Weird climate control issue - temperature drift

[TGrits10]

In short, when on Auto, the climate control system seems to read lower and lower temperatures over time, or at least behave as though it does, so I have to keep setting it to a lower and lower temperature to account for the discrepancy.



For example, say I want it 74F in the cabin: I set it to 74, and for the first 5 or 10 minutes it does its auto thing and maintains 74. Then eventually it backs off the fan/cooling and it starts to get warm and I notice and turn it to say 72, and it ramps the cooling up a bit and we get back to 74, but soon enough I have to turn it down again, and again, all the way to 60/max if the drive lasts long enough, just to maintain more or less that 74 I wanted. One long highway drive in particular it stood out because the last hour or so of the trip I had it set to 60 and needed the seat ventilator on to keep from sweating, i.e. it wasn't even maintaining 74, because I ran out of room to turn it down lower. It always seems to have plenty of cooling available, if I turn it down a lot at once it'll ramp the fan up and blow cold like a champ...it just chooses not to. The question is why??



I've looked around the forums trying to see if anyone has posted about something similar, so far I've come up empty. Could it be a temperature sensor going bad in a way that it progressively loses calibration over time? But then resets back to normal for the next drive? Same question if it's the HVAC computer brain gradually going off. Anyone seen something like this before?

[Qsilver7]

Can you also reveal where you have your stratified air dial set (the dial located in the center dash vent)? It is important to know since that dial can affect the temp from the dash vents warmer/cooler depending on if the dial is in the red, blue, or neutral (half way between).

The image below is from the f01 7 series which has TWO stratification dials…but the design concept is the same on the f06/f12/f13 even though they only have a single dial. In fact BMWs have had this stratification setup for decades…my 1991 735iL had this feature.

If this is not the issue…then this can be ruled out as the culprit:

[Opie55]

Those dials are connected to a motor? I always thought I was manually moving a flap.

[TGrits10]

I keep the stratified air dial (so that's what that thing is called) at neutral at all times. Never been a fan of having really-cold air blow on me, and I figure setting it to warmer would be counter-productive in summertime cooling? I dunno at any rate I've always set it to neutral and left it there.

[Qsilver7]

Originally Posted by Opie55

Those dials are connected to a motor? I always thought I was manually moving a flap.

I think it depends on the model...some may be connected to a bowden cable that manually moves the stratification flap...and some may be electronic.

As mentioned in my reply...the info above was from the 7 series.

Originally Posted by TGrits10

I keep the stratified air dial (so that's what that thing is called) at neutral at all times. Never been a fan of having really-cold air blow on me, and I figure setting it to warmer would be counter-productive in summertime cooling? I dunno at any rate I've always set it to neutral and left it there.

Thanks for the update...looks like the stratification dial is not the culprit. But in regard to its use...it can be helpful to temper the air warmer or cooler in odd situations. Like here, it was 36° this morning but this afternoon it will be in the 70s. I have the HVAC temp set at 70°F. Depending on conditions...it could be really hot or cool when I get into the car...and I may have to temper the air blowing on me a bit cooler or warmer. The stratified air dial allows that to happen without having to change the temp of the entire cabin. Doing that could start the ping-pong effect where I would have to keep changing the temp or fan speed as the system keeps adjusting to the new set temp.

The stratified air dial allows you to maintain the set temp...but if you get a chill or start to overheat...you can change the temp of the air from the dash vents to accommodate a temporary need. This can also happen to passengers sitting on the sunny side of the car vs the other side that may be in the shade. The front & rear stratified air dials can help the passengers that are in the sun or shade adjust the temp of the air blowing on them without having to change the set temp on the HVAC display.

[TGrits10]

Originally Posted by Qsilver7

But in regard to its use...it can be helpful to temper the air warmer or cooler in odd situations.

The front & rear stratified air dials can help the passengers that are in the sun or shade adjust the temp of the air blowing on them without having to change the set temp on the HVAC display.

This! Funny how I've spent my whole life being annoyed by the conflict between solar radiation and automatic climate control, but never thought to mess with the little temperature jiggy that BMWs all come with. A new weapon in my arsenal...thanks for pointing it out.

[TGrits10]

I have one thread of thought that goes about like this: something in the system's brain is gradually heating up, and the hotter it gets, the more it skews the behavior. It shouldn't be overheating, but either there's a cooling fan that's not running, a ventilation path that's blocked, or a component running hot. If that's the case, it strikes me as unlikely to be a sensor, possible to be a simple blowing out of the cobwebs, but also possible to be a dying HVAC controller ($$$).

[Qsilver7]

Originally Posted by TGrits10

This! Funny how I've spent my whole life being annoyed by the conflict between solar radiation and automatic climate control, but never thought to mess with the little temperature jiggy that BMWs all come with. A new weapon in my arsenal...thanks for pointing it out.

Have you ever gotten a check control message about low coolant? Low coolant can also mess with the HVAC's air temp.

The main coolant reservoir is located in the fender and the fill tube is in the right rear of the engine bay (as you stand in front of the car.

There's no level sensor...so you have to measure down into the fill tube to determine if the coolant is at the correct level. I believe its like between 4-6 inches from the top.

[TGrits10]

Originally Posted by Qsilver7

Have you ever gotten a check control message about low coolant? Low coolant can also mess with the HVAC's air temp.

The main coolant reservoir is located in the fender and the fill tube is in the right rear of the engine bay (as you stand in front of the car.

There's no level sensor...so you have to measure down into the fill tube to determine if the coolant is at the correct level. I believe its like between 4-6 inches from the top.

Heh...
Short answer: Very much yes, but it's all good now and the AC is still acting up.


Long answer (feel free to go TLDR, it's irrelevant to the AC conversation): The drama surrounding my car's cooling system over the last year is an interesting book with multiple chapters, let me see if I can summarize effectively:
-Turbo coolant lines started leaking at the connection points about a year ago, finally (due to busy life and much unrelated drama) was able to replace them recently, good to go now.
-Coolant reservoir was punctured by a friend's driveway brain fart while reversing and eventually replaced at a BMW dealer by insurance (along with some body work).
-while that was going on, evidently another coolant line went kablooey (see this thread if you're interested: https://www.6post.com/forums/showthread.php?t=2004588), so thinking the reservoir replacement had not been done properly or they had missed another damaged component, back it went to the dealer, found the culprit, I paid them ~$300 for identifying the problem and holding my car in limbo for weeks while ignoring my texts and phone calls, and I replaced the broken line.
-One way or another, all this drama killed the aux cooling pump for the turbos, so a rush order from FCP Euro and ANOTHER long night in the garage later, that's replaced as well.
-Now-paranoid me has been checking for signs of leaks and popping the two reservoirs to check levels daily, so far so good. Might be feeling safe enough to hand back my FIL's spare car I've been borrowing...