AC Fan slows down during acceleration

[vkot3366]

Hey everyone! Having an annoying problem with my 2014 328d (F30) that I bought the other day (this one is already out of warranty). When I press the MAX AC button, the blower fan speeds up to its max speed. However, when I start accelerating, the fan slows down to almost the slowest speed, and when I let go of the throttle, the fan speeds back up to its max speed. Could this have anything to do with the temperature outside being almost 115F? I checked to make sure the air is not just being redirected to the windshield or feet; the fan just seems to be slowing down. Any way to troubleshoot or fix the problem? Any help is very appreciated!

[MarcoZandrini]

Two possibilities:
- voltage is dropping when you accelerate
- feature and not a bug!

Google "BMW F30 hidden obc functions." I know on the E46 there's a hidden menu that has various "tests" including system voltage. There may be one for your car. Alternatively, use a digital bolt meter (you have one, right?) and have someone monitor the voltage while you accelerate.

[vkot3366]

Two possibilities:
- voltage is dropping when you accelerate
- feature and not a bug!

Google "BMW F30 hidden obc functions." I know on the E46 there's a hidden menu that has various "tests" including system voltage. There may be one for your car. Alternatively, use a digital bolt meter (you have one, right?) and have someone monitor the voltage while you accelerate.

It looks like the system voltage is about 13.4V when ignition is on. Is this normal? Or should the voltage be above 14V?

Funny thing is when I turned on the car and started the voltage test, the voltage was over 14V, but after about a minute, possibly because engine fan kicked in, voltage dropped to 13.4V. Is my battery dying? Or could it be alternator problem? It also looks like I only have this problem when the temp outside is over 100F.

[MarcoZandrini]

That's quite a drop in voltage. I'm wondering if the alternator is in need of a new voltage regulator.

[MarcoZandrini]

Go to www.realoem.com and enter the last 7 characters of your car's VIN. Hit enter and bookmark the page. Scroll down to engine electrical to find the alternator. See if the voltage regulator is replaceable. I know that in the E46 BMW sourced alternators from Bosch (yay) and Valeo ( ). I'm wondering if BMW did the same with your car. The 2 brands of alternator are not interchangeable.

[shadowpuck]

when you were monitoring the voltage (and, admittedly, no idea how accurate that test is OR what the charging strategy is on a modern BMW like this one) did you have accessories on such as AC, radio, etc? in other words, if there's load on the system i'd expect a drop.....but, if that's running with nothing on (doors closed, etc.) that might be normal based on charging strategy....admittedly, not sure!

[vkot3366]

I spoke to a mechanic, and he said this is in fact a feature and it is normal. Thanks to everyone for the input!

[bmwdirtracer]

"A FEATURE?"

How exciting, I love new features! Do the headlights go dim too? THAT would be really cool!

[MarcoZandrini]

Chris, yup they dim while accelerating so that the proton-based energy goes to the engine.

[ptarditi]

I would check in vehicle settings to see if the vehicle is set up for eco pro mode.

[bmwdirtracer]

Oh, my., Paul, that's really sad. It was bad enough that the N63 cars eat batteries because the alternator only charge while coasting..... The more they "improve" these things, the more I dislike them.

http://www.roadandtrack.com/car-cult...ns-bmw-n63-v8/

[thejlevie]

When at idle system voltage should be about 13.6v, rising to ~14v at 2500rpm or above. If the battery is low, below 12.6v, you will see slightly lower voltages and adding a large load, like the HVAC on max and/or the headlights on, may drop the voltage even lower as the alternator is being loaded by the low battery being charged and the extra load. Driving at highway speed with the A/C off for 20-25 minutes will fully charge a good battery, or you can put it on a charger overnight.