baufan2025 Posted September 21, 2022 Report Share Posted September 21, 2022 Hello, I have a 2013 Ford cmax and a few months ago I changed the AC compressor. It's been blowing fine at 43 degrees since June. Today I'm seeing an issue where if I'm sitting still it blows cold but when I accelerate it warms up to about 75 blowing out of the vents. Then when I'm driving it will alternate where it'll blow cool then warm then cool then warm. Any ideas? Quote Link to comment Share on other sites More sharing options...
cr08 Posted September 22, 2022 Report Share Posted September 22, 2022 How's the outside air temp reading from the vehicle itself? This can impact HVAC operation if it is flaking out and it'll be really obvious if it is as the displayed ext temp will fluctuate quite a bit. The exterior temp sensor is in the passenger side mirror and is the little nub that sticks out the bottom. If it is faulty, only replacement is replacing the entire mirror housing. It can in some cases be temporarily messed up by water/moisture making its way up inside the housing due to a car wash or such. Blasting some compressed air up into the housing around the sensor -may- help these cases. Another thing would be to check for any stored codes with a competent code scanner (Forscan with a good MS-CAN capable OBD tool like the OBDLink EX is my preferred go-to). The HVAC system is just as intelligent as the rest of the car and monitors various stats/pressures that -could- pinpoint a cause if there are any related codes stored. This is not guaranteed though. What are the weather conditions when this happens? Does it ever seem to happen more often during hotter and more humid weather? There's another well known condition where the evap core in the cabin will freeze over due to a faulty evap temp sensor. Usually this happens when it is very hot and humid out. It will more cause the airflow to slow and not cool -as well- as normal, doesn't outright switch to warm/hot output, and usually requires manual intervention and will not clear up on its own. This I doubt is what you are running into but figured I'd rule it out here. Those are the immediate things I can think of to check on. Quote Link to comment Share on other sites More sharing options...
baufan2025 Posted September 22, 2022 Author Report Share Posted September 22, 2022 Thanks, my external temp sensor has shown --- since I bought the car a few years ago. The guy at express said he voltage checked the sensor in the mirror and it's reading fine so he suspected wherever it plugs into. Not sure where that is. Quote Link to comment Share on other sites More sharing options...
baufan2025 Posted September 22, 2022 Author Report Share Posted September 22, 2022 On 9/21/2022 at 8:07 PM, cr08 said: How's the outside air temp reading from the vehicle itself? This can impact HVAC operation if it is flaking out and it'll be really obvious if it is as the displayed ext temp will fluctuate quite a bit. The exterior temp sensor is in the passenger side mirror and is the little nub that sticks out the bottom. If it is faulty, only replacement is replacing the entire mirror housing. It can in some cases be temporarily messed up by water/moisture making its way up inside the housing due to a car wash or such. Blasting some compressed air up into the housing around the sensor -may- help these cases. Another thing would be to check for any stored codes with a competent code scanner (Forscan with a good MS-CAN capable OBD tool like the OBDLink EX is my preferred go-to). The HVAC system is just as intelligent as the rest of the car and monitors various stats/pressures that -could- pinpoint a cause if there are any related codes stored. This is not guaranteed though. What are the weather conditions when this happens? Does it ever seem to happen more often during hotter and more humid weather? There's another well known condition where the evap core in the cabin will freeze over due to a faulty evap temp sensor. Usually this happens when it is very hot and humid out. It will more cause the airflow to slow and not cool -as well- as normal, doesn't outright switch to warm/hot output, and usually requires manual intervention and will not clear up on its own. This I doubt is what you are running into but figured I'd rule it out here. Those are the immediate things I can think of to check on. I will say it's always been a little flaky over the years I've had it. If I drive it, it will be ice cold and then when I turn it off whether I'm somewhere for 10 minutes or 2 hours It always takes 15 or 20 minutes to start cooling again. It did that before and after I changed the compressor on it. Quote Link to comment Share on other sites More sharing options...
cr08 Posted September 23, 2022 Report Share Posted September 23, 2022 On 9/22/2022 at 8:15 AM, baufan2025 said: Thanks, my external temp sensor has shown --- since I bought the car a few years ago. The guy at express said he voltage checked the sensor in the mirror and it's reading fine so he suspected wherever it plugs into. Not sure where that is. I can pull up the workshop manual and post full PDFs/pictures tomorrow, but the ambient temp sensor goes through one connector with the rest of the mirror/housing wiring and that sits on the inside of the door up near the mirror itself. I would definitely pull codes (again, with the proper tools. This will be stuff that is on the MS-CAN bus which most generic scantools won't know anything about) if you can as it could also help a little bit in pinpointing the problem or at least confirming if something has or is failing. I'd give about a 90% guess this ambient temp sensor could be the main or at least majority of your problem. The automatic climate control system on these vehicles heavily relies on the ambient temp sensor during normal operation to determine how to operate the heat/AC. Unfortunately AFAIK there's no way to straight up bypass it and make it fully manual. One thing you can try as a workaround/test, though I can't guarantee if it will bypass the ambient temp checks completely, is using Max AC or turning the temp all the way to LO. I -THINK- it's been mentioned maybe a few times by other owners with faulty temp sensors that this works in the short term but my memory is a bit fuzzy on that. Quote Link to comment Share on other sites More sharing options...
baufan2025 Posted September 23, 2022 Author Report Share Posted September 23, 2022 Thanks, I will definitely look into that. Quote Link to comment Share on other sites More sharing options...
cr08 Posted September 24, 2022 Report Share Posted September 24, 2022 Here are a couple PDFs from the workshop manual detailing where the ambient temp sensor is wired as well as the brief wiring diagram for it. 2013_cmax_ext_mirror.pdf 2013_cmax_front_door_trim.pdf Quote Link to comment Share on other sites More sharing options...
baufan2025 Posted September 27, 2022 Author Report Share Posted September 27, 2022 On 9/24/2022 at 12:19 AM, cr08 said: Here are a couple PDFs from the workshop manual detailing where the ambient temp sensor is wired as well as the brief wiring diagram for it. 2013_cmax_ext_mirror.pdf 933.13 kB · 3 downloads 2013_cmax_front_door_trim.pdf 728.15 kB · 3 downloads Thank you Quote Link to comment Share on other sites More sharing options...
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