92 Overheating, rad fan not working

I'm baffled. Car (92 grand caravan 3.3 litre) started overheating one day, hot and in traffic. Changed: thermostat, fan and temp sensor - fan still not running. Relay is good and has twelve volts going to it, but when I plug the fan directly into the hot side the voltage gets dragged down to zero - not even a flicker of fan movement. Fan runs when connected to battery.

Old fan was way hooped, ratted, seized and had a loose shaft. Is the computer in the car not allowing the new fan to run for some reason, i.e. thinks the old hooped fan is still installed? Or what? Yes, like other threads I've read I'm about ready to hard wire it to the fan and put a manual switch on it for those hot in traffic days, though I would really like to get it running properly. Any help greatly appreciated.

Antrobud

Check this thread out.
http://www.automotiveforums.com/vbulletin/showthread.php?t=237841