Air Conditioning, Air Velocity, Decreases with Time On

AC temperature is fine. Nice and cold. Actually, I experimented recently, by giving it a shot with some after market stuff that claims to make the AC air even colder. Guess what, it did slightly!

Problem now is, after a drive with the AC on(no matter what the fan speed is set to), say at least a 20 minute drive, the air velocity out of the vents significantly decreases after that consecutive with AC on, 20 min drive. When this happens, for your reference sake, fan speed 3, would feel like fan speed 2, and 2 would feel like 1. Sound wise, it almost sounds like something is restricting internally. The blower is spinning like it should, and sounds healthy, but the air simply ain't coming out of the vents like it should bbe.

Interestingly enough, while this symptom occurs, if I shut off the AC compressor via the AC button, but leave the fan on, within a mere 1-2 minutes, I can feel air velocity coming back as it should! At that point, I switch the AC button compressor back on, and all is good for another 15 minutes about, until I have to repeat all that again to make it work fine.

I've thought about water or even ice accumulation somewhere within the air network, choking the system, but the question I ask to some of you is where is this occurring? If that is the case that is.

What you describe is almost certainly ice forming on your evaporator coil.

This is often caused by not having enough charge in your AC or having the orifice or expansion valve (depending on the system), not passing enough liquid in to the evaporator coil.

You get a single small very cold spot, instead of evaporating (boiling) going on in the entire evaporator section. As ice forms, the point just moves further down the tube, until the whole coil is iced up.

DanG

Just to add to what Dan wrote, I suggest that you have it checked by a A/C shop (in the north they are usually A/C & Radiator shops). Don't try to just add more refrigerant. In fact the charge may not be the problem, which the A/C shop would figure out quickly), but the specific weather conditions and the fan speed settings you are using may be the only problem.

I'm watching this one as another thread is currently ongoing with a similar issue. In my case, I had my system checked with one of those electronic R134 contraptions, and it passed. I've heard there is a sensor that's supposed to kick the compressor off, so maybe that sensor is getting hot, failing, and allowing the compressor to freeze the evaporator.

Is something like that possible?

Bayou_Flyer said:

I've heard there is a sensor that's supposed to kick the compressor off, so maybe that sensor is getting hot, failing, and allowing the compressor to freeze the evaporator.

Is something like that possible?

Click to expand...

You can observe your compressor with the A/C on full blast. If the clutch activates and the compressor cycles on and off, then you shouldn't have a bad sensor. I agree with Dan. I think he nailed it.

Check the other thread. It may give you some insight.

http://forums.tdiclub.com/showthread.php?t=214723

wczorn said:

You can observe your compressor with the A/C on full blast. If the clutch activates and the compressor cycles on and off, then you shouldn't have a bad sensor. I agree with Dan. I think he nailed it.

Click to expand...

I had my system emptied, vacuumed, and refilled. No blockage, no leaks, and good pressure. If the evaporator is freezing, maybe the compressor is NOT cycling occasionally to prevent ice blockage. I keep my AC turned up so that the ourput is around 45-50F and the low flow still occurs.

I did find a TSB for the Mk5 that mentions a Evaporator Vent Temperature Sensor -G263- that is causing a low-flow/evap freezing condition. Do Mk4 systems have a similar sensor?

Are the fans running........I had a problem with mine the Fans weren't running.......VW replaced the fan under warranty

Unless they replaced the expansion valve or expansion orifice during servicing, even replacing the compressor can be ineffective.
Many places skip this, because these components may be under the dash and hard (time consuming) to get to.
There are two major causes of this type failure, low freon in the system, and low freon delivery to the coil. Not really, really low, just marginally low. The expansion valve or orifice is what determines the delivery rate. A small piece of metal, rubber or teflon can foul the expansion orifice.

I will try to look at the Bentley control loop for my MkIV tonight for the vent temperature sensor.

DanG