Auxiliary Fan + Cooling Temp Sensors

bchulett · Dec 17, 2003

According to the 1991 Jeep FSM, the Auxiliary Fan on a XJ switches on and operates when the air-conditioning is used ... or when the AC is off, the coolant temperature reaches 190°F.

Background:

1. The Aux fan works when I turn on the AC.

2. The Aux fan does not come on until the coolant temp hits about 215°F.

3. The dealer tells me the Aux Fan comes on at 215°F ... but I have reason to believe they're incorrect.

When my XJ was new, the Auxiliary Fan would switch on and operate exactly per the instructions in the FSM ... right when the temp reached 190°F. I would hear it come on.

I've suspected for years the cooling temp sensors may be faulty ... the only problem is the FSM is unclear which cooling temp sensor is ultimately wired to the Aux Fan ... the sensor in the "Block", or the sensor in the "Thermostat Housing".

The only thing the FSM says is the Aux Fan is turned on by the "engine controller" through a relay located in "power distribution center"---Okay ...

BUT ... the FSM covering the "Engine" under section "Cooling System" only references a sensor in the "Block" .... and the only place I can find reference to a sensor in the "Thermostat Housing" is in the FSM covering "Electrical, Heating-A/C & Emissions" under section "Ignition"---neither describes the Aux Fan and sensors in-depth.

Questions:

1. Any ideas what cooling sensor tells the engine controller "I've reached 190°F" ... so the Aux Fan will switch on and start blowing wind ??

2. Why is a cooling sensor in the "Thermostat Housing" referenced in the "Ignition" section of the FSM ?

This is probably clear as mud ... but any help would be appreciated.

DaveB · Dec 17, 2003

If I have deciphered the schematics correctly the sensor in the head at the back of the engine only controls the temperature gauge, and the sensor in the thermostat housing goes to the engine controller. If you wanted to test it unplug the sensor in the head and see if the fan will turn on after it warms up.
Dave
86/92 XJ
84(mixed up with 93/99/87/78/96) CJ8

Wayne Sihler · Dec 17, 2003

The Temp sensor in the thermostat housing is wired to the ECU.The normal operating temp for the 4.0 is 195* and as the dealer stated the Aux fan comes on at 215-218*.Most gauges will read 210* as the normal.
As to why it is listed in the "ignition"section I can only guess ,but the ECU recieves all the info and enables the engine to start.
If the high temp is a problem then it is time for a new radiator/fan clutch and flush.
HTH
Wayne

bchulett said:
According to the 1991 Jeep FSM, the Auxiliary Fan on a XJ switches on and operates when the air-conditioning is used ... or when the AC is off, the coolant temperature reaches 190°F.

Background:

1. The Aux fan works when I turn on the AC.

2. The Aux fan does not come on until the coolant temp hits about 215°F.

3. The dealer tells me the Aux Fan comes on at 215°F ... but I have reason to believe they're incorrect.

When my XJ was new, the Auxiliary Fan would switch on and operate exactly per the instructions in the FSM ... right when the temp reached 190°F. I would hear it come on.

I've suspected for years the cooling temp sensors may be faulty ... the only problem is the FSM is unclear which cooling temp sensor is ultimately wired to the Aux Fan ... the sensor in the "Block", or the sensor in the "Thermostat Housing".

The only thing the FSM says is the Aux Fan is turned on by the "engine controller" through a relay located in "power distribution center"---Okay ...

BUT ... the FSM covering the "Engine" under section "Cooling System" only references a sensor in the "Block" .... and the only place I can find reference to a sensor in the "Thermostat Housing" is in the FSM covering "Electrical, Heating-A/C & Emissions" under section "Ignition"---neither describes the Aux Fan and sensors in-depth.

Questions:

1. Any ideas what cooling sensor tells the engine controller "I've reached 190°F" ... so the Aux Fan will switch on and start blowing wind ??

2. Why is a cooling sensor in the "Thermostat Housing" referenced in the "Ignition" section of the FSM ?

This is probably clear as mud ... but any help would be appreciated.

8Mud · Dec 17, 2003

Just reading through the thread, doesn´t seem logical, that the aux fan would come on, about the same time the thermostate opens (195 F).
Got to also remember the temp. gauge sender, is at the back of the block and the temp. sensor is at the front of the block.

lndrvrxj · Dec 17, 2003

I had the same problem. well I had to replace the radiator because of a hole in it and once i did that everything works the way it should, now my jeep does not go over 210 when i am wheelin on a 95 degree day. i thought that the sensor was the problem to but if it ain't broke don't fix it right......

