Date registered: Aug 2008
Vehicle: 1997 S500 Coupe
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If I understood correctly, he has 2 S420's on the lot and you got one that runs perfect but has a heater problem, while the other one has a water pump problem?
Keep the heater problem one, the duo valve is usually the culprit. You can take the part apart, test it with some 12 volt electricity wires and see if it opens up and closes. The part is cheaper than a water pump and is less work to replace, I did it myself and this is the first car I've ever worked on.
What you can do is check if the duo valve heats up on both cores (look at the duo valve and you will see it has two chambers, both should be hot.). If one of them isn't hot, then your duo valve has a problem. Ofcourse, the best way to test is it to take it out and check in my opinion. From there, you clean it up. If cleaning it works, great! If not, decide if you want to buy a new one or used one. I will warn you, I had 3 used duo valves fail on me within a month time. Only after those 3 messed up valves. I ended up finding my original one and taking all the duo valves apart and replacing the rubbers and working parts all into one to get a working one, since I was tired of spending all the money for new used ones. (Getting a new one right off the bat would've saved me the trouble).
If the duo valve works (via electricity inspection, it should make a clicking sound on both sides), then you might have a problem like the original poster did, with a messed up sensor.
Hope I helped, good luck!
Moved to new account "Perkunas"