jeepster1991 Posted December 5, 2009 Share Posted December 5, 2009 I have a '91 Comanche 4.0 I just replaced the dummy lights with real gauges. Not realizing that the Sensor on the thermostat housing does more than just monitor the temperature of the coolant. What I need to know... is there any way to fool or bypass the computer into thinking that original sensor is still there. When starting the truck it stalls or almost stalls for about 30 seconds and runs really rough for a minute or 2 after that. If i reconnect the original sensor it starts fine, but then the auxillary fan doesn't kick on when needed and the truck runs way too hot. The reason i replaced the dummy lights is because the water pump went out and the motor got really hot before any warning light came on. I would really appreciate any input or thought anyone may have on this matter. Link to comment Share on other sites More sharing options...
mvusse Posted December 5, 2009 Share Posted December 5, 2009 Auxiliary fan should come on above 210 degrees, I believe. Up to 245 degrees is within operating specs, ~195 is nominal. What do you call "really hot" and "way too hot"? Link to comment Share on other sites More sharing options...
jeepster1991 Posted December 5, 2009 Author Share Posted December 5, 2009 My main concern isn't the aux fan. Without the original sensor hooked up it just runs the entire time, never shuts off at any temp range. I'm mostly worried about how the truck wants to stall when first started without the original sensor hooked up. Once it "warms up" it runs fine but i'm learning it takes 30 seconds to about 3 minutes befor the computer "regulates" itself. I really need to know how I can hook up the original and the new gauge so the computer proccesses the info from the original but i can have the new one to monitor the temp accuratey i can't depend on the dummy light alone. My truck was bone dry before that light even flickered and it got hot enough to burn off most of the oil and residue around the valve cover damn near looked like it was on fire. Know ideal how hot it really got because i had no actual temp gauge on till now. Link to comment Share on other sites More sharing options...
jmderyke Posted December 5, 2009 Share Posted December 5, 2009 Are these aftermarket gauges or other jeep gauges? If i was in your shoes id keep the sensor thats for the ecu in its location, and just find a new location for the gauge sensor. Jeep usually had two coolant sensors one for the ecu and the other for a gauge. Is one hiding from you maybe? Try the block drain bolt, or some inline 6cyls had a sensor location on the very back of the head, drivers side right near the valve cover for coolant sensor. If one is not there maybe its been closed with a allen bolt. Some jeeps had coolant sensor on the intake manifold (not to be fooled by the MAP sensor). If still no location for sensor id just drill a hole and tap some threads on the thermostat housing and install it there. Maybe pick one up from the junkyard first just incase it won't work of you mess up. good luck Link to comment Share on other sites More sharing options...
Eagle Posted December 5, 2009 Share Posted December 5, 2009 TIME OUT! You replaced the WRONG sensor. The sensor in the thermostat housing controls the ECU, which (on the '91) in turn controls the aux fan. BUT ... the sensor for the gauge on the dashboard for the 1991 models is still located in the back left corner of the head, NOT in the thermostat housing. Link to comment Share on other sites More sharing options...
Oizarod115 Posted December 5, 2009 Share Posted December 5, 2009 TIME OUT! You replaced the WRONG sensor. The sensor in the thermostat housing controls the ECU, which (on the '91) in turn controls the aux fan. BUT ... the sensor for the gauge on the dashboard for the 1991 models is still located in the back left corner of the head, NOT in the thermostat housing. :smart: beat me to it eagle! Link to comment Share on other sites More sharing options...
RLCollins Posted December 6, 2009 Share Posted December 6, 2009 What about my problem with my 1989 4.0 I-6 and the auxiliary fan thermal switch that is located in the side of the radiator on the old closed system. The newer 1993 XJ open system that I replace the old system with does not have a threaded hole in the radiator for this switch. Someone said that the old switch was an on/off switch whereas the new one in thermostat housing is a information switch. :dunno: are they both run by the ECU? :hmm: Link to comment Share on other sites More sharing options...
jeepster1991 Posted December 6, 2009 Author Share Posted December 6, 2009 Thank you all for your input, i think i'm gonna try tapping a location on the thermostat housing to have both sensors hooked up and hopefully solve my situation. I'll let you all know how it works out. Thanks again!! Link to comment Share on other sites More sharing options...
