I just replaced the ACC Monovalve in my 93 300D.

Unlike the 617, the monovalve is not rebuiltable. U have to replace the whole unit. As of Jan 93, the Monovalve and the Aux pump r one unit. So the relacement unit cost me $527 delivered to the door.

Before Jan 93, replacement unit is $103. The Monovale and Aux Pump r 2 separate units.

Replacement is straight forward. There r some pointers that may help the next person who has to do the job.

1) Remove the negative on the battery
2) Remove 2 electrical connectors - monovale and aux pump
3) remove 3 hoses near battery. U will lose about 1 gallon of coolant.
4) The expansion tank has to come off. There r 2 hoses below it that connect to the unit.
5) Remove 2 hoses under aux pump.
6) Remove the 2 10mm nuts, in fender, that hold unit
7) Slide out unit, disengaging bottom 2 hoses
8) Use nose piece plier to force out 3 rubber holders from old unit - put on new unit.
9) When putting unit in place, slide bottom hose (below ex tank) first before silding the top hose in.( Let the top metal pipe from unit jut below ex tank ) Then connect upper hose below ex tank.
10) Top up with coolant. Drive and check, for leaks and add coolant as needed.

Add on anything else that may the job easier for the next person.