TNCave Posted December 15, 2013 Share Posted December 15, 2013 OK, so I have my new clunker that I'm getting back into shape after years of abuse and neglect. I'm new to MJ's, and jeeps really, but when I bought the truck, I opened the hood and noticed one of many "jumper" wires is one running from the ballast resistor to the cooling fan. I had spent many hours reading this forum before I bought the truck, and knew that the ballast resistors were notorious for going bad and assumed it was a bad ballast. When I got home, it dawned on me that the fan should be thermostatically controlled, so it must be the opposite, running the fan off the fuel pump ballast circuit. Sure enough, that's what it is. So my fan is on all the time. I was at the Nashville Pull-a-Part today and noticed three other jeeps, one Comanche and two Cherokees that all had "jumper" wires feeding the electric fan from a variety of other circuits. I'm assuming this is a common issue with Jeeps of this era, is there a common part that is to blame. I haven't dug too deeply yet, but I'm assuming there should be a thermal switch that turns the fan off and on. Perhaps that is a common part to fail, but I can't figure why people wouldn't just replace it if that's the case. Any help with this would be great. Thanks Link to comment Share on other sites More sharing options...
relyt120 Posted December 15, 2013 Share Posted December 15, 2013 the temp switch for the fan is in the radiator and it switches on a relay , also the ac activates it too. Many people wire up a relay and switch to the dash to manually control the fan if they don't want to deal with fixing the factory wiring. Furthermore, if you ever swap a renix radiator it is common that the replacement won't have the bung hole for the fan switch so some people get an HO t stat housing and run a switch there instead. I like the manual switch myself. good luck! Link to comment Share on other sites More sharing options...
Eagle Posted December 15, 2013 Share Posted December 15, 2013 I'm assuming this is a common issue with Jeeps of this era, is there a common part that is to blame. I haven't dug too deeply yet, but I'm assuming there should be a thermal switch that turns the fan off and on. Perhaps that is a common part to fail, but I can't figure why people wouldn't just replace it if that's the case. Any help with this would be great. No, this is not a common issue. In fact, after owning and playing with XJs and MJs since I bought an XJ new in 1988, I have never encountered one with a jumper to the auxiliary fan. I know that some owners install a toggle switch in the cabin so they can turn on the fan manually when wheeling, but that's generally a matter or preference rather than necessity. The fan is controlled by a relay that should be located on the inner fender, next to the air intake box. More than likely the relay is messed up and the previous owner was too lazy or too stupid to fix it properly. Link to comment Share on other sites More sharing options...
Rohls Posted November 1, 2014 Share Posted November 1, 2014 I'm assuming this is a common issue with Jeeps of this era, is there a common part that is to blame. I haven't dug too deeply yet, but I'm assuming there should be a thermal switch that turns the fan off and on. Perhaps that is a common part to fail, but I can't figure why people wouldn't just replace it if that's the case. Any help with this would be great. No, this is not a common issue. In fact, after owning and playing with XJs and MJs since I bought an XJ new in 1988, I have never encountered one with a jumper to the auxiliary fan. I know that some owners install a toggle switch in the cabin so they can turn on the fan manually when wheeling, but that's generally a matter or preference rather than necessity. The fan is controlled by a relay that should be located on the inner fender, next to the air intake box. More than likely the relay is messed up and the previous owner was too lazy or too stupid to fix it properly. ...maybe a silly question: I have a 92 MJ...could you post a pic of where t cooling fan relay is? I don't think my fan is working and believe the PO may have thrown this relay away... Where is the temp switch that activates the cooling/aux fan? I've heard radiator, but a diagram/photo would be nice. I've installed the relay for the cooling fan, but don't think it changed much, so I'm looking at other options. ALso, does the AC motor have to function for the fan to turn on....? All the bits are there for my AC, but unplugged the power to the AC clutch because it needs a coolant refill and I don't need it anyway. Thanks! Link to comment Share on other sites More sharing options...
