I bought my 08 jk rubicon new in 08. It has 54k miles on it. Mostly because every time I drove it it broke down and id feel really bad dumping it on some unsuspecting kid... biggest pos I've ever owned and I've had some with 400k on them

But anyway, what tells the fan to come on. I know the fan is good. I can command it to come on via my Launch tool or I can hit the AC button and make it run. I also know the coolant temp sensor is good because i can watch it on my scan tool. Plus I put a new brass Mopar OE one in it.

 

Just some thoughts. It sounds like you can make the fan turn on which shows the wiring seems to be OK. I assume since you use a tool to turn on the fan the wiring from the ECU to the fan must be good. Can you monitor the signal for when it starts to get hot showing the temp sensor triggered the signal to the ECU. I know this is obvious but the fan won't come on right away unless the temp goes up which may take a few minutes while idling. If the fan doesn't come on while idling it will probably overheat.

 

Yeah. I was watching the live data from the temp sensor. I also had the fan command state pulled up. I let it hit 210 and both the command hi and Command low stayed off. The ecu knows the temp is high enough. But for whatever reason it don't change the command. Each vehicle is different and I thought maybe there was a secondary sensor that may be bad somewhere. Someone who knows the 3.8 better than me might know.

All the wiring,relays etc etc have to be good or I couldn't change the Command and make it run. Ive worked on enough Mopar at my shop to not underestimate the TIPM causing this. But the pcm/ecu isn't commanding the fan on

There is a sensor going in to the waterpump that I'm not sure what is. Pretty sure it's cam position though. The coolant temp sensor goes into the intake on the opposite side.

Everything else is good. I have a full shop. I put dye in the coolant. No leaks. I did leak down test, less than 2% on each cylinder. I did combustion gas/ block test. No problem. Just the fan won't command on

 

I don't have the wiring diagram but I think the TIPM is involved as well. I am in-between homes so I can't access my books.

 

 

It looks like the fan should have turned on. I am not sure if the .72 is correct or not. If the sensor is new, the wiring is good (sensor out and PCM out) and you have tested the fan itself then it seems like the PCM is not signaling the fan to turn on. Curious, does the temperature display move up to reflect them temperature? I think that comes from the PCM which would show that the sensor is sending the signal to the PCM.

 

Yeah. The sensor seems to be correct. It also shows the same temp as the old sensor I replaced. The old one would jump around a bit more. That's why I changed it. The fan didn't work with the other sensor either. Also when I trigger the fan via the AC the temp goes down to 190-ish like it should so I believe its right. The pcm is seeing the 200+degrees so the wiring must be fine too. But yeah I think the pcm is getting the correct temp. But for whatever reason isn't sending a command to start the fan. I dont know why. On the picture if the live data i posted the fan should be commanded on. But isn't. And yes the temp guage is showing correct

 

Something fishy about "Engine Coolant Temp Voltage". First the range high limit is 20V on a 12V vehicle - that is not the end of the world though and is common in industry due to accuracy not requiing proportional operational range scaling from absolute low to absolute high. At 0.72V for 212F in this case a 20V read would be in the thousands of degrees whereas the range max is 375 which should be 20V - that is what is on the reader. A reading of 0.72V at what is getting to be a dangerous temperature seems out of range to me. The fact that the AC turns on the fan tells me that side of the program in the ECU is working and it is very rare that an ECU will simply delete code by itself if even at all. Have you verified and cleaned grounds from battery to body and body to engine? A good starting point.

 

That makes sense as well. Grounds are a typical problem for intermittent issues but it comes on with the AC, odd?

 

But the temp sensor does not turn on the AC. The sensor issue and scaling is what I am driving at here. Great input tho'