Ok to start, I have cracked a few radiators in the last few years. I realize that is not completely abnormal. Today I was working on my ZJ when I saw first hand what is happening. There is some kind of problem with my e-fan setup. It will not turn on when the A/C is on. So, what happened was, at idle with the A/C on, all of the sudden the A/C compressor was engaged, and struggling to turn. Almost immediately after, the radiator started spraying freakin' hot coolant all over.
My assumption is that without the fan to cool the evaporator, pressure (and heat) build up quickly and cause this problem. This contributes to, if not causes, the radiator cracking problem. Well, my e-fan setup is almost exactly stock from the '98 5.9L. I have the dual coolant temperature sensors in the radiator hoses to the high/ low relays. My problem is that I can't figure out what is going on electrically with the A/C clutch signal to turn on the fan whenever the A/C is on. It seems as though the signal is high (+12v) while the clutch is disengaged, and low (0V) when the clutch is engaged. Specifically, I measure the PCM A/C clutch out at +12v whenever the ZJ is running, which to me just sounds stupid. Maybe it's a problem with the A/C panel itself, or does that signal goto the A/C panel after the PCM and accumulator switch.
This is following the wiring diagram in the crappy Haynes manual. I'm hoping someone else has had this problem and could possibly identify a wire color in the harness or otherwise confirm or deny what I interpret as happening...