So while messing around with Zener diodes, I kept running into the problem of non-linearity in the breakdown voltage at the low currents that are used to read the coolant temperature sensor. I originally expected a clean Zener breakdown voltage; i.e., I expected a 770 mV diode to have a Zener voltage at very close to 770 mV regardless of the current flowing through it. But this was not the case. With the 2,200 ohm sense resistor inline, the actual Zener voltage fluctuated somewhat with the changing resistance of the coolant temperature sensor (which is simply a NTC thermistor). This is why I originally gave up on the Zener diode.
But, I decided to try a circuit anyways, and actually found a completely new control strategy I like even better than the original. Essentially, the Zener diode wired in parallel to the NTC thermistor is used to ensure the fan always is running at least it's minimum speed, and then as the NTC thermistor dops resistance (rising radiator outlet coolant temperature), eventually it takes over, ramping the fan to a higher speed. The issue I expected was that it would be a harsh cutoff, given that I would have to set the minimum coolant temperature to turn on the fan to 160F just to get it to run at minimum state most of the time. However, what I actually found is that the non-linear response of the Zener diode allows the fan to ramp up non-linearly as the coolant temperature increases.
The setup includes the following:
- Set "fan on" temperature to 160F
- Set "fan max" temperature to 200F
- Coolant temperature sensor in radiator outlet (same as previous)
- 1N6006B Zener diode wired in parallel with GM coolant temperature sensor (NTC thermistor), wired in forward direction (stripe towards sensor ground).
Below are the data from a test I performed, with generic resistors of fixed resistances used in place of the thermistor. Temperatures are calculated using a formula for a GM temp sensor. Fan RPM is calculated from sound frequency. Note that this test was performed with the engine off (resting voltage around 12.4V), so the amperage is about 5-10% more than what you'd see with the engine on.
Edit: Turns out the forum software doesn't like copy-pasted Excel, so the below is provided as a screenshot.
View attachment 561586
Effectively, this system generates an output in which the fan is always on at the lowest speed, and non-linearly ramps up as radiator outlet coolant temperature rises. At the lowest speed, this system only uses about 60W. The fan ramps up only minimally in response to coolant temperature until about 170F, at which point it ramps aggressively, maxing out at about 200F.
This setup, although it uses a small amount of power constantly, seems to be even more efficient than the previous linear setup. 1st, the fan, when running at minimum speed, only uses about 60W, or about that of your cabin blower at its lowest speed. This comes out to about 0.2 gallons every 1,000 miles in a worst-case scenario (versus a fan that doesn't run at all). 2nd, the fan responds gently to moderate increases in temperature occurring during normal driving conditions, and even at 170F, the fan is still consuming less power than your cabin blower takes to run at it's highest speed. Ramping to high power only occurs when coolant outlet temperature begins to spike above 170F, which would be indicative that the radiator is nearing its rejection capacity, and only running max power in a near-overheat condition.
With this setup, there is no risk of damaging the A/C compressor either, and overall, the fan runs much quieter, since it stays at low RPM the vast majority of the time. It may also present a benefit by lowering underhood temperatures, though this I have not tested quantitatively. I have not noticed any fan speed cycling as a result of this setup.
Edited as I messed up the Zener diode info. I used a fairly high voltage Zener diode in the forward configuration, not in Zener configuration. Updated part number and link.