The Great S8

I have this intermittent issue on my 2015 Q7 where the HVAC gets called by the ECM to turn off the compressor, and the air coming out of the vents stops cooling. This usually occurs when it is hot, and after the car is warm. It does not always happen, so it has been a bear trying to catch it with VCDS.

When this happens there are some things that are always present:

1. VCDS always shows Compressor Shut Off Code 12 (Shut-Off requested by Engine Control)
2. There are no error codes present - none, nada, never.
3. When driving, I can reactivate the compressor by switching the tranny into Neutral, and coast down a hill to cool down the cabin. It also works if I stop and switch to Park.
4. When I switch back to Drive, or Reverse it turns off the compressor again.
5. Each time I switch from Park/Neutral to Drive/Reverse, I can see the Shut Off Code go from 0, to 12, and vice versa.

I used VCDS to check the fans, and they both turn on/off as needed. Last week I took it to Audi and they could not replicate it (no surprise) and said the refrigerant pressures were fine.

There is very little info online about this problem, and none of it ever mentions why the ECM would turn off the compressor, other then maybe a vacuum leak. The argument is that the ECM wants more power for the engine, but I cannot find any documentation out there that describes the programming.

Does anybody have insight as to what conditions could cause the ECM to do this? I would watch the engine data, but there are so many data points that could be read, I wouldn't know where to start.

Log attached

drjonez

If it is only in drive, it may be something related to requested torque from the ECU. Has the ECU been tuned?

The Great S8

Thanks for the reply drjonez, that is an interesting question. There are no tunes on the ECU as far as I can tell, power etc seems all factory and similar to our 2012 Q7. I'd like to get a JHM tune but that is for another conversation.

'10Q7TDI_Prestige'

It could be a simple vacuum leak sub-symptom. I just had to R&R my brake vacuum line connection at the firewall exit point, where it meets the engine vacuum line, etc., as that was causing myriad issues to occur, as well as braking assist loss, etc. FWIW, I think all these cars are going to develop a vacuum leak at that same location, and it occurs gradually. Rodents also love to chew on vacuum lines, FWIW.

I repaired the vacuum leak at the connection point between the two lines, and all my sub-symptoms went away. This car has numerous vacuum lines/connects on it besides the brake booster connection point, which can get fragile from engine heat over time, so it's good to take a look (and listen) for vacuum leaks with the engine cover removed.

It's not uncommon for cars with a high-performance mode to shut-down the A/C compressor temporarily when max performance is being requested from the engine, but it should come right back on it's own, and not require user interventions, so that indicates you've got an unrelated vacuum leak or perhaps a internal thermo-switch/feedback loop in the A/C system. I once had the radiator fan control resistor fail in a different car, so it wasn't the fan itself, but only the electronic controller for the fan, but gave similar results...the fan wasn't running as necessary to cool the engine, so the A/C compressor got shut down by ECU, etc.

You'll figure it out...just be systematic about it and really think about the details of weather and how you are driving the car when it shuts down the A/C compressor. If the system charge is good, as per Audi, and it works like gangbusters otherwise, then it is likely to be a simple solution. Do pay attention to the HVAC fan itself, as those are known to go out on this car (just replaced one in our '15), and be sure your cabin filter is clear of debris/been changed out, although the audi dealer would have tried to sell you one of those, no doubt.

The Great S8

Thanks for the help. The HVAC really does work like Gangbusters except for when it switches off. The AC in my '15 Q7 works waayy better than the one in my wife's '12 Q7. Last month I drove from San Diego to Tucson twice and it hit 110 in the desert, but it worked like charm the whole time. Only when I got back to SD did this issue start popping up again.

I was thinking about replacing the vacuum lines, but it seems like any failure point might be in the actual hard plastic fittings, like a valve versus the flexible tubing. I will try to find the fitting at the firewall you mentioned. Thinking I should monitor the vacuum data streams in VCDS while I am driving... just not sure which ones.

I have a VCDS HEX-NET, so I think I can connect my phone via wifi to monitor the shut off codes, and have my laptop hardwired to monitor the vacuum at the same time.

SleathyS5

Have you had your battery replaced recently? i know if you have a slightly lower power battery it can cause some accessories to intermittently work such as A/C, Overhead lights, dash lights ect

'10Q7TDI_Prestige'

Agree on the battery status check; there are also a number of sensors in the HVAC system designed to protect the system from overpressure/underpressure situations, etc. Generally, performance calls from the engine can also shut-down the A/C clutch temporarily as a means to get max power output, but not sure if that applies to your situation or not. Regardless, the ECU on this car monitors just about every possible parameter there is on the car...it's all linked together. You can get really odd things happen with a vacuum leak...just sayin.

Whenever I've had A/C go down in other cars, it has always been either low refrigerant, a busted hose, or the radiator fan stopped working, etc. On our Q7, I actually went into VCDS and looked up the HVAC system live data to see all the temp monitors it has and what temps it was showing internally, which helped me determine it was getting plenty cold internally, so that wasn't my issue. Classic tells for low refrigerant is the temp on passenger side front will be colder than on driver side front, etc.

Best way to determine yours is to make a chart of the observed data you have on when it shuts down; the fact that you can coast down a hill in neutral (pushing air through the front with low load/less heat from engine, etc.) may indicate you've got a coolant circulation issue, or the car thinks it is too hot, etc., if you are for sure those radiator fans and their variable speed resistors are good. It's often the resistor that fails, not the fan, etc. Cast a wide net; the low voltage sensed will shut down the HVAC blower and other 'unnecessary' power drains. Low voltage could mean a weak alternator, or even just a bad ground strap issue, etc.

The Great S8

I did have Audi inspect the battery, since I was getting those "quiescent current" codes in VCDS. They said the battery was fine, and could not explain the cause of the codes. I have not had any other battery-related bugs occurring, so I don't think it is battery-related.
Funny that ever since I originally posted this, my problem has mostly gone away. I assume it's because it hasn't been as hot, and so I imagine I won't be seeing it until next summer. I will be going over the vacuum lines in the meantime. Thanks for the input!

'10Q7TDI_Prestige'

One final thought; I just replaced all idler pulleys, tensioner, drive belt on one of ours, and if your belt is slipping on the A/C pump pulley, or the pump itself is approaching end-of-life, etc., or has a clogged orifice tube, etc., all reasons for reduced A/C under a higher load scenario.

Of course, Audi dealer once tried to sell me a new A/C compressor when I asked them about a running noise from engine bay; funny, they didn't mention the heavily corroded crankshaft pulley that was just about ready to fall off our TDI engine. The A/C had never been a problem and still isn't on that car.

I will share that the best 'upgrade' I've done on my Q7 so far is to get all the glass on the car ceramic film covered (even the windshield) and the sunroofs are ceramic film covered to reject heat during the summer. Makes such a HUGE difference. Also, putting film on the sunroofs protects you and yours in case of the unlikely event where the glass on one explodes, which does happen, btw...look it up.

outerunknown

I am having a dog of a time with my HVAC in this thing.
warm/cool/hot, makes no difference. It is arbitrary at best.
VCDS and Launch have no fault codes for the HVAC barring an air quality sensor.
the AC cools very well and suddenly stops cooling. 5 minutes or 50. Some days it runs perfectly. Some days it refuses to cool at all. Whilst running on a flat road, cooling and then not. Traffic or no traffic.
gas levels are good. Pressure is good. Changed the HP/Thrust Switch. No difference.

Sometimes it comes on briefly for a few minutes after a start, but it’s largely hit or miss. More of the latter.

has anyone else experienced this?