per the 2003 service manual, it is controlled by the PCM(?) to a SSR.
I don't have a oscilloscope at home to verify it.
You are totally correct here!
I eventually found in the 2003 KJ Service Manual a very brief description of the Radiator Fan Operation:
There is a Solid State Relay for the fan located near the front left fender.
Pin #1 on this Relay gets +12 volts from Fuse #2 40 Amp in the PDC, Pin #3 gets Ground, Pin #2 supplies pulsed +12 volts to the Fan Pin #1. Pin #2 on the Fan goes to Ground.
Pin #4 on the Relay gets its control from the PCM C2 Pin #12.
The Relay then supplies Pulsed Width voltage to the Fan ie. for Slow Mode it will supply a short pulse of +12 volts and a longer period of no voltage. For medium speed the ratio will be something like 50% ON and 50% OFF and probably at maximum speed the PCM turns the Relay on full time. Like turning a light switch ON/OFF rapidly!
I can only find this system in the 2003 Manuals.
My experience of these Solid State Relays in Computer and Peripheral applications is that they work well in controlling speed but they can break down slowly which cause weird problems.
So this is not relevant to the 2004 KJ in this post but for 2003 3.7 L KJ Fan problems this relay should possibly be replaced!
So I have learnt something new here today...I have not worked in this area on the 2003 3.7 L KJs. Interesting point in the Description of the Radiator Systems they describe the Diesel Engine, the 2.4 L Gasser, the 3.7L Gasser and a 4.0L Gasser!!
Probably lifted off the Manual from one of the other Jeep Models...not unusual!