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Overheating 2001 Jeep

A while back I was driving back home from work and had to go through a very congested tunnel, long story short, I found that my car was over heating to the point that I had to pull over as soon as I got out of the tunnel to let it cool down. Once I got home my dad and I looked everything over and saw that the electric fan would not turn on when the temperature started to increase. Something similar happened to us previously and the solution was to replace the fan relay located near the right fender. Replacing this did not solve the problem. We checked to see that the fan wasn’t the problem by connecting it straight to the battery and it worked just fine. The only other thing we could thing of was to replace the temperature sensor located near the thermostat. So we replaced the sensor and the thermostat just in case. It seemed to have been fixed until I drove through the tunnel again and it overheated, not so much that I needed to pull over, but it was definitely climbing above normal operating temperatures. We have also replaced the fuse for the fan that’s located in the power distribution center. My dad says he’s idled the car a few times with mixed results: He’ll idle the car with air-con on and the fan will turn on immediately to help with cooling, he’ll then try it again a while later and the fan won’t turn on causing the temp to rise. We’re really stumped by this one, we’ve replaced pretty much every component we can think of and it still isn’t working reliably. The only other thing I can think of is that the fan has some circuitry on it that may be damaged, but I feel that is unlikely.

Car description:
2001 Jeep Grand Cherokee Laredo
~136K miles

Parts replaced:
-Fan relay
-Fan fuse in power distribution center
-Water temp sensor

Any advice would be greatly appreciated,


" the solution was to replace the fan relay located near the right fender"

This doesn’t sound right. The Grand Cherokee uses a solid state fan relay mounted on the right frame rail below the right headlight. The fan relay is difficult to access. The fan relay needs to be bolted to a metal suface to absorb heat from the relay. If the fan relay is left loose the fan will stop working when the relay gets hot.

You seem to be on the right track. There’s a lot of things that can cause a car to overheat. But if the fan isn’t coming on when the temp guage is reading “high” fixing that is the first place to start. Here’s a couple of ideas to try

  • The second temp sensor you installed might also be faulty. You may just have had bad luck. Or you’ve accidentally replaced the wrong temp sensor. There’s usually several temp sensors on cars. I’m not familiar w/your make/model, but usually the fan temp sensor is a simple two-terminal temp activated switch that screws into the waterjacket somewhere. When the switch turns on (due to the water temp raising to a high enough level), the switch closes and either grounds the fan circuit directly, or it grounds a relay selenoid, which in turn, turns on the fan. In any event, if you idle the car in the driveway, the fan should eventually turn on. On my car it takes 5-10 minutes of idling. You can watch the das board temp guage too. If it get 3/4 of the way up the scale and the fan hasn’t turned on, something is probably wrong.

Testing the temp sensor is usually pretty easy. Take it out of the car and into your kitchen. Put it in a pan of water with a thermometer, and turn on the heat. Using an ohm-meter, verify the switch closes as the water heats up. I think the turn on point for my car anyway is around 180 degrees. Your shop manual will tell you what it is for your car.

  • There’s a chance the wiring to the fan or associated circuits is faulty. The most likely would be a connector. Often time there’s a connector near the fan, and it’s located in a spot that battery acid will drip on it if the battery is leaking. That’s the first place I’d look. If not, get out the schematics and probe your way ohmically through the wiring to see if you’ve got good connections throughout the circuit. If you don’t already know how to do this, don’t have access to the schematics, etc, take the car to a shop that does.

Not sure of your Jeep, but most cars turn on an electric fan to cool the radiator all the time the AC is on. Does you car have only one fan? Most have 2 fans and some have 3.

Your problem is when the AC is not on, meaning the temp sensor could be bad, that relay, or even one of the fan motor(s) is bad. You are on the right track, keep at it.

2 suggestions:

  1. Check to make sure that the engine has plenty of coolant
  2. It sounds like you may have a bad water pump.

When I said the relay next to the right fender I did mean the one that’s behind the headlight and terribly difficult to access, I was just being general. I didn’t know that it needed to be attached to the metal fender to dissipate heat as you said so I’ll make sure to check tomorrow.

Also, I don’t think there’s anything wrong with the water pump or coolant level because as I described, sometimes it will work without a problem keeping the engine at normal temps with A/C on. I would be almost certain that it’s something with the fan or some of the sensors that are controlling it. We’ll check the mounting of the relay tomorrow.

I’ll also make sure to check to see if any battery acid may have corroded the wiring between the fan and the relay because I think the wires may actually pass under the battery.

Thanks for all the advice, I’ll report back anything I find.

As soon as I asked about the relay being properly secured to the frame rail, my dad mentioned that the first time we had to replace it, he noticed that there were 2 screw needed to hold it down but one was missing. He though nothing of it and just used the 1 screw as before.

Yesterday he found a matching screw and secured the relay properly. He tested it out afterwards and so far it looks promising. He’ll test the car more vigorously later today. I really find it funny how often puzzling problems often have simple answers.

Thanks for all the help,

It looks like @Nevada_545 nailed this one. Experience is proved to again be the key to the sol’n. This is why the Car Talk forum is so helpful – there are a lot of experienced mechanics who volunteer their time to help others here.

What you describe is not a relay, but a fan control module, and yes it does need a good ground and being mounted directly to metal helps the heat sink work.