I stumbled across this site while goofing off at work. It shows how to enter the diagnostics mode using the ignition.
I thought it was useful, some of the gearheads might already know this stuff.
I tried it and it works.
Here is the site.
Vintage Jeep Parts - Jeep parts and Jeep accessories for civilian and military Jeeps - Extensive On-Line Catalog.
At the top,click on Tech Library and then Wrangler YJ Trouble Codes.
There's alot of other good stuff on there as well.