Papa Jo,

Logic would suggest that each pump would be dedicated to the two main heating loops we have. Your findings are correct. Whenever the Webasto is on the pump connected with the brown wire is always on. I just verified this with the schematic and then turned on the interior heat and both pumps came on. If you only have the basement heat selected then only the brown wire connected pump will come on. When you have the interior heat on the blue wire connect pump will ALSO come on. BTW, this is not clearly evident when looking at the plumbing drawing.

So, why did fuses blow when you had both pumps jumped together? It would seem that the total load for both motors running at the same time exceeds the fuse ratings. Try this, disconnect the interior pump and connect the main pump to the blue wire. Now set your interior heat and thermostat to call for interior heat. This should activate the main pump and the system should run without the popping (boiling) sound and the fuses should not blow. If this works, then you need to determine why the main pump is not being supplied power. You have probably looked at the same drawing I did. The Webasto signals the main pump to come on. See if the Webasto is indeed signalling the main pump. If that is good, you will have to look at the main control board in the ECC where I believe you'll find the relay that energizes the main pump circuit.

You asked about the impeller. It is simply pressed on the motor shaft. Here's what the impeller looks like:



Impeller.jpg

I have replaced one of the two pumps and the job isn't hard. I turned off the engine coolant valves near the engine. Unlike Reagan, I had a lot of coolant escaping. I used pinch clamps on the hoses to limit coolant loss. I bought my pump from Midwest Bus Parts. I also bought a repair kit for the pump. As you can see from the photo, I have yet to get around to putting the repair kit in. My pump had a leaking shaft seal.