"If I unplug the pre-pump fuse, shouldn't I hear some kind of change? It runs exactly the same with the pre-pump fuse in or out."
At the fuse holder, you can test ("hot wire") the tank pump by itself. Remove fuse 4 and jumper 12V* to the output (right side ) contact. The tank pump should be running.
* Get +12V direct from battery at any fuse 6—10 input (left side) contact.
Jumpering 12V* to fuse 4 input (fuse still out) should run the Main pump.
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Bruce Young, '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.
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