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Coolant Temp Sensor

54K views 27 replies 16 participants last post by  Steve Pick 
#1 ·
Just wanted to post a little FYI, because I had trouble finding this info here and in the matrix.

I had been having intermittent cooling problems in my 2008 for a little while.

It started with the coolant boiling in the overflow bottle after the jeep was up to temperature and I would park it. It happened after shut down most of the time.

It progressed along with colder temperatures to my dash gauge needle being at 1/4 when I could tell the jeep was up to temperature. Consequently, when the needle would reach the normal middle position, the jeep would be overheating. This would only happen if the jeep idled for more than say 30 minutes. Last saturday, while on the Toy run in downtown Chicago, it overheated to the point of coolant bubbling out of the overflow spout on the bottle. The jeep cooled down immediately back to 1/4 on the gauge as soon as I started driving again.

As part of all the troubleshooting I did, I replaced the thermostat, and the radiator cap. I burped the radiator multiple times. I reseated the thermostat housing a couple times, and finally looked up torque specs and did it one more time.

In the end, it was the coolant temp sensor. It lives right behind the thermostat on the block. $6 retail at the dealer, 10 minutes to change. As soon as I changed it and fired up the Jeep, It heated up and the gauge went to the middle as it should have. Went for a test drive and it stayed where it should. Idled it after the drive and the fan cycled, again as it should. What was happening - the damn $6 sensor was giving the ECM a false reading, so the fan was not activated when it should be. Hence, overheat on long idles, but not when driving since it is the winter and air passing thru was plenty to cool things down.

I thought about changing it at least a month ago. Wish I had done it then. Lesson learned.
 
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#3 ·
Does yor fan run as soon as you fire it up? my fan cycles way too much burned up the stocker replaced the temp sender, the fan unit and the circuit breaker but it never overheated on me. It still cycles way too much and only in high speed no low speed i think i have a gremlin no codes either.
 
#5 ·
No. My fan wasn't running enough due to the false readings. Now it runs when it should.

I suppose there is a possibility your sensor is delivering readings that are too high, making the fan run too much!
 
#14 ·
Another thing you guys can try if your having overheating/running ho issues is to remove you fan and shroud then hose out the radiator and a/c condenser from the back. Mine was running for about two weeks I did this and it fixed the problem. You wouldn't believe how much crap gets trapped in there took me almost two hours to clean it all out. Just be careful with the coils no high pressure just the pressure from the hose nozzle should do the trick.
 
#18 · (Edited)
Thanks for this thread. It may have just confirmed what I was thinking.
Silly system though. A failed sensor is to show over heating and cooling fan usually has a sensor on the radiator. Obviously the engineers of the 3.8l forgot that they should have some fail safe technology built into the systems to protect the motor.

Yeah I know very old thread but that is why old threads don't get deleted.

Part number for sensor to go here!

Update - Stealerships want $50 for the part not $6. I don't know where to go to get it at that price so I guess I will open my wallet as I need it now.
 
#20 ·
Actually there are several failure modes. If you lose the ac pressure signal or the ect circuit goes open/shorted the cooling fan will come on high. There is a strange logic where the iat, ect and ambient temp sensors are checked against each other and any two sensor signals can override the third if it deemed faulty.

Starting your JK can cause the fan to cycle on high, low or both, don't try and figure it out. A/C pressure, engine heat soak and transmission fluid temps all come into play. Some JK's do it and some don't. Make sure you have the latest software updates for cooling fan controls, especially on Pentstar JK's. Some Pentstar JK's were shipped with missing or wrong TIPM fan control software. Early JK's have updates to help not catching on fire.

If the ect sensor were to get out of range and read low temperature your somewhat screwed because it is within the acceptable range and won't set a fault turning on the fan; however it will normally throw a 118, 218 code(if i remember correctly) Turning on the ac can help because the fan will normally come low with the ac on(220 psi).

Remember this tip: If you have a fan control problem unplug the 3 wire ac pressure switch next to the compressor, this will command the high fan on till you get it figured out. This works on 3.8 and Pentstar JK's.

If you have a bad fan motor, voltage divider or relays they need to be fixed immediately. If you get mud in the condenser or fan motor their days are numbered. We replace lots of condensers, you can and should flush the condenser out on a regular basis but it is difficult.

Finally early JK's to about 2011 used a black plastic ect sensor, they are junk. In te last few years Chrysler upgraded all replacement ECT sensors to brass which are much better and have a low failure rate compared to the plastic sensor. The plastic sensor ran about $10 and the brass one is about 3 times that. With MoPar's policy to increase prices every month they may be more now. I've seen some MoPar parts double in the last two years, especially interior parts.






Thanks for this thread. It may have just confirmed what I was thinking.
Silly system though. A failed sensor is to show over heating and cooling fan usually has a sensor on the radiator. Obviously the engineers of the 3.8l forgot that they should have some fail safe technology built into the systems to protect the motor.

Yeah I know very old thread but that is why old threads don't get deleted.

Part number for sensor to go here!

Update - Stealerships want $50 for the part not $6. I don't know where to go to get it at that price so I guess I will open my wallet as I need it now.
 
#21 ·
Most excellent info.
Unplugging a pressure switch to get fan running all the time was the best tip. That would have helped me. I parked it until I purchased the switch.
My new switch is brass. Your explanation on price difference helped me feel more at ease about my $50 price tag.

Thanks MOTECH.
 
#26 ·
HI my next step is to replace the sensor but Ive replaced the water pump and thermo stat on my 08 wrangler what am I overlooking? the problem is is keeps reading on the temp gauge danger overheating. But the radiator isnt boiling the fan is working and overflow is cool to the touch and the radiator cap gets warm of course but could remove it bare handed and not blow out when i do. Any thoughts??????
 
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