2008 SI AC temperature varies
 

the temperature of the air blowing from the center dash vent varies 10 F over 15 seconds. This occurs at idle as well as at highway speed with cruise control set. Is this normal?

Yeah that's normal. The A/C compressor engages and disengages frequently causing the air temp to go up when it's disengaged and down when its engaged. It's like when you have the a/c on and then turn it off, the air remains cool for a short time before it starts to warm up.

The A/C compressor doesn't run all the time to save fuel and prevent the cabin from getting too cold, more to save fuel on Honda's. Their A/C systems aren't the most powerful but are very efficient, my mom's Ford Fusion will freeze you on a hot summer day.

If you wanna test it to be sure sit and idle with the A/C on and put your thermo in the vent. You will be able to hear the A/C compressor click when its engaging/disengaging along with a small RPM surge. If you hear a click and the temp changes that's why.

Thanks Anibis,

I certainly haven’t experienced the cabin getting too cold! Anyway to keep the compressor on?

Yeah I don't think any Honda has ever had a cabin that's too cold. I have black interior and the A/C just can't keep up on hot days, it keeps me cool but not cold.

There's no way to keep it on. You may be able to rig it up to a switch but I'm not sure, never tried.

What temperature are you getting? Saying "it varies by 10 degrees" means nothing. Measure with a physical thermometer, not an IR temperature gun. It should go down close to 40 F. The inside of the car will get cooler if you switch over to recirculated air once the temperature inside is colder than that outside.

In very hot weather the compressor would run constantly anyway. It's controlled by the air temperature leaving the evaporator, it should only stop after the evaporator gets so cold that it is going to frost up.

Thanks for your response.

I guess I wasn’t clear in the original post, so let me try to explain. I’m measuring the supply air temperature with a Fluke 52 thermometer and a type K fast response bead thermocouple. When I say the temperature varies 10 F in 15 seconds I mean that it will go from say 45 F to 55 F and back to 45 F in 15 seconds and this will repeat non-stop while driving. The longer the car is driven the lower the temperature will go, but the 10F swing in 15 seconds is consistent. This happens regardless of the temperature outside or cabin temperature, and with the dash controls set in normal or recirculate (max). It is most notable at a constant engine speed like when running 55mph with cruise control. I’ve also noticed temperature varies wildly with changes in engine speed, but I had assumed this was normal on these high rpm engines…perhaps not.