How to Lower Your A/C Temperature

This intent of this modification is to lower the cycling temperatures of the stock A/C system when needed, but still allow immediate reversion to the original calibration when not. As the Fit’s air system was designed for minimum fuel economy loss when in operation, the typical temperature maintained by the system is higher than most with vent temperatures kept above 50 degrees F via rapid compressor cycling on days exceeding 85 degrees. To modify the temperature range, the signal that the computer relies on to control the compressor must be altered: the resistance of the thermistor on the evaporator. However, the proper resistance value must be chosen that does not let the computer push the evaporator temperature below 32 degrees F, or ice could form and block it, potentially causing damage as the refrigerant will boil more and more slowly and may reach the compressor itself still in liquid form.

On my GD3, the thermistor Honda used is a 2-wire high resistance unit that has a somewhat linear change in resistance with temperature. This modification is not suitable for use with the 3-wire switched type, which completely blocks or passes current on a 36F to 41F temperature range, and changed resistance values will have no effect.

To calculate the resistance value we need to aim for, I consulted a service manual which listed the following thermistor resistance values on a graph: 30k ohm @ 32F, 18k ohm @ 50F, 12k ohm @ 68F, and 9k ohm @ 86F. According to the graph, if on a cool day the system shuts off at 42F (vent) as I measured with fan speed at 2, then the evaporator is likely closer to 38F, with the resistance about 24k ohm. To lower the temperature, the resistance must be lowered slightly, so that means the new resistor(s) go in parallel.

Based on the calculations on this website, as I was too lazy to do the math myself, I chose a 200k ohm setup to start as this would lower resistance to 21.4k ohm. At this value the computer would assume the evaporator is about 6 degrees warmer than it actually is, if the graph is correct, bringing the evaporator into the 32F-34F range. To provide adjustability, I added a 100k ohm micro potentiometer to allow fine tuning.

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Items Required:

-pack of 100k ohm resistors, 1/2 watt
-100k ohm micro potentiometer, linear type
-22 ga. wire spool
-high-temperature square adhesive zip tie mounts, plastic, and zip ties
-soft touch on-off pushbutton switch
-quick-disconnect spade or bullet connectors, insulated
-wire splice taps for 22 ga. wire
-multimeter to test your new connections and the potentiometer
-soldering iron and electrical solder
-shrink wrap of various sizes
-electrical connector lubricant to prevent corrosion

cost, minus the multimeter, about $36

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Assembly:

This is simple so step-by-step isn’t really needed. Solder the resistor(s) you’re going to be using to the potentiometer on the two leads that make the most resistance with the knob turned clockwise fully. That way, like a volume knob, up has a higher temperature and down a lower one. If you wish to reverse that, then use the other lead, but that’s up to you.

Once you get it soldered up, start shrink wrapping. Keep all metal parts apart so nothing can short out. Also a layer or two of larger wrap will strengthen the resistor wires and keep it from bending, as well as cover up the leads on the potentiometer.

On my setup I have one 100k ohm resistor on the line, and the other 100k ohm provided by the potentiometer. If I need it, I will add another 100k ohm resistor on another wire between spade connectors and move the range to 200k-300k adjustable, as right now at the max of 200k ohm it’s close to too cold. Can always add or take away with the spades, but if soldered it’s permanent.

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Installation:

This part is all up to you. I chose this location as it is nearly invisible, easy to reach when driving, and require no holes to be drilled.

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The zip tie mounts holding the potentiometer in place. Notice it can be easily adjusted by just popping this cover off. Need the removable connectors if you want to be able to remove this panel (highly recommended). Remember to put some electrical lubricant into these to prevent corrosion.

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This is where it’s tapped, behind the glove box. Don’t even have to remove the glove box as it’s totally accessible from the bottom once you pop the panel off (two snaps, much like the driver’s side). You know it’s the right wire if it’s got that grey insulation and it follows around the back of the evaporator to right next to the accelerator pedal, where it goes into the evaporator assembly. This is the wire you test for resistance to see if your thermistor is the linear resistance or switched type. Also remember to use some electrical lubricant here too. If you want additional protection, use electrical tape.

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Don’t forget to test your resistances to make sure you don’t have a short or broken wire. With the button not pressed, it was 9.8k ohm; pressed, 9.3k ohm. Perfect, no shorts.

Observations:

First test, ambient air 77F. Temperatures taken with my old wall thermometer at center vent, HVAC sent to vent only, window vents are open, recirculate off. I have my cabin filter removed for now as it clogged with dirt, to be replaced soon. I may have to add another 100k ohm resistor and crank down the potentiometer to compensate.

At fan speed 2 and button not pressed (OEM resistance), the compressor cycles:
-at 55 mph, on at 50F, off at 42F.
-at idle, on at 41F, off at 38F. This was weird how it went down.

At fan speed 2 and button pressed (modified resistance), the compressor cycles:
-at 55 mph, on at 36F, off at 32F.
-at idle, steady at 33F.

At fan speed 1 with button pressed (modified resistance), the compressor will cycle at idle with fan speed at 1 when it drops to 32F, about one complete cycle every 90 seconds. At fan speed 4 and engine idle, it stays steady at 36F.

As this is likely cold enough to form ice, I may add another resistor if it becomes an issue. It does form a small amount of condensation on the vents. However if it does not drop that far in hot weather, I’ll leave it as is because 90% of the time I’ll just have it in normal mode for the fuel economy and just use the cold mode for initial cabin cooling.

Picture taken instant compressor turned off with button on and fan speed on 1:

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