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Replacing the Auxiliary Water Pump

on the W126 1985 300SD

by whunter


The Aux water pump is there to flow hot coolant through the heater core at low engine RPM.
Without this pump, most drivers would never have heat.



Here's the procedure:
  1. Open hood.
  2. Disconnect and remove battery.
  3. Remove weather seal from false bulk head = blue line in attached picture #A.
  4. Remove four access panel screws = red dots in attached picture #A.
  5. Remove access panel = pink line in attached picture #A.
  6. Remove the silver processor box from bracket and lay it back on the bezel.
  7. Remove the three hoses where they pass through bulk head.
  8. Trace the aux pump wire to the plug and disconnect it.
  9. Remove the wire from mono valve.
  10. Remove two 10mm bolts holding mono valve, you may need pliers to hold the bottom nut unit.
  11. Remove the upper hose from the mono valve.
  12. Remove the water feed hose going through false bulk head to aux pump.
  13. Remove the two 8mm bolts supporting the aux pump isolation bracket.
  14. Remove the pump and mono valve assembly as a unit, use great care.
  15. Note orientation of aux pump to mono valve.
  16. Remove aux pump from mono valve.
  17. Remove aux pump isolation bracket from old unit and install on new unit.
  18. Install new aux pump to mono valve with correct orientation.

Reverse order # 14. - #1. and you are done.

Mine was nasty; it took two hours in the shop with all correct tools.
I hope yours is easy.
Have a great day.

- whunter

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