The black control unit is an Auxiliary Idle Control Module, also known as an Auxiliary Power Control Module (APCM) that was available for the pre-'04 Ford vans with the 7.3L diesel. There may have been a few early model '04's with the 6.0L diesel that had this option, SMB began installing a simple "on/off" idle control switch on the knee panel in front of your right knee to replace this bulky APCM.
The APCM controls the idle speed. You can program it to operate automatically, or operate it manually. The most common mode used was the Charge Protect, which would increase the engine RPM to help charge batteries faster. You must have the van in Park, the parking brake set, and your foot off of the brake pedal in order for it to operate (just like the '04 and newer 6.0L with the SMB installed "on/off" switch). There is usaully a two second delay before the RPM is increased once it is programmed to come on.
Here is a link to the owners manual for the APCM; https://www.fleet.ford.com/TRUCKBBAS/no ... manual.pdf
By the way, if you have the unit but don't know how to connect it, the connector is taped to a large wire harness that runs vertically just to the left of the Ford fuse panel under the dash. Ford did offer a mounting bracket to secure it to the engine housing, but you would lose the two left side cupholders, so most folks just let it rest on the engine housing. A few clever owners designed some nice mounting methods.