I looked in earnest for the 5HP24 sub-variant from a 4.6 when I was planning the motor swap in my 2002 E53 and had zero luck. I also looked for the torque converter thinking I could at least use that in my 4.4 gear box. I came up empty on both accounts though. Fortunately so for the TQ situation as I later learned that the 5HP24 in the 4.6 has a different bell housing to accommodate the larger diameter TQ that comes with the 4.6 transmission.

After picking the brains of several 540 guys on the other forum who have done the 4.6 swap I decided to just bolt up the new 4.6 to the 4.4 gear box, squirt the 4.6 data into my 4.4 DME and get on with it. Hopefully your fix will be easy and inexpensive, once you find out the cause. But if worse comes to worst, you could do the same as I did since 4.4 specific 5HP24's are readily available. I even saw somewhere info on a shop in NJ (I think) that has a big torque rebuild kit for it. Although, I'm not sure what they do for the TQ since the amount of torque a TQ can handle is a function of diameter.

At any rate, my E53 is running great with the 4.6 motor bolted to the 4.4 ZF. Who knows, it could fail tomorrow, but there are guys who have been running this combo for years with no problems.

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Current
Original owner 2002 E53 X5 4.4i to 4.6i swap
2026 G80 M3 6 spd
2025 G06 X5 50e
Former
1972 Audi Fox
1986 Saab 900S
1996 BMW Z3
1998 BMW E36 M3 Sedan
2004 BMW E46 M3
2006 Audi A3 Quatro
1993 Mopar 318 Jeep Grand Cherokee
2015 V6 Jeep Grand Cherokee