Eagle Posted November 1, 2014 Share Posted November 1, 2014 The discussion in this thread was about a '90 MJ -- that was a Renix design, with AMC/Renault electrics. Your '92 has a completely different wiring system, designed by Chrysler. On the '87-'90 Renix system, the aux fan is controlled by a temperature switch in the driver's side radiator tank, which sends a signal to a relay on the driver's side inner fender, which in turn controls the fan. There's a second feed from the A/C circuit that turns on the fan when the A/C is operating. On the HO models, the temperature sender is located in the thermostat housing rather than the radiator tank. The sender transmits a signal to the ECU, and the fan is controlled by the ECU. I don't know if the HO aux fan has a relay. Link to comment Share on other sites More sharing options...
Rohls Posted November 2, 2014 Share Posted November 2, 2014 The discussion in this thread was about a '90 MJ -- that was a Renix design, with AMC/Renault electrics. Your '92 has a completely different wiring system, designed by Chrysler. On the '87-'90 Renix system, the aux fan is controlled by a temperature switch in the driver's side radiator tank, which sends a signal to a relay on the driver's side inner fender, which in turn controls the fan. There's a second feed from the A/C circuit that turns on the fan when the A/C is operating. On the HO models, the temperature sender is located in the thermostat housing rather than the radiator tank. The sender transmits a signal to the ECU, and the fan is controlled by the ECU. I don't know if the HO aux fan has a relay. Doh! :) Thanks; I should have known. Temperature switch and thermostat are brand new (100 miles). So I just hot-wired my fan...and it spun up fine. I checked the relay receptacle and it is getting 12V with the key on ACC. Last thing I can think of is the relay is bad...swap relays? But then how do I know it's functioning? Should I 'hot wire' the relay by connecting the switch outputs to the fan...? Link to comment Share on other sites More sharing options...
Eagle Posted November 2, 2014 Share Posted November 2, 2014 I checked the relay receptacle and it is getting 12V with the key on ACC. Last thing I can think of is the relay is bad...swap relays? But then how do I know it's functioning? Should I 'hot wire' the relay by connecting the switch outputs to the fan...? Which terminal in the relay socket did you test? Remember -- a relay is an electrically controlled switch. For the relay to be "ON" there has to be 12 volts to the input of the control side (terminal 85 or 86), and 12 volts to the input of the load side (terminal 30). Link to comment Share on other sites More sharing options...
HOrnbrod Posted November 2, 2014 Share Posted November 2, 2014 If you located the aux fan relay you know it's in the PDC (Power Distribution Center) on the right side engine compartment. The relays in there are all the same, so swap out the aux fan relay to test. Does your 92 have A/C? If so, turn it on and the aux fan will run if all is well. The only other way the aux fan will run w/o the A/C running is if the coolant temperature reaches ~230* or so to prevent overheating. Link to comment Share on other sites More sharing options...
Rohls Posted November 2, 2014 Share Posted November 2, 2014 If you located the aux fan relay you know it's in the PDC (Power Distribution Center) on the right side engine compartment. The relays in there are all the same, so swap out the aux fan relay to test. Does your 92 have A/C? If so, turn it on and the aux fan will run if all is well. The only other way the aux fan will run w/o the A/C running is if the coolant temperature reaches ~230* or so to prevent overheating. Thanks for the reply. I do have the components for the AC all hooked up, but it's not functioning...what's the easiest way to diagnose the AC? I tried turning the AC on and no dice. Swapped some relays, still nothing. Shouldn't the AC clutch engage when turned on? I imagine the AUX fan would spin up....but it didn't.. Thanks! Link to comment Share on other sites More sharing options...
