On the first gen trucks you would have to use a Y cable, that plugs in between the controller and harness, to give you a diagnostic port to work with. I have never been able to find one of these cables though. On an A1 truck with data bus diag you can plug into the trucks diag port and access the ctis. Not sure about 3126 A1. Never tried. The interface box is a Dearborn protocol adapter. The newer ones are usb. Most of the older ones are serial port to laptop.