There are 2 options to enable tracing of FTP transfers in Spool:
1) The TCPDRIV option "T" that will create a file named TCPTRACE in hold queued to the printer and a dynamically allocated sysout with the trace information
2) The TCPDRIV option "e" (lowercase e) that will create only the sysout with the trace information
The first one can generate big files depending on the amount of data that is being transferred, and it will show the data in the TCPTRACE file.
The second one is preferable as it will only produce a trace of the FTP commands and responses in the sysout.