1987 Cadillac wont hold charge have to jump off

Check your fuse in your fuse block. It's on the left side under dash, If not that there is an electronic module on the right side o fdash that may be going bad. Make sure there is ower to the alternator and not poor power. One lead should go t battery and other to the starter solenoid. The white or gray wire leads to the electronic module. But also check yoru battery grounds as the normal battery ground goes to the fender behind the battery. You migh talso hook a ground up to the engine block from the battery as the starter may be getting a poor ground causing the problem.

If all else fails take it to a qualified mechanic and see what they can come up with. It's'very hard to diagnose electrical problems here via the internet. Besides you could have a bad draw on your battery which if you go to the diy section on the site it will tell you how to check it. Also the battery may be the wrong size or not able to take a load and they will check for that as well.