1988 Jeep Wrangler hot engine cold radiator

I have a 88 jeep wrangler with a serpentine belt. However I think the engine a 4.0 might be from a cherokee possibly 93. After the mechanic chaned the waterpump recently it seems like the engine gets really hot and the radiator stays cold until I turn off the engine, then the radiator gets hot also. There is no thermostat in the car. I was wondering if the waterpump might work against the current due to the serpentine belt. if so is there anything I can do with the waterpump or do I have to change it completely? I just changed the temp sensor and it gave the same hot reading on the gauge. the radiator is brand new.