That is a really good data point. My on filter pump lasted 111,000 miles. The cobbled up wiring is the problem to the tank mounted retrofit and is most likely unlike the OEM in tank wiring. Without benefit of the schematic I think the retrofit wiring goes something like this:
The short run OEM wiring from the ECM to the on filter pump was able to manage without too much voltage drop but still, i suspect the pump starved for juice but not enough to cause failure UNLESS the connector got dirty or was not making good contact. Notice that the connector from the ECM to the original pump and now to the retrofit wiring harness is a square fitting without benefit of being water or weather sealed like the ones with the three rubber seals. Connectors in wiring are like bends and valves in piping, they create resistance and V still equals IR. When they are dirty, or not making good contact or corroded they have more resistance. Like a corroded flashlight battery. In low voltage systems there is not enough pressure / voltage to power through that resistance point.
In the retrofit they knew the connector to the original pump from the ECM was not enough to make it all the way back to the tank without unacceptable voltage drop, they probably also knew that they had a voltage drop problem causing premature failure of the filter mounted pump, so they had to rob some power somehow from somewhere else while still using the lead from the ECM to get the precharge and run timing coordinated by the ECM.
What I suspect they really should have done that would have been much simpler would have been to increase the size of the wiring to the filter mounted pump. Instead, to standardize parts count they went to a field retrofit using the existing wiring not understanding it is prone to install errors. I suspect this is why the new in tank pumps last a long time and the retrofit pumps don't. They executed a cobbled up patch instead of a fix. That hardly ever works out well. It is equivalent to patching potholes when the real problem is water intrusion and failure of the subbase.
What they did is to put a relay in the line that would add juice to the system at an intermediate point to aid the transmission of enough current at the right voltage back to the in tank pump.
The line from the ECM energizes the relay taking juice from the battery but also sends the same current from the ECM IN ADDITION to the current from the battery on down the line to the in tank pump. This is the reason the pump will run with the battery disconnected and with the fuse out of the line to the relay and with the relay completely disconnected. However, in that condition the pump operates at low voltage or low amperage taking power only directly from the ECM but leads to its early demise.
My theory is that in some retrofit installations they have gotten it wrong when it comes to the modification of the 14 pin connector behind the driver's side fender liner near the floor board. Not sure what they get wrong but I suspect the added power from the battery is not all making it to the pump in the tank.
That is my theory that remains to be somehow proven. I think it has to do with the wiring of the retrofit and low voltage.
Not knowing how your two retrofit pumps are wired in I can only guess but I guess that the reason your fuse blows is that they are sending all the current to the pump from the battery switched by a relay signaled by the ECM outlet that once went to the filter mounted pump. All that current on a small existing wire going back to the tank is causing an overheat and blowing the fuse.
The wires to the tank need to be bigger!!!!
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