I ran into a problem with my 87 fuse block that I chased for a week and a half! I wasn't getting any dash lights and my tail lights wouldn't work. That's a tell-tale sign of a blown fuse for the rear lights. When I checked the fuse block I found that 5 fuse terminals, for various fuses, had at least one broken side too them. This had to be fixed before I could continue. I went to the Pick-n-Pull and cut out an entire fuse block from an 89. From the donor fuse block I carefully removed the identical fuse terminal that needed to be replaced. I removed the broken ones from my block and spliced in the new ones. I tested continuity as I worked. I put everything back together, fired it up, and it still didn't work....
I checked every relay and every ground after I checked every fuse. After another night of troubleshooting I was at a loss so I stepped back and tried to figure out what I was missing. I realized that when I checked the fuses I didn't do continuity checks, just visual checks with a flashlight to make sure they weren't blown. Well the problem ended up being a fuse that was so dirty and caked with crap (how they get that way I'll never know) that it wasn't allowing for a good connection. I took the fuses out and cleaned them, one by one. I held the fuse so the metal terminals were laying on the edge of some scrap wood. I used just a squirt or electrical contact cleaner and a small metal-bristle brush to clean the terminals until they shined. Flip it over and do the other side the same way, wipe off the crap and you'll have nice pretty fuses. Re-install and you're on your way.
The moral here is that your jeep is as old as mine. Check the condition of your fuses before you assume the fuse block is bad. 3 volts coming from the 12 volt line means you have a bad connection (or a bad meter) or a dirty fuse. It's quick and easy to do. And make sure you have the correct size fuses in the correct slots.
1987 XJ 4.0 I6, Peugeot tranny, NP231, AA SYE, 8.25" 29 spline, Dana 30 (HP), Rusty’s 4.5 inch coils, BDS 5500 4.5" shocks, 5" junkyard bastard pack, Rusty’s Adj LCAs, RC Quick Discos, JKS BPEs (front and rear), Aussie Lockers (front and rear), Durango steering box, and Removable Doors!