87MJJeep Posted April 14, 2017 Share Posted April 14, 2017 Back in the winter, I let my Comanche idle for a while while talking to a neighbor about - The Comanche. The guy is probably 20-something, and he has a 2016 or so Wrangler. It was neat that he was digging the Comanche! I hopped back in and freaked when I saw the temp gauge getting up to 260. I immediately got it rolling, and airflow dropped the temp down to normal in a hurry. The electric fan will turn on if I turn on the AC, so I know it's okay. I am fairly certain the sensor that grounds at a set temp has failed. Where is this sensor located, and are their any brands recommended to buy or avoid? Is a Jeep part still available through Mopar? When I swapped to a full gauge cluster years ago, both the oil sending unit and temp sending units I got from Autozone failed within a year (coolant read super low on a normal temp engine, and the sending unit was leaking oil out the backside!). I figured I better ask for brand advice before getting this even more critical sensor. Thank you. Link to comment Share on other sites More sharing options...
87MJJeep Posted April 14, 2017 Author Share Posted April 14, 2017 I can't figure out how to post a photo. I just snapped two from the driver side of the block from underneath. Is the temp sensor the smaller one that is located higher up? I assume the lower/larger one is the knock sensor. As low as these are, I assume it's best to just remove the sensor and screw the new one in asap rather than bothering to drain coolant first. (on a cool engine!) Edit: I just disconnected the smaller sensor's plug (2 prongs), and I ran a grounded wire from the battery and touched each connector in the harness' plug, and the fan would not come on. I did this with the key ON and even with the engine running. Does this sensor circuit operate by grounding the circuit when the sensor reaches the ON temp? Is there a relay upstream for this circuit that is separate from the AC's control of the fan? Turning the AC on still kicks the fan on. Thank you. Link to comment Share on other sites More sharing options...
omega_rugal Posted April 14, 2017 Share Posted April 14, 2017 the sensor is the small one, also coolant pours out when you remove it, the other is the knock sensor and only screws in there, nothing comes out the hole the "AC on" and the "high temp" conditions to operate the efan come from 2 diferent places in the ecu, one can work while the other don`t like in your case... Link to comment Share on other sites More sharing options...
cruiser54 Posted April 14, 2017 Share Posted April 14, 2017 the sensor is the small one, also coolant pours out when you remove it, the other is the knock sensor and only screws in there, nothing comes out the hole the "AC on" and the "high temp" conditions to operate the efan come from 2 diferent places in the ecu, one can work while the other don`t like in your case... This is totally confusing..... The fan SWITCH is located just above the lower radiator hose on the radiator. Key On, unplug the switch and jumper the connector on the harness side. Fan should come on. Report back your findings and we'll figure this out. Link to comment Share on other sites More sharing options...
omega_rugal Posted April 14, 2017 Share Posted April 14, 2017 ok let me re explain the ECU activates the solenoid by grounding the coil, i assume through a NPN transistor, the current to its base can come from 2 different pins form the ECU, one that the ECU enables when the AC is on, the other when the engine temp goes too high 2 diferent way to activate a relay Link to comment Share on other sites More sharing options...
cruiser54 Posted April 14, 2017 Share Posted April 14, 2017 ok let me re explain the ECU activates the solenoid by grounding the coil, i assume through a NPN transistor, the current to its base can come from 2 different pins form the ECU, one that the ECU enables when the AC is on, the other when the engine temp goes too high 2 diferent way to activate a relay Your assumption is wrong for Renix era Jeeps. Link to comment Share on other sites More sharing options...
omega_rugal Posted April 14, 2017 Share Posted April 14, 2017 then what is it? the fan work with ac on... unless the temp sensor or the A/D circuitry that translates the analogue signal temp to the ECU is defective, i can`t think of anythignelse right now... Link to comment Share on other sites More sharing options...
cruiser54 Posted April 14, 2017 Share Posted April 14, 2017 then what is it? the fan work with ac on... unless the temp sensor or the A/D circuitry that translates the analogue signal temp to the ECU is defective, i can`t think of anythignelse right now... It's a Renix. The coolant Sender is at the back of the head to work the dash gauge. The coolant temp Sensor is on the driver's side of the block to tell the ECU the engine coolant temp. The fan Switch is located in the lower part of the radiator to tell the fan to come on through a relay at about 208*. When the AC compressor cycles, it signals the fan to come on. The ECU only gets 1 signal and that's from the AC coming on so it can tailor the idle control for the load. Link to comment Share on other sites More sharing options...
