Trying the KSD 9700 Bimetal Temperature Switch
To try and control the temperature more precisely, I bought a KSD9700 temperature switch. Comes in both NC and NO varieties, and has a metal housing with a bimetal switch inside. Rs. 120 for a single switch.
I disconnected the existing radiator mounted switch and installed an 80°C rated normally open switch. The housing of the switch is metal, so I used a ziptie and some RTV704 to temporarily affix it to the brass tee.
Unfortunately, this, too, didn't quite work.
While the switch triggers and closes at ~80°C, it is rated to reset at 55±15°C. That's a huge margin and not good enough for precision control. The cooling system would get up to temperature, but then the fan would run so long it would cool down too much before turning off.
Trying the W1209 Temperature Controller
The W1209 is a commonly available Chinese module with a microcontroller that turns a relay on/off based on input from a 10K NTC thermistor. I figured I'd give it a try, and was pleasantly surprised.
It's inexpensive, responds quickly, retains settings with power loss, has a diode for reverse polarity protection between +12V and GND, is quite configurable, and consumes 35mA when running. All for Rs. 350.
The lower radiator hose temperature, measured at the brass T-junction where it feeds the turbo, differs quite significantly from the temperature read at the top of the loop near the temp sender, and they're not directly proportional to each other.
For example, 67°C at the temp sender means 80°C at the lower hose, and this relationship changes if you're using boost, if the car is idling, etc.
No wonder the bimetal switch was triggering "too early."
After observing the temperatures for a bit, I configured the W1209 as follows:
Setpoint - 93°C
P0 Heating/Cooling Mode - Cooling
P1 Hysteresis - 4°C
P2 Upper Limit - 100°C
P3 Lower Limit - 80°C
P4 Calibration - 0°C
P5 Delay Start Time - 0 min
P6 High Temp Alarm - OFF
It'll try to keep the temperature of the lower hose between 89°C and 97°C.
It gets 12V with ignition on from the radiator fan relay, GND from the chassis nearby, and opens and closes the relay to control the fan via K0 and K1. The thermistor is attached to the brass tee with a ziptie and RTV 704.
Since the W1209 comes as just a bare PCB, I put the whole thing into its ESD safe baggie and stuffed it nearby.
Observations
It seems like the cooling system has plenty of capacity to operate passively, without a fan connected at all, as long as boost doesn't come into play. I drove around, ran some errands, and idled with the AC on and off for a total of 45 minutes with the fan disconnected and the temperature needle never even got to the halfway mark. Yes, I have an 82°C thermostat installed that I have verified to work.
As soon as you begin to use boost, the temperature begins to rise and starts to creep up past the halfway mark. Figures - there is an oil-to-water heat exchanger, the turbo itself is cooled with both oil and water, and the lower radiator hose directly tees off into the turbo water inlet line.
In practice, the W1209 works well, and the temperature lives at or slightly below the halfway mark on the factory gauge, and above 80°C at the upper temp gauge. Oil pan temps are now finally averaging 80°C.
There is a slight difference in the way the car behaves when the coolant temperature is above 80°C. Idle quality is perhaps smoother, and the turbo seems to begin spooling earlier in the rev range at ~1800rpm compared to >2000rpm.
I had to loosen the AC idle up VSV screw, because as soon as the temp exceeded 80°C, idle with AC would drop to 500rpm. Again, seems like an issue with the idle control solenoid.
Conclusion
The car takes a long time to warm up to the point that the radiator fan needs to kick in, but when it does, hopefully this more precise temperature control has a positive effect on fuel economy, longevity and performance.
Overall performance feels slightly better, at least in my 25km of testing. Pleasingly, the entire system seems to maintain temperature uniformly in a traffic jam.
The highest oil pan temperature I saw was 92.5°C.
Tiny GIF of the board doing it's thing.
This video is sped up; in real time, the fan is only on for 11 seconds to cool from 97°C to 93°C.
The red LED to the right of the temperature indicates that the relay on the board has closed, which means the fan is on. It turns off once the temperature set point has been reached.
Once the fan is off, the temperature stays at 91°C for about a minute before it begins to climb. It takes several minutes for the fan to kick in again.
This is what a good cooling system needs: headroom!