My Cherokee Overheats When AC is On

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This is a letter I received recently from a woman who getting tired of her 1998 Jeep Cherokee overheating:

Hello! I own a 1998 Jeep Cherokee SE (not a Grand) 4.0L I6 with 4WD. The consistent problem I am having is with the cooling system, and I am wondering if it is just the car or if something can be done about it.
Basically in warmer temperatures, I cannot use my air conditioning or vent air for more than a few minutes before the engine overheats, especially when I get off the highway where I have been running the air. I try to stop before the warning light comes on since I can begin to smell the coolant before that point. When I get out of the car, the coolant is dripping out underneath the car (can't pinpoint from where - everything under the right front side of the car seems to covered with coolant). Under the hood, the overflow tank is also overfull/backed up. I usually wait until the car cools off a bit and add a liter of coolant mixture to the overflow tank after it empties. That seems to be enough to run the car back to wherever I need to be. I have only had to have the car towed when one of the radiator hoses burst.

I have had a coolant flush and both radiator hoses and my thermostat replaced. It has not seemed to make a difference under these conditions so I have resorted to not using my air since I have been stuck more than a few times. Do you know if this is just a typical problem with this Jeep model/engine? My current mechanic isn't sure what else to do other than replace major stuff so I'm open for any suggestions for steps to take before getting to that expensive point.

Emily A. - Ann Arbor, MI

To answer the first and easiest question - No, this problem isn't just "your car" and you don't have to live with it! You've taken the right path in terms of fixing maintenance parts like hoses and the thermostat first, but with a little troubleshooting you might have found your problem a little sooner. I'm not one to knock other mechanics, but if your mechanic hasn't checked the electric cooling fan for proper function then he is remiss. ​​

All of your symptoms point to an electric cooling fan that isn't working. When you're traveling at high speed, there is enough air passing through the radiator to keep everything cool on its own. But you slow down and so does that air, which is why they added an electric fan to help out. My guess is you either have a bad fan motor or a bad sending unit. This is very easy to figure out. The fan itself can be tested directly by running a jumper to it (this bypasses all of the switches and adds electricity directly to the fan.) If it comes on, your sending unit, almost always attached to the radiator itself, is bad.

My money would be on the fan, and here's why. Your electric fan is activated in two different scenarios. The first is based on your engine's coolant temperature. If it reaches a certain point, the fan comes on to cool it off. The second time the fan comes on is with your AC. Your air conditioning creates extra work, and heat, for yur engine. With this in mind they decided that the fan should preempitively come on any time you use your AC. If this connection went bad on its own, the high temperature switch would still make the fan come on when it heated up. Since nothing is making the engine cool down, I'd say the fan isn't working. But like I said, a little diagnostic work will save time and money, so be sure your mechanic goes through the motions.