The cooling fan never seems to switch on when the car is already plenty hot, and perhaps it just waits too long. Here's my idea for a solution to lower the switch temperature, but I haven't tried it yet. The chilton manual says that the PCM uses a voltage to measure the resistance of the ECTS in order to decide when to switch on the fan, which is supposed to be at 190 degrees. My idea is to put a 500 Ohm resistor in parallel with the sensor, which has the effect of lowering all the resistances, and thus it should fool the PCM into switching on at a lower temperature.

 

What is your reason for wanting it to happen before 190 degrees. I don't think the car will perform correctly if not brought up to the proper temperature.

 

I want to adjust the sensor because I think that the car is really much hotter than what the PCM is gathering. The temperature gauge on the instrument panel will show past the 1/2-way mark and the fan still doesn't switch on.

I think the measurement is already flawed, so I'm trying to compensate. I believe the gauge on the panel is more accurate than the ECTS.

 

What year S coupe?
What brand of ECTS is installed?
Are there signs of corrosion or dried antifreeze on the contacts of the connector?
If you have a scanner that provides real-time data, simply monitor the PID for engine coolant temperature. That way you will know exactly what temperature the fan kicks on at.

If the car is 96 or newer, the signal from the ects also drives the gauge on the cluster.
For all we know, that gauge might be out of adjustment or faulty.

My 97 SC2 fan doesn't kick on until the 3/4 mark.. it's been like this since the day I drove it off the lot. 23 years ago.
My 95 SC2 kicks the fan on somewhere between 1/2 and 3/4. All 25 years I've owned the vehicle.

Neither vehicle overheats.

And the dash gauge behavior,, just like the resistance response of a thermistor, is not linear.
The gauge response is also heavily damped.

Remember that the rate at which the temperature of the coolant rises and drops depends on the efficiency of the cooling system. If your radiator and heater core are half plugged with crud from corrosion or your water pump fins are mostly eaten away, the coolant flow may be insufficient or shall I say much less than optimal.
You are sampling a single point in a physical system and assuming it is representative of the entire system. If the system is not behaving reasonably close to design, then this assumption breaks down. Any conclusions you try to draw regarding system design arer then also of questionable validity.

There are a lot of potential factors that may be combining to produce the end result you see.

My first step would be to look at the connect your contacts on the ECTS pigtail.

Then read the real-time data and see what temperature the PCM indicates the fan is turning on at.

Please let us know what you find.