Rohls Posted November 2, 2014 Share Posted November 2, 2014 I checked the relay receptacle and it is getting 12V with the key on ACC. Last thing I can think of is the relay is bad...swap relays? But then how do I know it's functioning? Should I 'hot wire' the relay by connecting the switch outputs to the fan...? Which terminal in the relay socket did you test? Remember -- a relay is an electrically controlled switch. For the relay to be "ON" there has to be 12 volts to the input of the control side (terminal 85 or 86), and 12 volts to the input of the load side (terminal 30). Thanks, the 12V was coming from the 30-87a circuit....the 'hot' one. I guess my question becomes how do I check that the temperature switch is functioning? Can I just 'jump' the 85/86 circuit to test if that kicks on the AUX fan...? The temperature switch in the thermostat housing is brand new...but maybe the signal wire isn't connected? Hmmm//// Link to comment Share on other sites More sharing options...
Eagle Posted November 2, 2014 Share Posted November 2, 2014 If you located the aux fan relay you know it's in the PDC (Power Distribution Center) on the right side engine compartment. The relays in there are all the same, so swap out the aux fan relay to test. Does your 92 have A/C? If so, turn it on and the aux fan will run if all is well. The only other way the aux fan will run w/o the A/C running is if the coolant temperature reaches ~230* or so to prevent overheating. The aux fan relays on my '88 XJ and my '88 MJ are on the inner driver's fender, right next to the airbox. Link to comment Share on other sites More sharing options...
Rohls Posted November 2, 2014 Share Posted November 2, 2014 If you located the aux fan relay you know it's in the PDC (Power Distribution Center) on the right side engine compartment. The relays in there are all the same, so swap out the aux fan relay to test. Does your 92 have A/C? If so, turn it on and the aux fan will run if all is well. The only other way the aux fan will run w/o the A/C running is if the coolant temperature reaches ~230* or so to prevent overheating. The aux fan relays on my '88 XJ and my '88 MJ are on the inner driver's fender, right next to the airbox. Yeah, thanks...I don't have those. my AUX fan relay is in my PDC. So how do I verify the AUX fan is switching on when it is suppose to? Link to comment Share on other sites More sharing options...
HOrnbrod Posted November 2, 2014 Share Posted November 2, 2014 Looks like TNCave (Renix) started this thread then Rohls (92 HO) stole it. :yes: The below is directed to Rohls: Your A/C will not turn on if it's low on freon because it can't build pressure to trigger the low pressure switch. When triggered this switch supplies the ground signal to the aux fan relay coil which turns on the fan. W/o the ground signal - no aux fan operation. Can I just 'jump' the 85/86 circuit to test if that kicks on the AUX fan...? Yes. Link to comment Share on other sites More sharing options...
Eagle Posted November 3, 2014 Share Posted November 3, 2014 If you located the aux fan relay you know it's in the PDC (Power Distribution Center) on the right side engine compartment. The relays in there are all the same, so swap out the aux fan relay to test. Does your 92 have A/C? If so, turn it on and the aux fan will run if all is well. The only other way the aux fan will run w/o the A/C running is if the coolant temperature reaches ~230* or so to prevent overheating. The aux fan relays on my '88 XJ and my '88 MJ are on the inner driver's fender, right next to the airbox. Yeah, thanks...I don't have those. my AUX fan relay is in my PDC. So how do I verify the AUX fan is switching on when it is suppose to? My bad. I went away, and when I came back I forgot we had switched from discussing a '90 to a '92. Hornbrod is the man for HO stuff. Link to comment Share on other sites More sharing options...
Rohls Posted November 3, 2014 Share Posted November 3, 2014 Looks like TNCave (Renix) started this thread then Rohls (92 HO) stole it. :yes: The below is directed to Rohls: Your A/C will not turn on if it's low on freon because it can't build pressure to trigger the low pressure switch. If your A/C will not come on, neither will the aux fan because the aux fan relay will not get the ground signal from the low pressure switch to trigger the aux fan relay coil. Can I just 'jump' the 85/86 circuit to test if that kicks on the AUX fan...? Yes. Alright! Thank you! You're the man...! Gonna try this out soon! I apologize for the thread hijacking! :banana: :banana: :banana: . :MJ 1: . :banana: Link to comment Share on other sites More sharing options...
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