Guess I better chime in here.

The AC cycling is controlled by one thing, provided its system pressure and temperature sensor conditions are met - what temperature the brain sees at the evaporator thermistor. Every fit is slightly different, but most will turn ON the compressor when the thermistor is about 50F, and OFF when it is down to about 40F. As I said, there are variances, as some will get close to freezing while others like mine will run in the 45F-55F range. Any gunk buildup on the thermistor will lower this temperature because it delays the response. It is a higher temperature setting than most cars - for example, the civic of the same year would cycle off at 36F and on at 40F (it has a temp switch, not a variable resistance thermistor). When mine kicks off and the air blows out 55+F, I was fairly upset (and thus made 'the button' of system domination, a 200k ohm leak circuit that dropped the temp 10F).

If you run the fan low, the evaporator will cool quickly, and thus run a short duty cycle - as low as 10 seconds on, 30+ off depending on how fast the ambient air warms the evaporator.

If you run the fan high, it will take the system much longer to cool the evaporator, and if the ambient air temp is high enough, it won't cycle off at all.

When running recirculated the already-cooled air in the car will slow the warming of the evaporator, resulting in longer warming and shorter cooling times, and thus better fuel economy. Personally I'll run in recirculate most of the time and every half hour or so run fresh for 5 minutes to change things out.

 

A very interesting and helpful thread. I was wondering how to maximize fuel economy last week, as I had a long drive down the I-95 in very hot conditions.

Bringing together what Steve244 and polaski say, I assume that the heater core adds heat downstream from the evaporator, so that compressor cycling will be a function only of ambient temperature and fan speed (and, of course, set point).

If this is right, then to keep the compressor off as much as possible (and thus save gas) it would be best to leave the temperature dial all the way to the blue side so that no engine heat is added and then control cabin temperature using the fan speed. A high fan speed would keep the evaporator warm so that the compressor would keep running and so would remove the maximum amount of heat from the cabin, and thus would be the best setting for very warm days. On cooler days the fan would be set to low, which would result in a colder evaporator sooner, with consequent cycling off, and thus fuel savings.

Does this sound right?

 

Even on a supercharged engine the A/C can be felt cycling on and off.

 

Yes. Along with using recirculate.

The main thing though is the comfort of you and your passengers. On mild days I find myself pulling heat through by adjusting the temp knob. I only use recirculate when I feel it does a better job of cooling.

Polaski has a great thread on adding the "leak circuit" to alter the characteristics of the temperature limiting thermistor. I'm tempted. If I didn't have covered parking at work I'd be doing this. As it is the A/C was up to the job over this freaky summer.

 

I can't believe people actually contemplate the AC this deeply. When it's hot in the car, I turn the AC on and cool it down. When it isn't hot, I don't turn the AC on. If some extra gas is used while the AC is on, so be it. I can't sufficate in extreme heat. It's the price of being more comfortable. And the compressor goes on and off too. Beyond these facts, is micro management really needed to run the AC????

Dan

 

Thanks Steve244 -- that’s handy to know. I take your point about comfort being the most important thing. Still, I’m the sort of person who likes to know what’s going on, and I’d always wondered about how Honda had configured the temperature-adjustment knob. Now I know, and will keep this in mind next time I'm on a long drive on my own.

 

It's right up there with sex and food. Well living in the south it is anyway.

I think people are contemplating saving fuel. I think that's obsessive, but I understand it, especially in a high mileage economy car.

 

This Summer has been very hot with many days in a row of 100+ degree heat and humidity that raised the temperature heat index to levels you would have to go into a wet sauna to experience... The louvers on my hood, dark tint across the top of my windshield, total coverage on the windows and cheap Mugen copy vent/visors worked well to allow my car to be comfortably cool with no incidence of frost building up on the evaporator even while operating in the closed circulation setting on the 2nd or 3rd fan setting..... Using the Electronic Throttle Controller In the EC 5 setting and staying at 70MPH or a little lower when it was possible to do without hindering faster moving traffic has resulted in 33 + MPG consistently even with jumps to over 115 MPH when passing large trucks on two lane roads... This is the first summer out of 5 that the A/C has performed as well in what has been the hottest summer since I have owned the car....