Eric,

As always, Steve prefectly described the way the system works.

There is one thing I would like to add, since the auxilliary water pump came up as a possible cause of your problem. If the auxilliary water pump gets a little flaky, probably from a slight amount of cooling system contamination, it can draw too much current which will blow your climate control push button unit in the dash, a part that is even more expensive.

It might be a good idea at this point to check the pump for current draw. If it draws more than 1.3 amps, it would probably be a good idea to replace it. As Steve said it could be why you're having varying temperature, and if it draws too much current, it can and probably will blow the pushbutton control unit.

Good luck,

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Larry Bible
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles