What's wrong with my car's air conditioner?

[zenith]

@Rick

There are a few errors in your post zenith.
First off as I said before the A/C system does not use coolant.

Secondly sweating or frost is largely a function of humidity. If you are in LA in July (or Phoenix, or any number of other places with low humidity) you will see little or no frost. The presence or absence of sweating is at best a very poor indicator of system performance.
Low pressure does not always mean low refrigerant, it means a low temperature at the evaporator. Temperature and pressure are directly related. R-12 was almost exactly 1:1, pressure equaled temp, so if your low side pressure was 28PSI you evaporator temperature was 28F. R-134 is very close but not exactly 1:1 between temperature and pressure. Anyway with a jumper in place, the lowest pressure you will see is a function of air flow across the condenser, humidity, and ambient temp. I have seen 4 PSI on a completely full system with the switch jumpered and the cooling fan running.
R-134 fitting do not look anything like a tire fitting, and I don’t think I have ever seen a pencil type gauge for that type of system. here is a picture of a R134 service valve (3rd one down). It looks more like an air line quick disconnect than it does a tire valve. Also the schrader valve is recessed on a R134 fitting, and all the connectors I have seen for R134 have a screw wheel to extend or retract the schrader depressor like this one
Not at all like a tire valve.

Sorry, I misread the OP. In a 1998 Jeep, the refrigerant used would be R134. My reply referenced pressures and fittings used prior to Chrysler’s 1993 changeover from R12. I actually found a bad REFRIGERANT pressure switch in an R12 system in a 1991 Chrysler product using the method I detailed, but R134 IS a different animal and does use different fittings and thus my old tire-valve type guage won’t work. Also, you’re right that very little sweat/frost will form at low humidity. As a resident of a place that’s nearly 100% RH nearly 100% of the summer,I forget that some people from arid climes actually think that 95 degrees@ 10% humidity is hot while I think it feels no worse than 70 feels back home.

Bad editing left one mistaken reference to “coolant” when I meant “refrigerant”.

I need to more closely RTFQ and more carefuly edit replies in the future.