Emissions control system - RA1400KE2D030X(14-0030)

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D. Oxygen sensor heater
The oxygen sensor contains a heating element (47) which is supplied with voltage under certain conditions. This ensures that the operating temperature of the active sensor ceramic (44) is maintained at a constant level. In addition, the operating temperature is reached more rapidly.

Testing oxygen sensor heater

For testing, separate plug connection and connect into the amp-meter cable. For test see Diagnosis Manual Engine.

Fig 1: Identifying Heater Oxygen Sensor
Courtesy of MERCEDES-BENZ OF NORTH AMERICA.
1. Oxygen sensor heater up to 08/89
  The heating element of the oxygen sensor is supplied with voltage via terminal 87 of the fuel pump relay and is heated for as long as the fuel pump is running.
2. Oxygen sensor heater as of 09/89
  1st version up to approx. 12/89
  
  The fuel pump relay is fitted with an additional connector for the oxygen sensor heater with the designation 87H. Consequently, the cable assignment at the relay was also modified. Actuation is performed as before as the additional connector 87H is linked via a bridge to the connector with the designation 87.
  
  Fig 2: Oxygen Sensor Heater Circuit Diagram (1st Version)
  Courtesy of MERCEDES-BENZ OF NORTH AMERICA.
  
  2nd version as of 01/90
  
  From this date on the bridge between terminals 87 and 87H is no longer fitted. The electronic circuitry in the relay was modified as a result. The cut-in temperature for the oxygen sensor heater is +20 °C and is determined by the coolant temperature sensor.
  
  Fig 3: Oxygen Sensor Heater Circuit Diagram (2nd Version)
  Courtesy of MERCEDES-BENZ OF NORTH AMERICA.
  
  NOTE: Only the new fuel pump relays are available as replacement parts. This means on vehicles manufactured prior to 09/89 actuation of fuel pump and oxygen sensor heater continues to be performed by connector 87. Connector 87H does not perform any function on these vehicles.
3. Oxygen sensor heater, engine 103.984 (model 129)
  The oxygen sensor is heated from a coolant temperature of 20 °C. The voltage supply is effected by engine systems control unit, terminal 87SH.
4. Oxygen sensor heater, engines 104, 119
  The heating capacity of the oxygen sensor is:
  
  Engine 104 up to 08/89 heating capacity 12 W
  
  Engine 104 as of 09/89 heating capacity 18 W
  
  Engine 119 heating capacity 18 W.
  
  The KE control unit supplies a control signal for the oxygen sensor heater to the engine systems control unit which then controls the oxygen sensor heater. The engine systems control unit supplies an information signal regarding the oxygen sensor heating current (feedback) to the KE control unit for diagnostic purposes.
5. Oxygen sensor heater, engine 103 as of 1991
The heating capacity of the oxygen sensor has been increased to 18 watts.

Actuation of the oxygen sensor heater is performed from a temperature of approx. +20 °C by the KE injection system control unit (N3) via the oxygen sensor relay (K35).

The oxygen sensor relay (K35) on model 124 is installed in the fuse/relay box in relay position "E", on model 201 in front of the electrical center in the relay box in relay position "F".

Fig 4: Identifying Fuse/Relay Box
Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Fig 5: Wiring Diagram Oxygen Sensor Heater (Engine 103 Model 124)
Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Wiring diagram oxygen sensor heater

Engine 103 model 124

Fig 6: Wiring Diagram Oxygen Sensor Heater (Engine 103 Model 201)
Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Wiring diagram oxygen sensor heater

Engine 103 model 201