Electrical problem
1999 Jeep Laredo 6 cyl Two Wheel Drive Automatic 96,000 miles

I just replaced the radiator in my 1999 Jeep Grand Cherokee. After everything was put together all was fine except the cooling fan would not run. I turned on the AC and let it heat up to operating temp and still wouldn't work. I checked the voltage coming from the relay connector to the fan and had 12.8 volts. I bypassed the connectors and still it would not operate. I ran it to the battery and it ran fine. What could be the problem?

check the Cooling Fan Control (Electric Fan Thermostat) also check the Coolant Temperature Sensor theyare what makes the fan turn on

I have looked at the relay in the PCM, the relay under the passenger side headlight, but I have not heard of the electric fan thermostat. Where is it located? Also, if
it has 12+ volts at the fan connector what else is required for it to come on?

ok now do you have 2 fans 1 electric and 1 mechanical or do you have just the electric fan reason asking you can put a mechanical fan on it

Electric fan only. I checked the leads from the relay and had 12.8 volts. When I hooked them to the fan leads I had no voltage. If there was a short in the fan would it cause the relay to break the voltage?

check the continuity in the wires going from the relay to the fan you may have a break in one of the wires and not letting the current through

Attached alligator clip leads to relay leads and had 12.6 volts. Took off connectors and directly wired relay leads to fan. No voltage. Hot wired fan to battery and it runs.
Something (Relay?) is getting kicked when the fan is connected to the relay leads.

you said you changed the relay correct? if so check to see if you are getting voltage in wires from relay to fan