93 cherokee low spark

I just bought my jeep about a week and a half ago and drove it for about 5 days before it stopped running. The day before it would take a while for it to start. It's a 93, 4.0, auto. At first I had a week spark at the dist. , so I replaced the coil. Still the same. It cranks fine. I made sure that it hasn't jumped time. I tested the wires going to the dist. and read 12 volts. Then while cranking it would go to -6 volts. Is that normal? I checked the wires going to the dist. sync with a test lite. The only way the lite would come on is if I hooked it to the pos. on the battery.The middle wire showed good ( the test lite came on ) but on the orange wire, the lite bearly came on. My volt meter showed 3 volts (or-3, I don't remember). I got the same reading at the cps. I have baffeled every machanic that I know with this. My next option is the computer. Any suggestions? Please help me. Thank you