My grand cherokee is overheating..

it sounds like the problem is with your fan not running. I would try and see if you have power at the fan and test the fan to see if it isn't bad.

Only overheating at idle while standing still is most probably a cooling fan problem. If you have the electrical cooling fan, then first check its fuse. If ok, than there are two separate relays (high speed and low speed) in the power distribution center under the hood. (this is for 2004 model only). Swap both relays, it can be that the low speed relay is bad. If this helped, than you will have to replace the bad relay. If this didn´t help, then put 12V directly on the fan motor (watch hands and loose clothing) to be sure the motor is not bad.

Hi there:
DTC P1491 - RAD FAN CONTROL RELAY CIRCUIT

DTC P1491 is for the cooling fan relay control circuit as you've found already. Start by checking the fuse for the fan relay. Its in the box under the hood, a 40 amp fuse.

If you've got a couple jumper wires, you can unplug the fan connector and carefully jump the fan to see if it works. The fan connector is on the right side of the back of the fan shroud. Unplug it and jump the black wire to ground and the green to positive. You've got to be very careful doing this though-you are in very tight quarters, its not hard for your jumper wires to touch each other in the connector and short together, and if you use small diameter jumper wires they will get hot quickly when the fan comes on.

If the fan doesn't come on when you jump it, or if it runs slow or sounds bad, you've got a bad motor and will need to replace the fan assembly. One thing that does happen somewhat often, is a bad fan motor will burn up the relay. If the fan motor is drawing too much current it will overload the relay and burn it up. This is why its so important that if the fan motor runs slow or makes noise that you replace it, or you may end up burning up relays frequently.

If the fan runs and the fuse is good, its very likely you just need a fan relay. This is a pulse width modulated relay, not the on/off clicker type you'd find in the fuse box. This unit is capable of infinite speed control. It gets very hot in operation, so its mounted to the body sheet metal to allow heat to dissipate through the sheet metal. The relay is mounted under the right headlamp held down with two 8mm screws. To get to it you have to remove the front bumper cover and headlamp bracketry to access the relay. These relays are pretty common.

Check and test this possible causes:
radiator fan ctrl rly circuit wire harn intermittent defect
radiator fan ctrl rly circuit wire harn observable defect
radiator fan relay ground circuit open
rad fan relay control circuit open
radiator fan relay control circuit short to ground
rad fan control relay defective
PCM defective

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You probably have an electric radiator fan that isn't working properly. First thing to check would be fuses, then swap relays if no blown fuses are seen, and finally check at the fan connector for power. If everything is good, you need to replace the fan motor. It isn't very fun trying to figure out how to get the fan shroud/fan out. The 4 bolts holding it to the radiator are in the worst spots and it will prob take an hour or so to get the bolts out before you realize that it's super tight in there. Good luck

Check the cooling fan motor and of course the fluid level. If the temp is changing thru the heater it could just be low on coolant , leaking from ? Two wheel drive have an elecrtic cooling fan only, make sure its spinning up when temp is 3/4. If not its probably the fan motor. Can be checked with a test light .

have you changed the thermostat? is your fan kicking on? if your fan is not kicking on you need a temperature sensor.

have you checked the current with a test light? what about the temperature sensor? the fan will only come on when the temperature gets to a certain point.. try checking the temperature sensor, maybe plug came off, but you should get check engine light if that was the case. fan motor might have gone bad.

IT COULD BE THE FAN FUSE OR FAN RELAY, IF THE FAN IS DRIVEN BY THE ENGINE TRY CHECKING THE FAN CLUTCH. IF THE FAN SPINS FREELY WHILE IT IS COOL, THEN WAIT FOR THE ENGINE TO HEAT UP AND SHUT THE ENGINE OFF AND TRY TURNING THE FAN BY HAND. IF THERE IS LITTLE OR NO RESISTANCE THEN REPLACE THE FAN CLUTCH. IF THE FAN IS ELECTRIC THEN REPLACE THE FUSE AND RELAY LOCATED UNDER THE HOOD IN THE FUSE PANEL. HOPE THIS HELPS. PLEASE KEEP ME POSTED. THANKS FOR AN HONEST RATING AND FOR USING FIX-YA. BEST OF LUCK.

It sounds like you have changed everthing already. It could be an airpocket in the cooling system.

You can test the system by disconnecting the engine temp sensor that turns on the fan, the engine computer will go into failure mode and turn on the cooling fan, if the fan does not run check for voltage at the fan with a 12 volt test lite..