I know you can read trouble codes by turning the key on and off in a certain sequence. Trouble codes are stored in the computer's memory whenever an electronic component on the car fails a test administered by the central computer (ECM?). These trouble codes will give you a starting point to determine what, if anything, is wrong with your vehicle. I'm not sure what the sequence is exactly, and you'd need a decoder list anyway for the codes that you read to make any sense. I believe the chilton manual that I bought for my ZJ has this list, as well as instructions for reading the codes from the ECM. I would recommend that you go to your local autoparts store and buy the Chilton manual for your vehicle. Typically they are around 25 dollars; and worth every penny. If you are really tight on funds, then you may be able to check out the Chilton manual from your local library.
1993 ZJ Limited 5.2 - no mods - Red with Gold (what were they thinking) trim.