Temp Sensor

[creekscout]

I would like to  know where the temp sensor is located for the electric fan in my 92 mj with a 4.0L. I know there is a sending unit in the thermostat housing but I do not know if that is for the elec. fan or temp gauge or both. Also I would like to know what temp they are set to turn on the fan. At 220 my fan does still not come on yet it does come on with the defrost & a/c.

[ParadiseMJ]

On a 90 or older the temp switch for the e-fan is in the radiator, the sensor for the ECU that provides info for A/F ratio is in the lower block, driver side.   The fan SHOULD come on at 218-220. 

 

On a 91+ the sensor/switch is in the t-stat housing.  It tells the computer to turn the fan on, and provides info for A/F ratio etc.  I don't know if the temp threshold is the same as the Renix.

 

Both have the sender in the rear of the head for the gauge.  That changed in the later 90's too.

[cruiser54]

On a 90 or older the temp switch for the e-fan is in the radiator, the sensor for the ECU that provides info for A/F ratio is in the lower block, driver side.   The fan SHOULD come on at 218-220. 

 

On a 91+ the sensor/switch is in the t-stat housing.  It tells the computer to turn the fan on, and provides info for A/F ratio etc.  I don't know if the temp threshold is the same as the Renix.

 

Both have the sender in the rear of the head for the gauge.  That changed in the later 90's too.

Extremely well stated.

[creekscout]

When you replace the sensor in the thermostat housing can I use teflon tape or will that affect the grounding of the sensor?

[cruiser54]

When you replace the sensor in the thermostat housing can I use teflon tape or will that affect the grounding of the sensor?

I just use Permates #1 or 2, or teflon paste thread sealant.

[ParadiseMJ]

Extremely well stated.

 

I have moments...mostly after hours/days/months of trying to figure stuff out.

 

OP: 

Using Teflon tape ot PTFE thread sealer is fine if the sensor is a two/three wire sensor. One wire sensors ground through the block like our gauge/light sender...or in the case of one wire tranny temp sender...the steel trans lines, so need direct contact with metal

[creekscout]

How do I test my sensor while it is still in the thermostat housing?

[HOrnbrod]

There really is no accurate way to check the sensor unless you monitor the resistance vs. the coolant temperature in the thermostat housing. On most all HOs (including mine) the aux fan won't kick on until 230*-235* on the temp gauge. And I did try a new sensor, no difference. That's too high for me, so I wired in a dash switch that applies a ground to pin 31 of the ECU to energize the fan relay. This is what the thermostat housing sensor does to turn on the aux fan at high temp.

[creekscout]

Don, that is exactly the solution I am looking for. Will the fan still function with the temperature as well as with the switch? Will there be any problems with the CEL coming on. This would only be an on/off switch not a three way right?

[HOrnbrod]

The aux fan still functions as it always does; comes on with the A/C, and will come on @ 230* or so if you don't switch it on manually because you are using the same circuit, just applying the ground to the ECU earlier. Yes, it's a simple on/off dash switch. I used a rear window defrost dash switch from an XJ for mine. PM me an email address and I can send you the wiring diagram.