HELP!!! Changed Alternator, Engine turns over but won't start

Hi,

Over here from the Short wheel base forum. I did a favour for my sister(always regret favours for family) and changed the dead alternator in her 91cherokee 4.0l. Now the engine cranks but won't start, it's turning over quickly, so I know the battery still has enough charge. I disconnected the battery over night as well just incase the computer had a code stuck in it or something. Tried it this morning and still the same thing. No check engine light or anything?

Any ideas?

Thanks in advance for the help.