I have a 2002 Grand Cherokee Loredo 4.0. I noticed is over heating when I put on the AC.
1. If I'm idling standing still it starts to overheat going to the red. The Anti Freeze starts to bubble up and the hoses starts to get really hot that you can't touch them with your hands.
2. If I'm driving on the highway I get cold AC air.
3. If I don't put on the AC the Jeep won't overheat at all.

Replaced new this week
1.Water pump
2.Fan Relay sensor
3.Thermostat
4.Heat sensor relay on top of the thermostat housing.
5 Radiator Cap
6. Radiator

I noticed today when I turn on the AC the fan will turn on for a few seconds and then turn off. Turn back on after a few seconds later it will start up again. I took out the fan and ran it directly to the battery it ran well directly to the battery.

 

Hiw is your coolant mixed... 50/50? Also does it overheat at idle with the AC off? Check your fan clutch and make sure it's still good.

 

My coolant mix is fine I'm not losing any coolant. It don't over heat when AC is off. But once I put on the AC it will overheat and I can see the needle on the temperature climbing.

 

His WJ doesn't have a fan clutch, only an electric fan.

What fan relay SENSOR did you replace? Did you replace the fan relay itself? Here's a picture of where it is;
It's underneath the headlight mounting panel.

 

Hey Dave Yeah I replaced the one under the passenger headlight the one you displayed. My jeep been eating these relays up like cookies. I even bought the dealer one that cost me 80 bucks. It lasted me a few months and then they go bad. Chrysler has a known issue with these and until today they haven't done anything about this problem. I replaced that one in your pic and the sensor that sits on top of the thermostat housing. I am thinking that maybe I am having a problem with the AC because that is when it over heats. I can see that the compressor turns on not sure if I have a AC leak some where or maybe the vacuum bottle has gone bad. But if I don't put on the AC the Jeep don't overheat.

 

If you're toasting relays, your fan motor is drawing too many amps. You may need to replace the fan. The fan runs on high speed whenever the A/C is on, otherwise it runs slow until the temp gets above 210*, then drops down to low speed at 200*. My fan relay was the original one until 170K miles, then it crapped out. Buy Mopar parts for replacement of any sensors and relays. The quality is much better than ANY aftermarket parts.

That temp sensor on the thermostat housing is for the gauge. However, the PCM reads it's temp and controls the fan relay accordingly. That damn PCM does just about everything in your WJ! It's actually the alternator voltage regulator as well, controlling both voltage and charge rate. It also communicates with the Body Control Module, the Transmission Control Module, and the ABS and airbag systems. It has as much computing power as a PC! It's evolution started just controlling spark timing and fuel mixtures, but has gotten more complex as time went on.

 

One thing i did notice is the fan looks like it ain't running at high speed. Once I put it directly to the battery you can hear the fan at full power. My mechanic told me that the fan I have is fine. He said the problem I am having is coming from the AC it can be a leak some where or maybe I don't have AC gas. I'm going to go pick up another dealer relay tomm. Maybe this after market one is not pushing out the right signal to the fan. I'll keep you updated on what happens because I know there are plenty of people who probably has the same problem as me. I'll go pick up a new fan as well to see if that is the problem. If is not at least I added a new part in the truck one less thing to worry about for the now.

 

It's true you may not be getting a fan signal from the A/C compressor. I believe it comes from the clutch engagement. IDK if THAT goes thru the PCM or not.

JMHO, but don't buy a fan until you check it out. They are expensive. At my local P&P, there isn't a single one to be found.

 

Hey Dave thanks for your help. It was the fan after all. I had a feeling it was because I wasn't getting the power I usually get in the past. I put the AC on let it idle for about 30 minutes and it didn't over heat. I hope this post will help other as well who has the same problem as my 2002 Grand Cherokee Loredo.