John has it right.. thats the engine comoputer.. (ECU, ECM)..
the DT-530 is a HEUI engine just like the DT-466E, T-444E, CAT-3126, VT-365 engines were.. HEUI engines were also made for ford by navistar for the Powerstroke 7.3 and 6.0, while your DT-530 is totally diffewrent mechanically than the likes of everything but the DT-466E, electronically you can follow many of the same troubleshooting procedures as you do for the ford power-stroke 7.3 and 6.0.
its ULTRA-IMPORTANT if you remove that computer for any reason.. that when you put it back, the electrical connector bolts are Torqued EXACTLY like the spec on the unit says.. too much torque and you can flatten the pins or deform the plastic connector, too little torque can cause bad connections, gremlins, ARCing (which can ruin the computer)..
your DT-530 is likely connected to an electronic transmission such as an MD-3060 pushbutton shifter transmission. there will be a separate computer that handles the tranbsmission. called a 'TCM'.