Planetcat · Dec 17, 2003

My '96 FSM, ditto's what Wayne said - sensor in front of engine w/waterpump, ECU controlled temp. I looked for months for a way to adjust the 215* setting on the aux fan, but couldn't find an ECU programmer.

bchulett · Dec 18, 2003

8Mud said:
Just reading through the thread, doesn´t seem logical, that the aux fan would come on, about the same time the thermostate opens (195 F). Got to also remember the temp. gauge sender, is at the back of the block and the temp. sensor is at the front of the block.

Well, on the surface it doesn't seem logical. But, if you stop and think about it .... if I was designing the optimum HD cooling system I think I would want the Aux Fan to operate proactive ... not reactive.

In other words ... it should come on right before the thermostat opens ... and begin to assist the viscous fan ... cooling the fluid returning from the block as it flows into the radiator---a proactive approach to optimizing the cooling system to maintain the 195° operating temperature for extreme conditions.

I agree my HD radiator has seen its best years ... but irrespective of that fact, "the system" should still function per the sensor output and signal to the ECU----assuming the sensor is sending the proper signal when it reaches 190°F ... correct ?

That's why I suspect it may not be functioning properly per the FSM specs.

I bought a new thermostat sensor from the dealer today ... I'm swapping it out to see if it makes a difference.

Thanks for all the input... I'll report my findings.

Bryan C. · Dec 18, 2003

bchulett said:
In other words ... it should come on right before the thermostat opens ... and begin to assist the viscous fan ... cooling the fluid returning from the block as it flows into the radiator---a proactive approach to optimizing the cooling system to maintain the 195° operating temperature for extreme conditions.

If the fan comes on before the thermostat opens, then there is no coolant flow in the radiator, and the fan will have nothing to cool. The normal operating temp range of 215 to 195 is mostly designed for emissions as a hotter running engine will burn fuel more efficently. Chrysler has a bad habit of just copying the previous year FSM. Try to find a later model year FSM and see what it says. The 190* temp spec for the fan is from the 87-90 era renix system with the fan switch in the radiator. The 91 and up system works via the PCM and a relay. You cannot change the PCM programming to adjust when the fan comes on. I think your system is functioning normally and you may have another problem that is causing your Jeep to run hot. Check the radiator for a restriction or any leaks in you cooling system before trying to reinvent the wheel. If you really want to control the fan, just wire up a switch inside your Jeep and control it manually when you feel it should be running. Tap into the ground side of the fan control relay and use a toggle switch to actuate the relay.

Bryan

bchulett · Dec 18, 2003

Bryan C. said:
If the fan comes on before the thermostat opens, then there is no coolant flow in the radiator, and the fan will have nothing to cool.

Please clarify ... my coolant temp sensor is in the thermostat housing monitoring the coolant on the radiator side. From my perspective, it's just sensing the temperature of the coolant before and after the thermostat opens up ... switching the fan on and off as required----vehicle not moving ... sitting there at idle.

Correct me if I'm wrong ... the thermostat primarily senses the coolant from the engine side ... where the coolant is measured and reported via the gauge on the instrument panel.

I don't understand why the radiator wouldn't have any coolant in it just because the temp is at 190°F and it switches on the Aux Fan----there's no air in the system. It seems to me the coolant in the radiator should be well below the operating temperature in the block ... therefore requiring a thermostat.

??

Bryan C. · Dec 19, 2003

I think what I am trying to say is that the thermostat is used to maintain the minimum engine temperature and the fan controls the max temp. The radiator has coolant, but no coolant flow when the thermostat is closed. If the fan came on before the thermostat opens, the engine would continually be getting a charge of cold coolant once the thermostat finally opens, which may or may not allow the engine to get fully warmed up. This may cause the engine controller to kick the fuel system into open loop and that is a big no-no when it comes to the EPA emission requirements. This may even make your vehicle fail your next emission test too, that is a big deal here in California. I understand what you are saying about keeping it cooler which is adventageous to us off-road, but not good for the average driver or vehicle. Try using an adjustable fan control system or wire up a manual switch.

Bryan

91xjtim · Dec 19, 2003

On my 91, the cooling fan had always kicked on @215ish. I know when the gauge got right above 210 it would kick on until it got down to 180-190ish. granted it hasnt gotten above 210 since I installed new radiator & fan clutch.

markg · Mar 10, 2007

if the gage was removed from the dash(just the temp)would the fan still come on at the correct temp?i would think it would but im just checking because mine is out and i cant remember my fan coming on lately.if i turn the ac on it should come on correct?you say tap the ground side of the relay to manually turn it on w a toggle switch.how exactly do i do this ?just toggle betweed a body ground being open or closed?
thanks for all of the info!

Auxiliary Fan + Cooling Temp Sensors

bchulett

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DaveB

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Wayne Sihler

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8Mud

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lndrvrxj

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Planetcat

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bchulett

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Bryan C.

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bchulett

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Bryan C.

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91xjtim

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markg

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