omega_rugal Posted April 14, 2017 Share Posted April 14, 2017 so there`s a second sensor that controls the efan... mine doesn`t have it so i couldn`t tell... it HAD and efan but it was wired directly to the ignition as my MJ came from factory with no AC... Link to comment Share on other sites More sharing options...
cruiser54 Posted April 15, 2017 Share Posted April 15, 2017 It should have had an e-fan SWITCH in the radiator regardless of whether it had AC or not. Link to comment Share on other sites More sharing options...
omega_rugal Posted April 15, 2017 Share Posted April 15, 2017 It should have had an e-fan SWITCH in the radiator regardless of whether it had AC or not. it doesn´t, there´s is a spot where it shoudl be but is not even drilled... Link to comment Share on other sites More sharing options...
Airborne Janitor Posted April 15, 2017 Share Posted April 15, 2017 It should have had an e-fan SWITCH in the radiator regardless of whether it had AC or not. it doesn´t, there´s is a spot where it shoudl be but is not even drilled... Does your 4.0L have the closed cooling system using a radiator with no radiator cap and the coolant reservoir on the passenger firewall, or is it open with a radiator cap on the radiator? My 89 MJ was converted over to the open system before I owned it and the fan switch was removed, only the pigtail was sticking out of the harness near the airbox. I was able to find an aftermarket radiator that has a filler neck/radiator cap and the bung for the fan switch sensor so I can still maintain the fan switch with the upgraded cooling system. Mine had an electric-fan and no AC from the factory also. Link to comment Share on other sites More sharing options...
omega_rugal Posted April 15, 2017 Share Posted April 15, 2017 it was closed cooling from factory with no ac, the radiator doesn´t have a switch nor a place to screw one, some PO installed an efan but wired it through a relay actuated by the key on signal, said fan fell off it´s mounting last year so i remove it, so far the mechanical fan is doing well, except for the noise is unbeareable... Link to comment Share on other sites More sharing options...
Airborne Janitor Posted April 15, 2017 Share Posted April 15, 2017 it was closed cooling from factory with no ac, the radiator doesn´t have a switch nor a place to screw one, some PO installed an efan but wired it through a relay actuated by the key on signal, said fan fell off it´s mounting last year so i remove it, so far the mechanical fan is doing well, except for the noise is unbeareable... Is there a pigtail for the fan switch it around the airbox wiring harness? Should be a two prong weatherpack connector. Link to comment Share on other sites More sharing options...
87MJJeep Posted April 15, 2017 Author Share Posted April 15, 2017 the sensor is the small one, also coolant pours out when you remove it, the other is the knock sensor and only screws in there, nothing comes out the hole the "AC on" and the "high temp" conditions to operate the efan come from 2 diferent places in the ecu, one can work while the other don`t like in your case... This is totally confusing..... The fan SWITCH is located just above the lower radiator hose on the radiator. Key On, unplug the switch and jumper the connector on the harness side. Fan should come on. Report back your findings and we'll figure this out. Hi Cruiser, I jumpered the connector with a paperclip, and the fan came on. Does this confirm the switch is the problem? I have had on the back burner the task to replace the radiator and all coolant hoses after deciding if I keep it closed system or convert to open. If the switch is the issue, will it carry over to a new radiator that is open? Or should I get an OEM closed radiator and use one of those parts where I add a filler/cap between two sections of upper radiator hose? Is 12V running through that switch, or by jumpering it was I just grounding the circuit? Depending on radiator advice and when I will do the replacement/swap, I am considering running a temporary manual switch to the connector. Link to comment Share on other sites More sharing options...
cruiser54 Posted April 15, 2017 Share Posted April 15, 2017 Yes it does. There are radiators out there that take a cap and have the provision for the fan switch. I've purchased them at Napa. I'd have to study a diagram to verify if it grounds or has power running to one side of the switch. You could do that with the key on and probing the harness connector also. A recommendation for when you convert. Eliminate the heater control valve. Use 97 and later heater hoses. Napa 11038 and 11039. Get a coolant recovery bottle from Napa. 730-4514 Radiator cap 703-1698 Link to comment Share on other sites More sharing options...
Jeep Driver Posted April 15, 2017 Share Posted April 15, 2017 You could get an HO housing and insert a thermo switch and wire the fan independently. And forget about it. Your 12v should be running through a relay, not a switch. Your housing will look different than mine ........however........ Link to comment Share on other sites More sharing options...
87MJJeep Posted April 15, 2017 Author Share Posted April 15, 2017 Thank you for the conversion advice and switch confirmation, Cruiser. Thanks for the option, Jeep Driver! Link to comment Share on other sites More sharing options...
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