A/C compressor caused engine to stall?

[crazyquik22023]

Was on the way back from a 4 hour trip and engine stalled out on the at low rpms on the highway. Pulled over and tried cranking over engine and couldn't get it to crank. Starter just does a small crank, kind of like when you have a weak battery. Pulled out VCDS, no fault codes, battery voltage is good. After doing several more attempts to crank I finally got it to crank over and it ran fine. Drove it to local auto parts store to get battery tested and everything checks out good. Only thing I can tell that is wrong is I have no A/C now. I am wondering if some how the compressor was seized up which stalled the motor, and then cranking it over enough times with the starter broke it free. Anyone ever heard of something like this happening before?

[Turbo510]

The AC pump can't stall the engine because it is driven through rubber parts that will shear off if the compressor requires too much torque to turn. The engine stalling and not cranking sounds like an electrical problem, maybe the ignition switch.

[crazyquik22023]

The AC pump can't stall the engine because it is driven through rubber parts that will shear off if the compressor requires too much torque to turn. The engine stalling and not cranking sounds like an electrical problem, maybe the ignition switch.

I have to disagree with you there. If it were ignition switch related I would assume a loss of communication with other electrical components and also it was sending a signal to the starter evertime I attempted to crank after it initially stalled. Everytime I have seen an ignition switch go bad even intermittently it would not cause the type of symptoms I am seeing. Also A/C was blowing cold air before stall and then after stalling that was the only thing that is not working.. Seems like too big of coincidence for the A/C in some way to not be related to the stalling issue.

[Cybersombosis]

Next time it happens, remove the AC belt and see if it starts. You will then have your answer.

[Kevin C]

Check the rubber drive links in the pulley, if the compressor seized they may have sheared off.

[crazyquik22023]

Next time it happens, remove the AC belt and see if it starts. You will then have your answer.

For sure I think that would be a great idea. I think it was a one time thing though. I am doubting it will happen again, but that would be a good way to tell for sure if it does happen again. This just happened a few hours ago and I have started and stopped then engine at least 20 times since then. Drove about 200 more miles as well.

Check the rubber drive links in the pulley, if the compressor seized they may have sheared off.

I will give it a look. Is it possible to see that without removing the pulley from the compressor? From what I remember that pulley is completely covered.

[Cybersombosis]

If the hydrostatic clutch failed and seized, then broke free and will no longer engage the compressor, that would explain why no more A/C.

[crazyquik22023]

If the hydrostatic clutch failed and seized, then broke free and will no longer engage the compressor, that would explain why no more A/C.

I agree completely. That was my initial thought as to what happened. I was just curious if anyone has had something like this happen before and shed some light onto my theory of this being the case. I myself have not heard of a seized a/c compressor clutch being stuck engaged causing the motor to stall then breaking free, but it was just my first logical thought as to what could have happened in this instance. In theory it makes sense and I can see it being the cause, but I have just never heard of such a thing before.

[old guy]

The A4 B6 A/C compressor doesn't have a clutch. It is permanently engaged. There are rubber pads that connect the pully to the compressor. If the compressor locks up it will usually kick the drive pads out.

[Cybersombosis]

Ah I see. Ya I had a customer call about the screeching sound of the belt for a B5 and when he unplugged the clutch, all was good. He could drive the car again. Good to know about the B6.

[crazyquik22023]

The A4 B6 A/C compressor doesn't have a clutch. It is permanently engaged. There are rubber pads that connect the pully to the compressor. If the compressor locks up it will usually kick the drive pads out.

Interesting. It still has to technically work like a clutch though right?

[old guy]

It's a variable displacement compressor. AKA swashplate compressor. Always turning but not always compressing.

[Turbo510]

No clutch. As old guy says, variable displacement, meaning the piston strokes can vary from nearly zero to their maximum. At zero stroke the pistons don't move so the refrigerant isn't compressed. A benefit of this arrangement is that a only a simple orifice is needed to efficiently discharge the refrigerant into the evaporator. The pressure difference before and after the orifice is the same as across the inlet and outlet of the compressor, and this is used to adjust the tilting swash plate.

[crazyquik22023]

I see. So are you saying my theory of the a/c compressor "seizing up" then breaking free is not viable?

[Kevin C]

I see. So are you saying my theory of the a/c compressor "seizing up" then breaking free is not viable?

The parts inside the compressor are always spinning so they can seize and increase turning torque. Also, the compressor is never actually set for zero displacement, its always pumping a small amount so that it circulates lubricant to prevent seizing. The displacement level is low enough that the drag is tiny, but its still always pumping. Did you ever check out the compressor?

