I have a 98 Saturn SL2 with 140k miles. The engine cooling fan does not come on in normal driving. It does work when the air conditioner is running. I have changed the radiator cap, replaced the sender and replaced the wiring clip on the sender. I checked the relay and it is operating correctly. The gauge in the car reads correctly. The car runs in a normal range when driven on the highway but will begin to heat up in traffic. Radiator fluid levels are ok.
I was told I might change the thermostat but I don't see how that would effect the fan operation as the engine comes up to normal tempature and hold there during normal driving.
You might be chasing a ghost here. We know the cooling fan and relay work if it is coming on with the AC. You said you replaced the sender. I assume you mean the Coolant temperature sender (ects) and it's connector. Good move. The older sensors were made with a composite/plastic tip that would crack after a while and leak coolant onto the connection. Did you replace it with the updated brass tipped design from saturn. The jury is still out on the reliability of aftermarket ects.
The gauges on the saturns aren't exactly precise. The one on my 96 SC2 stays around 3/8ths to a half most times. My fan kicks in at about the 3/4 piont and runs untill the gauge goes back to 5/8ths.
Your gauge might show it slightly different. The key is that it stays consistent. Meaning that the fan comes on and goes off at the same spot on your gauge.
I thought my fan was not coming on also. I was just impatient. It took almost 20 minutes from a cold start on an 85 degree day for it to engage at the spot on the gauge that I previously mentioned. Give it a try. I would not worry even if it does not come on until the gauge hits 7/8ths.
One way to tell if a thermostat is hanging open is by the output of the cars heater. If you are getting good heat in the same amount of time from start up the thermostat should be functioning properly. If the heat is slow coming on or does not get hot enough that would mean that the engine is running too cool.
96 and up saturns have only one ects that controls the gauge and sends signals to the PCM (computer) that turns the cooling fan on.
95 and older had two sensors. One controling the gauge and the other sending the signal to the PCM. I believe that is what Jack is refering to.
If the fan still does not come on double check the wiring. I I remeber correctly the PCM sends a 5 volt signal to the ects and depending on temperature the sensor sends back 5 volts or less.