Monovalve/Duovalve and where and explain the cooling system
 

I own a 2008 ML 320 with heating issues. It has thrown an auxiliary pump code and I found the pump, or what I thought was the pump on the frame rail near the alternator. But the pump is good. I pulled it and ran it, no problems. I have bypassed it and problem is the same.

A few days later I heard what sounded like a noisy pump under the hood and found a second identical pump under the expansion tank. It is vibrating so something is happening, but I haven't pulled it yet. Why two pumps?

As I read hear it sounds as if my intermittent only heating problem is more likely to be the duovalve or monovalve. I think I have a duovalve. Can someone tell me where it is located?