A/C duct issues?

[ptheland]

Are there any common places where the A/C and ventilation ducting commonly comes apart?

I was driving down a bit of rough road (I-405 South, just past the Sepulveda Pass for those that are familiar with LA Area freeways) when I noticed that the air had stopped coming out of the dash vents. I can still hear the fan running, and the sound changes as usual when selecting different fan speeds. I get a general sense of cool air in the dash area, but it's definitely NOT coming out of the vents. The air was flowing normally earlier in the drive, so it's some kind of fairly sudden failure.

My guess is that something fell apart in the duct work, causing the air to leak out behind the dash. I'm wondering if there is a common place this happens. I'm hoping all of you off-roaders will have a bunch of experience with rough roads and will be able to help out a city-bound cousin.

I haven't had a chance to do any troubleshooting yet, so any tips would be greatly appreciated.

 

[Bob Laps]

Have u tried it again since?...I've read on here about something freezing (condenser?) and it working again after it thaws...it happened to my 2017 and after I stopped at Starbucks and came back out it was fine and has been fine since

Bob

 

[ptheland]

Have u tried it again since?...I've read on here about something freezing (condenser?) and it working again after it thaws...

Ummm ... errrrr ... aahhh ... no. Oops.

I’ve read the same and completely forgot about it. I’ll start with that and report back.

 

[4wheeldog]

Ummm ... errrrr ... aahhh ... no. Oops.

I’ve read the same and completely forgot about it. I’ll start with that and report back.

I had the same thing happen. I was concerned enough (since it was damned hot......Over 100 degrees, and I was traveling with dogs) that I pulled over and shut down to check things out.
Almost immediately, with engine shut down, air was again coming from the vents.
When I realized, I adjusted the temp to about 65. Worked great ever since.

 

[ptheland]

Well - I just went out to the van. And it blows air.

Must have frozen up. I did have the temp spun around to a pretty low setting. It didn't freeze with the air temps pushing 90 during the frequently traffic-jammed drive out. But the late afternoon drive home was closer to the coast with cooler temperatures and free-flowing traffic.

Move along. Nothing to see here. Just some old guy who appreciates the courteous reminders of things he's read before.

 

[Bob Laps]

Awesome

 

[OffroadHamster]

Common issue with the sprinter from what I have seen and experienced. I've had it happen a few times when the ambient humidity was right and the temp outside was right. Leaving the fan on and switching the AC off will thaw it out pretty quick while spitting water at you for ten minutes.

 

[ptheland]

Had the van out today for an actual drive. AC blow fine again. Kept the temp at 65-68.

 

[4wheeldog]

Well - I just went out to the van. And it blows air.

Must have frozen up. I did have the temp spun around to a pretty low setting. It didn't freeze with the air temps pushing 90 during the frequently traffic-jammed drive out. But the late afternoon drive home was closer to the coast with cooler temperatures and free-flowing traffic.

Move along. Nothing to see here. Just some old guy who appreciates the courteous reminders of things he's read before.

I have to say...…..This is the only vehicle I have heard of this happening to.
I have seen it with dehumidifiers, And it can happen to refrigerator/freezers.
But a road vehicle...….It is a first for me.

 

[ptheland]

I have to say...…..This is the only vehicle I have heard of this happening to.
I have seen it with dehumidifiers, And it can happen to refrigerator/freezers.
But a road vehicle...….It is a first for me.

I'd have to dust of some ancient learning, but my guess is that it has to do with the refrigerant charge.

The stuff I don't completely remember any more is how the amount of refrigerant in a system affects it's ability to cool. I think that it's a bit of a funny relationship.

At the obvious end of things, an A/C system won't work without any refrigerant. And it won't work much with way too much refrigerant. (I think that with too much, the pressure stays too high in the low pressure side, which makes it hard for the refrigerant to change from a liquid to a gas, which is how it takes heat from the air.)

If you graphed the amount of refrigerant in a system against the cooling it is able to achieve, it will start at zero cooling with zero refrigerant, go up then curve back down until you get to a point where you again get zero cooling, but with a bunch of refrigerant in the system.

My guess is that Mercedes has specified the amount of refrigerant pretty close (probably just a bit more) than the amount where you get maximum cooling. And that point is probably not too far above 0 C (or 32F). Lose just a little bit of refrigerant (which is something that is going to happen in automotive systems), and you drop temps to the point where the evaporator coil is freezing.

That gives you a lot of cooling capacity. The problem comes when you don't need all of that capacity. When it's 25 C (call it 80 F) outside, you don't need nearly as much cooling as when it's 40 C (or 100 F). Get to those slightly cooler outside temps with a slightly low refrigerant fill and things freeze up.

Part of me wants to say that fits the German engineering thought. Design the system to operate near peak performance, and don't worry too much that it requires more maintenance attention. But that's stereotyping. Just ask any Mercedes or BMW or Audi owner.

If all of that is correct - which is an extremely big "IF" - then perhaps a very slight overfill of refrigerant is the way to go. You get a bit more room for the slow leakage that is going to happen in any A/C system. The system can't get to quite as low a temperature, though. Which isn't always a bad thing. Slightly less cooling (slightly higher minimum available temperatures) mean that things are less likely to ice up.

I may just have to go and add a few ounces of refrigerant to my van.