Mercedes Benz repair: coolant servo 1979 450SEL, number search results, servo control

Question
My mechanic says that my servo is leaking slowly.the replacement cost is 900.00.does this servo control all the coolant,or is it just for heat.i live in arizona,so if it is a matter of the heat not working no problem!he said this controls all the coolant flow .my temp gauge reads fine.

Answer
Which do you have?
450SEL 6.9
450SEL

I am very confused, what servo?

After much searching of your year model, the only servo found is the cruise control servo.
I need to know if there is another name for this part?

Here are two parts that go bad under normal conditions.

PART NUMBER SEARCH RESULTS WITH PRICES
Vehicle 1979 Mercedes Benz 450SEL
Part: Auxiliary Water Pump
 Note:  Federal
This unit has 12" lead wire. Bosch 0.130.002.070     
 Chassis: 043634-
G3050-12429 Auxiliary Water Pump Bosch IN STOCK $165.21  
  
http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=merc...


Vehicle: 1979 Mercedes Benz 450SEL
Part: Heater Valve
Select one of the products from below  
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   Heater Valve
with Faucet
California
Some models may still use a metal valve & vacuum element. For these vehicles use 107 800 24 75 & 107 835 01 20 or 116 800 18 75 & 116 835 00 20.    Chassis: -043560
----------------------------------------
   Heater Valve
with Faucet
Federal
Some models may still use a metal valve & vacuum element. For these vehicles use 107 800 24 75 & 107 835 01 20 or 116 800 18 75 & 116 835 00 20.    Chassis: -043633
---------------------------------------
   Heater Valve
aluminum
California
used with 116 800 18 75. Some models may now use 107 830 05 84 as a modification.    Chassis: -043560
--------------------------------------------
   Heater Valve
aluminum
Federal
used with 116 800 18 75. Some models may now use 107 830 05 84 as a modification.    Chassis: -043639

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=merc...