I’ve recently encountered an odd AC issue with my 2007 Mazda3 that I cannot find any other message boards addressing. For a variety of reasons, I had occasion to drive 17 hours straight from the SF Bay Area to Albuquerque NM on one day and then 10 hours to Oklahoma the next day at the end of July. The first 6-7 hours were fine with the AC blowing as normal, on a 100+ degree day. The problems began to occur deep into the drive where the speed of the blower decreased substantially, regardless of the speed setting. The air coming out was cold, but at very low volume.
I suffered through the next 6 hours or so until about 7pm, when I pulled in for another tank of gas and bite to eat. Starting up the car the fan starting blowing normally , temp was still around 100+, and ran fine through the evening until I hit Albuquerque 7 or 8 hours late. Thinking I made it through the blast oven, I started the next day in fine shape with the AC working normally, until another 6 hours into the drive to Oklahoma, the fan again slowed dramatically. No improvement on the fan speed until later that after arriving in OK and checking into the hotel I went back to the car to get a bite to eat and the AC fan was working normally again. Might I add, no problems have resurfaced in the ensuing months, although I also haven’t had to drive it more than 1-2 hours at a time in the last 3 months.
Anyone have any ideas? Could lines freeze up after hours of usage and block the airflow? I have no idea.
Something is causing the fan to slow down gradually. After it’s turned off for a while, and then back on, it works OK. Does that sum it up?
Could frost be building up on the fan and in its housing, and then melting?
If you are in Recirc mode, that becomes more likely. The humidity in the cabin builds up, frost forms slowly, voila. My guess is if you have the AC in Fresh mode the problem will not happen, because the air in the cabin won’t get too humid.
I tend to agree with Testor, blower motor over heating.
However if you wish to experiment to rule out ice/frost on the evaporator, next time it happens turn off the AC, set to fresh air mode. If ice buildup is the problem it will rapidly melt and you will get full airflow again.
Thanks for the reply. You did sum up the situation correctly. Sadly the only way I can reproduce this is to run the car for 4-6 hours, and this last drive was the first time I’ve had it happen. Then again, this was the first time I’ve tried driving 17 hours straight in this or any car, over the past 20 years.
I suspect your conclusion is correct, as the 30-45 minute downtime while getting gas and food was enough time to defrost the coils (or however the cold coolant lines are exposed to the ambient air). The subsequent drive at night, was generally cooler and at altitude (4000+ feet) so likely less humidity and a slower fan speed, so less build up of frozen condensate.
I did try this approach sporadically and saw some improvement in airflow. That being said, the temp also shot up to 110 degrees within minutes and became insufferable. Was better to tolerate some cold flow that kept cabin around 92-95 degrees versus 110 or more with no cold air. The couple times I tried this though, for 5-6 minutes I had near normal airflow (heavenly at that point) before it would return to a degraded airflow.
I’m leaning towards what @Nevada_545 said. Turning the a/c off of recirc (or Max) can help with this.
On Max setting, the a/c recirculates the already cooled off air in the cabin. On normal a/c setting, it draws air from outside- which will be warmer- helping to keep the evap from freezing over.
you may notice a bit of temp difference, but not much: and much better than the almost zero airflow you will get from a frozen evap.
There is an evaporator temperature sensor that is monitored by an ECU that controls the compressor, this should prevent the evaporator from going below 33 F but they sometimes fail.
110 degree air is usually very dry, I suspect that your A/C system is not set to fresh air.
The three most common causes of evaporator icing are:
1- Air set to “recirculate” or “Max” instead of “fresh”
2- A/C temp set excessively low
3- Refrigerant charge is getting low
I get in peoples cars all the time and they set the temp to full blast low temp instead of setting a comfortable temp (like 70 degrees or similar) with an automatic temp controller. I am always amazed because the thermostat in the car works just like the one at home, yet people use them entirely differently. If you have manual controls, simply adjust the temp to some moderate setting once the car is cool enough.