[crazyquik22023]

The parts inside the compressor are always spinning so they can seize and increase turning torque. Also, the compressor is never actually set for zero displacement, its always pumping a small amount so that it circulates lubricant to prevent seizing. The displacement level is low enough that the drag is tiny, but its still always pumping. Did you ever check out the compressor?

Never got a chance to take it off yet. I would like to do it the "right" way and have the refrigerant evacuated and not just let it out into the o-zone. Have to take it to a shop to do that and haven't had the time yet and luckily its about winter time where I live so no need for the compressor right now. I might see if I have a chance over the weekend to get it evacuated and then I can take the compressor off and check it out. I've been driving it daily since it stalled though and no problems.

[Kevin C]

Never got a chance to take it off yet. I would like to do it the "right" way and have the refrigerant evacuated and not just let it out into the o-zone. Have to take it to a shop to do that and haven't had the time yet and luckily its about winter time where I live so no need for the compressor right now. I might see if I have a chance over the weekend to get it evacuated and then I can take the compressor off and check it out. I've been driving it daily since it stalled though and no problems.

If your motivated you can un-tension the drive belt and just give the pulley a spin. Super free wheeling says the drive couplers sheered. A light drag is normal, bumpy is pieces of the drive are still banging around.

[Turbo510]

If your motivated you can un-tension the drive belt and just give the pulley a spin. Super free wheeling says the drive couplers sheered.

My son's 2009 Q5's compressor quit one day, in summertime Phoenix of all places. We changed it out at his Air Force base auto shop, where a bay with lift is only $8/hr, and use of the AC evac and recharge machine is included. We spent all day, in a breezy shop with fans blowing the 115 degree air around. But the old compressor was exactly as you described; super free wheeling. The new replacement by contrast had the feeling of mechanical movement inside, and when pre-filling the PAG oil, made slurping sounds as we turned the pulley.

[john_gonzo]

These might be helpful...

Air-conditioning-compressor-swash-plate-vs-clutch

Bypassing-3-0-A-C-compressor-pulley-with-shorter-serpentine-belt
Info for 1.8 belt is in there too.

[Kevin C]

One thing that can kill a compressor is low refrigerant charge. Since the guts are always spinning it needs circulating lubricant. Low charge reduces lubricant circulation and can shorten or fail the compressor. All AC systems lose about 4 to 10 grams of charge per year. Not a lot compared to the total charge, but after 10 years its enough to reduce AC performance and lubricant circulation. Figure our cars are hitting the 20 year mark. Having the AC topped off / serviced can save money long term.

[crazyquik22023]

If your motivated you can un-tension the drive belt and just give the pulley a spin. Super free wheeling says the drive couplers sheered. A light drag is normal, bumpy is pieces of the drive are still banging around.

I'll give it a spin this weekend

These might be helpful...

Air-conditioning-compressor-swash-plate-vs-clutch

Bypassing-3-0-A-C-compressor-pulley-with-shorter-serpentine-belt
Info for 1.8 belt is in there too.

Great links

One thing that can kill a compressor is low refrigerant charge. Since the guts are always spinning it needs circulating lubricant. Low charge reduces lubricant circulation and can shorten or fail the compressor. All AC systems lose about 4 to 10 grams of charge per year. Not a lot compared to the total charge, but after 10 years its enough to reduce AC performance and lubricant circulation. Figure our cars are hitting the 20 year mark. Having the AC topped off / serviced can save money long term.

I know my charge was not low when/if my compressor has failed. Based strictly on low & high side pressures, not weight of refrigerant, I know my a/c was right where it should be just a few weeks before my "incident"

[crazyquik22023]

Checked the compressor, pulley spins with almost no resistance. I am willing to bet the coupler is toast. Probably won't end up changing it out until the spring. Will most likely just buy a used compressor. Thanks for all the input.

[Turbo510]

We bought the compressor for the Q5 brand new from discountacparts.com, for around $240 which included the receiver. Apparently very good quality, made in Taiwan I believe, works just fine.

[inliner311]

So this isn't a direct comparison but on my B5 S4, it would stall on the highway and I would pop it out of gear then just pop start it. It did this a couple times but it was real infrequent. Then one day it died while I was like a mile from my house. I had to push it to a side street. Friend was with me and need to get somewhere so call him an Uber. Uber took 15 minutes and I started trying to find a tow truck. The minute my friend gets into the Uber, my car starts up.

I was stumped on what it could be. I changed a couple things but it end up being the crank position sensor. What is happening to you seems oddly similar to what would happen to my B5 S4.

[inliner311]

Also the crank position sensor in a B5 S4 is the same as the one in a B6 A4 so it would make sense that when one starts to go bad they would have similar symptoms.