The 4.2L is pre-OBD 1. The computer does not record or throw codes, it only sends real-time binary signals to the diagnostic ports located on the passenger side of the engine bay next to the battery. In order to read the signals you need a diagnostic reader. However, you will have to make your own reader, as nobody manufactures a reader for the 4.2L setup. Furthermore, you need to understand what the signals mean in order to be able to make any sense of the info. The signal is binary - either a light in on or it is off for a circuit.
Fortunately, that info is available in the following website: Adventures Under the Hood
. That site is the best one to explain what does what in the factory setup for the 4.2L, and the various ways to go about testing bits and pieces without needing a diagnostic tool. I recommend you read the whole site. Good stuff that will help you figure out the 4.2L setup. If you plan on keeping the Carter BBD, I recommend building a diagnostic reader as it will give you quick and easy access to basic computer signals allowing you to verify computer related functions.
I'm assuming here that the computer is connected and the Nutter bypass has not been done. The purpose of the Nutter bypass is to essentially disable the stepper motor, and the computer controlled timing adjustments (basically making it a non-feedback carb). If the stepper motor has never worked since you acquired the Jeep, check that the bypass has not been done, and that the computer is still connected.
If the bypass has not been done and you are running an unmodified factory setup, you'll need to start by verifying that the computer is taking over and that the stepper motor is getting signals. Tips and tricks on how to do this without a diagnostic tool can be found in the website mentioned above.