usually its the CPS-the crankshaft position sensor, that gives people fits when they have a no start condition. Cute little sensor, mounted on the bellhouseing that tells the computer that the engine "is indeed turning over, add spark".
One of the funny tests/fixes people use, is to unplug it, and then plug it right back in- the theroy behind it is spooky- the sensor is a glorified hall effect style magnet- it sits in relation to the flywheels ring-flywheel turns, magnet senses the gaps in the ring, computer gets the OK to fire the plugs. When the sensor "magicly dies", it, in theroy, lose`s its magnatisim...unplugging, then replugging the connector supposedly wakes it back up....and signals a trip to the autoparts store for 2 new ones...one for the glove box, one for the bellhouseing....because there aint no autoparts store around when you only have one...and its bad.....and its snowing....and dark.
Your situation sounds like that....but without testing it, theres no real way to know for sure. You need a good OHMS/Volt meter- they should come stock with the renix jeeps(87-91) because you can test 99% of everything with one-
I`d go by the numbers-even if yours seems like its in a different area, since its more than likely ignition related-
test the CPS, the TPS- check to see if it has a functioning NSS-(nuteral safty switch-)- followed by the ignition box under hood, then on the lower column. The TPS isnt really a factor in starting, but i dont know if a bad one would kill the spark or not. The ignition dosent really go through it, as much as it uses it after the fact.
Once you own a renix for some time, you get to know these areas like second nature- when something go`s wrong, you`ll already know what areas to start testing- thats what makes the renix jeeps so good in my opinion-their easy to test and check once you know the tricks. Its not something super complicated like the newer jeeps with OBD11(and above) supercomputers
, nor an old jeep that you need a carburator degree to get to run because the jets are off a hair, and the condensor looks like a roasted mashmellow. You`ll not have to take a renix into the stealership like a new jeep...the sensors are basicly dumb, the code storage is minimal at best, and when things go wrong, you can fix them.
side suggestion- he`s got a 90? check the fuel injectors for leaks- its about time for that to start. Check ALLDATA for the recalls, because I know hes got a few comeing to him if someone else hasent done them yet. Go ahead and put aside money for a new fuelpump if its the original-and a radiator, but think about converting to the open style...