Engine service soon light?

Being an 88 that has OBD 1 on it and you can read the codes on those with a piece of wire. The code would tell you what system the computer sees an issue with. The codes however are limited in terms of what they really tell you.
https://2carpros.com/articles/buick-cadillac-chevy-gmc-oldsmobile-pontiac-gm-1983-1995-obd1-code-definitions-and-retrieval-method/
Tells you how to read them yourself. The connector looks like the attached image. It is commonly under the dash on the drivers side of the vehicle. To read the codes you just turn off the key, install a jumper wire (any conductive item works. I commonly used a paper clip) between the A and B terminals at the top right. Then turn on the key and count the flashes of the light. The code will repeat 3 times so a code 35 would be three flashes, then 5 flashes, then 3 - 5 and then it pauses for a couple seconds and moves to the next code. Normally you will see a code 12 first telling you the system is in diagnostic mode, then 12 at the end to tell you that you have seen all codes, then it will start over. Once you have the code(s) you look at the chart and see where to start.
Hope that gives you a start.