displayed by the fuel gauge in the instrument cluster not the trip computer, check the cables from the 4-pole connector at the instrument cluster (sockets 1, 2 and 3) to the 35-pole connector of the trip computer (sockets 6, 19 and 21) for open circuit and renew if necessary.
  
  Fig 6: Identifying 35-Pole Connector Of Trip Computer
  Courtesy of MERCEDES-BENZ OF NORTH AMERICA.
  
  If there is still no display on the trip computer, renew the trip computer control unit.
4. If the fuel gauge in the instrument cluster shows no value either, check the fuel gauge sensor and renew if necessary.
  Checking fuel consumption display
5. Start up engine.
  Use the keyboard to set the display to CONSUMPTION - MOMENTARY.
  
  Display: Fuel consumption in "l/h"
  
  Nominal value at idle: 1.5-2 "l/h"
  
  (Requirement: Engine must be correctly tuned).
  
  Fig 7: Identifying Fuel Consumption Display
  Courtesy of MERCEDES-BENZ OF NORTH AMERICA.
6. If the nominal value is not attained or if there is no display, for model 126 with CIS fuel injection system (up to approx. 07.85) check the potentiometer at the air flow meter (55-0422). For all other models, check the continuity of the yellow/violet cable for the consumption signal between the CIS-E control unit, socket 4, and the trip computer control unit, socket 24. If the cable is in order, renew the CIS-E control unit. If there is still no display, renew the trip computer control unit.
  NOTE: Failure of the fuel consumption display may also be caused by the installation of incorrect or incompatible components, e.g. trip computer coding plug, CIS-E control unit or fuel distributor.
  
  Check these components in accordance with the service documentation and install the correct component if necessary.
Checking speed display

Drive the vehicle on a roller dynamometer at a speed of 100 km/h.

Use the keyboard to set the display to SPEED - AVERAGE.

Compare the trip computer display with the dynamometer display.

(The speedometer has a lead).

Nominal value: 100 km/h ± 4 km/h

If the nominal value is not attained, renew the control unit.

Fig 8: Identifying Speed Display Screen
Courtesy of MERCEDES-BENZ OF NORTH AMERICA.
Models 129 and 140
NOTE: For testing, refer to the Diagnosis Manual BODY,

Volume 1.