For the past few months I have been running a little hot in traffic. I noticed that it was when I was not using the AC so I zeroed in on the aux cooling fan. The fan comes on when the AC is on but not off. I let the temp get up to about 230 and nada. I replaced the both temp sensors about a year ago (on the Tstat housing and rear of the head) so I don't think that it is the sensor.

Can anyone tell me if thee are two separate relays for the fan (one for the AC and one for when engine hot)? I am fairly sure that the sensor on the Tstat housing is for the fan, does anyone know if it is a sensor or thermal switch? Lastly, everything is running fine so I don't think that it could be the computer, input?

I will probably replace the Tstat sensor this weekend just to be sure it is not that. I am considering installing an independent temp control for the aux fan if I cannot narrow it down.


1992 Cherokee 2WD, 4.0L, Auto
228k miles

 

It there another sensor screwed on drivers side rad tank just above bottom hose ?

 

No. No sensors at all in the radiator.

 

Renix only.

 

If you unplug temp sensor on thermostat housing and start engine does it come on ?Its a trick on some cars to see if sensors bad .

 

Strange thing my radiator bought new for a 92 has the hole there I screwed my sensor in ,wasnt sure when they stoped them .

 

Update. I discovered that the aftermarket temp sensors for the T-housing are not accurate, most ohm at around 1K at 212 and should be under 700. I jumped a 5k variable resistor in parallel wiht the sensor and when the temp hit around 210 I adjusted the V-Res until the fan came on. I tweeked until I got the best on and off temps to keep it as steady as possible. I then measured the V-res and soldered an equal valus resistor in its place and it is back to running rock steadt between 195 and 210 (200 most of the time).

 

Very nice diagnostics work. Who was the offending brand if you recall?

 

Nice find Shadow. I have my fan hooked up to a relay and switch so i can turn the fan on at any time for cooling. Works great if I am on some rocks and want to keep her nice and comfortable.

 

I cannot remember but I bought one sensor from Rockauto and one from Advanced Auto. From my research this seems to be very common in aftermarket sensors.

 

If this is the same sensor that inputs to the PCM for spark control and injector pulse width, the sensor and resistor will be in range at some temps and probably out at other temps. The sensor is NTC and not linear.