Eagle Posted December 6, 2009 Share Posted December 6, 2009 What about my problem with my 1989 4.0 I-6 and the auxiliary fan thermal switch that is located in the side of the radiator on the old closed system. The newer 1993 XJ open system that I replace the old system with does not have a threaded hole in the radiator for this switch. Someone said that the old switch was an on/off switch whereas the new one in thermostat housing is a information switch. :dunno: are they both run by the ECU? :hmm: Both what? Both switches? The '87 thru '90 auxiliary fan is controlled by either the sensor in the radiator tank (for overheating) or by activating the a/c compressor. Neither one involves the ECU. Temp data for the ECU comes from a temperature sensor mounted low on the left (driver's) side of the block. In the '91 thru '96 HO system, there is no sensor in the radiator tank. The temp sensor in the thermostat housing provides data to the ECU, and the ECO controls the auxiliary fan. The Renix ('87 - '90) radiator tank sensor is nothing but an ON-OFF switch. The HO sensor in the thermostat housing is a variable resistor that reports the actual temperatore (expressed as ohms). It is not a simple ON-OFF switch. It is not, in fact, any kind of a switch. It is a temperature sensor. Link to comment Share on other sites More sharing options...
jmderyke Posted December 6, 2009 Share Posted December 6, 2009 mmm i think you should the sensor location near rear of head. the ecu could operate on different resistance readings of the sensor Link to comment Share on other sites More sharing options...
RLCollins Posted December 6, 2009 Share Posted December 6, 2009 Thanks for the input, is there an on/off switch available for the thermostat housing? I like the part about moving the auxiliary fan switch on/off switch from the end of the flow of water on the down side of the radiator cooling, back to the hotter part at the thermostat housing. This would allow the auxiliary fan to come on sooner than later in theory. does it matter what temperature thermostat is installed? what sould it be? is there a down side to a cooler one verses a hot one? I sorry for stepping in on someone else thread, but I figured it may help this person also Link to comment Share on other sites More sharing options...
mvusse Posted December 7, 2009 Share Posted December 7, 2009 The ECU needs the input from the sensor on the thermostat housing to operate properly. That is why it doesn't run right when it's disconnected. If you keep it disconnected you will most likely see a drop in fuel economy as well, since it will run in open loop all the time. If you want to be able to run the fan the whole time, just wire it (the fan) to a switch on the dashboard. Link to comment Share on other sites More sharing options...
RLCollins Posted December 7, 2009 Share Posted December 7, 2009 Eagle says that the ECU runs off the coolant temperature sensor on the engine block. So if that is true than the auxiliary fan thermal switch which is located in the radiator is for the auxiliary fan only. Since I have changed my old closed system to an open one, The new radiator does not have an threaded opening for the Auxiliary fan switch and I have to use the thermostat housing for a location for the Auxiliary fan switch. Do they make an on/off thermostaticly controlled switch for this purpose that will fit into the thermostat housing? this is a 1989 MJ 4.0 I-6 MPI. :dunno: Thanks for all the information :chillin: Link to comment Share on other sites More sharing options...
mvusse Posted December 8, 2009 Share Posted December 8, 2009 Sorry about my confusion. As far as I knew, only the 91 and newer had the sending unit on the thermostat housing, so when you posted this I assume you had a HO engine. I have never seen a Renix that came stock with a temp sending unit in the thermostat housing. Mine has one because I swapped it in. It will be used for the aux fan control I still need to build, as my truck never had one from the factory. Link to comment Share on other sites More sharing options...
Eagle Posted December 8, 2009 Share Posted December 8, 2009 The new radiator does not have an threaded opening for the Auxiliary fan switch and I have to use the thermostat housing for a location for the Auxiliary fan switch. Do they make an on/off thermostaticly controlled switch for this purpose that will fit into the thermostat housing? this is a 1989 MJ 4.0 I-6 MPI. You are searching for the Holy Grail of Jeepdom. Over the years, various people on NAXJA (which has been around several years longer than this site) have variously reported that some year of sensor from a Corvette would work. That didn't prove to be correct. Then someone said they found an obscure sensor from some model of Honda that would work. I don't recall if that one proved to be correct or not. Personally, I would do as I have already suggested to you -- clip off the old sensor and extend the wires to a toggle switch on the dashboard. Link to comment Share on other sites More sharing options...
HOrnbrod Posted December 8, 2009 Share Posted December 8, 2009 Why do we have two threads running simultaneously on the same subject?? :nuts: On the "other" thread, I posted up some info I remembered from NAXJA. Whether the Corvette switch works or not I don't know because I've never tried it. Also, there are plenty of "open system" radiators w. the pressure cap around that have retained the temp switch bunghole. I have one on my 91. Link to comment Share on other sites More sharing options...
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