Strange Warning Light Problem

Had just started driving my 1994 J30 with 94k when I noticed that the brake light, battery light and, the transmission lights were all on at the same time. In my experience with older cars, I thought that the alternator was shot. Took it into a mechanic and was told that the alternator was working and since it was a 94, he didn't have the connectors to check the codes.

While he was explaining to me his findings and I was paying the diagnostic charge, the car was left running. When I went out to the car, lo and behold, the warning lights were off. Me and the mechanic scratched our heads and I left.

I have driven the car around 300 miles since then and the lights have not come back on. However, I know these warning lights did not come on for nothing. Since I am planning a cross country trip, I am concerned that the problem that these warning lights were pointing to is going to leave me stranded somewhere.

Any ideas??? I am still leaning toward an alternator problem or perhaps an electrical gremlin.

Thanks in advance.

you can buy a obd-1 connector for your j30 for abt $90 and download a free program called z-talk and you can use a laptop to connect to your ecu and read codes. other than that i have never had all the lights suddenly come on while driving. im leaning towards your alternator going.