Yep, a standard relay can not be substituted for a PWM relay that the WJ uses. Hooking up a regular relay in place of a PWM relay will create a firestorm inside the regular relay and burn it out in very short order.
Think of pulse width modulating as basicly it turning on and off really fast. Doing this, can allow the fan to speed up slowly and avoid the sudden high amp draw when it first kicks on and speeds up like if you were using a regular relay. And also can be used to control its speed.
'93 YJ 4.0L, AX-15, NP-231HD/SYE, 8.8in rear, OME 2.5 lift, home made front hub conversion, hydroboost. Warn M10000 winch. Currently building a stroker.
'79 Cherokee Chief, 401 MPFI fuel injection, T-18/D-20 and hydroboost. Otherwise stock.