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Heating water circulation pump, location/task/function - GF83.20-P-2109GI

MODEL 163 as of 1.9.01 with code (580a) Automatic air conditioning 

Fig 1: Identifying Heating Water Circulation Pump
G04608910Courtesy of MERCEDES-BENZ USA
  Heating water circulation pump, position The heating water circulation pump (M13) is located at the front right in the engine compartment next to the coolant compensating reservoir.  
  Heating water circulation pump task The heating water circulation pump (M13) serves to maintain a constant flow of hot water through the heat exchanger in the event of low engine speeds and residual engine heat utilization.  
  Heating water circulation pump function   GF83.20-P-2102-01GI