Can I replace internal diff in Total Commander with a custom tool?

There are many ways to do this (here I'm using BeyondCompare as an example, it's the best I've found in this category, but the solution also works well with WinMerge):

A) Edit wincmd.ini under %USERPROFILE%\AppData\Roaming\GHISLER\ by default, and add something like this:

[Configuration]
CompareTool=C:\Program Files\Shareware\FileCmp\BeyondCmp\BCompare.exe

Now all comparisons are made using this app instead of the internal tool.

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B) Using TotalCmd's StartMenu (command "Change start menu..."), add these commands:

Title:

BC (Compare left-vs-right directories)

Command: %YourPath%\BeyondCmp\BCompare.exe

Parameters: %X%p %X%t

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Title:

BC (Compare current file under cursor vs same file in the other panel)

Command: %YourPath%\BeyondCmp\BCompare.exe

Parameters: "%X%p%x%n" "%X%t%x%n"

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Title:

BC (Compare current file in left vs current file in right)

Command: %YourPath%\BeyondCmp\BCompare.exe

Parameters: "%X%p%n" "%X%t%m"