W124 Front Windshield Fogging with AC? - PeachParts Mercedes-Benz Forum

JamesDean · #1 07-17-2013, 09:58 AM

Hey everyone,

So pretty much every day on my way up to work I run the a/c on low speed and without fail every day I get condensation on the outside of the window.

So i end up looking like this:

I'm just wondering if this is normal? I know in the 126's MB left the defroster pod open at 20% with the a/c on....

Not so sure about the 124s....

Anyway, let me know what you guys think..

jay_bob · #2 07-17-2013, 10:25 AM

Basic laws of physics = surface temperature below dewpoint -> moisture will condense on that surface. The cooled air coming out of the defroster is driving the temperature of the glass below the dew point.

I looked at the sequence of operations for the 124 ACC and it appears there are conditions where it is normal for the defrost flap to be opened slightly while operating in air conditioning mode at higher temperatures. This is to be expected, as you might be running the ac on a cool rainy day and have the temperature up about 22 to 24 deg c. In this case the system should be putting some dry air on the windshield to keep the inside from fogging up from interior moisture.

However once the temperature in the plenum drops below 8 deg c the defrost flap is supposed to be completely shut.

I wonder if you have a weak pod on the defrost flap. Remember that "safe mode" on the ACC is that the defroster defaults to open if it loses vacuum or control power. I suspect your defrost flap might not be shutting all the way even when the ACC is telling it to.

I will say in central SC where we have high humidity that this is a common problem for us. Just running the ac can drop the surface temperature of the glass below the dew point (and our summer morning dew points can be in the 70s) and cause the entire car to fog up all at once.

JimFreeh · #3 07-17-2013, 10:58 AM

Quote:

Originally Posted by jay_bob

Basic laws of physics = surface temperature below dewpoint -> moisture will condense on that surface. The cooled air coming out of the defroster is driving the temperature of the glass below the dew point.

I looked at the sequence of operations for the 124 ACC and it appears there are conditions where it is normal for the defrost flap to be opened slightly while operating in air conditioning mode at higher temperatures. This is to be expected, as you might be running the ac on a cool rainy day and have the temperature up about 22 to 24 deg c. In this case the system should be putting some dry air on the windshield to keep the inside from fogging up from interior moisture.

However once the temperature in the plenum drops below 8 deg c the defrost flap is supposed to be completely shut.

I wonder if you have a weak pod on the defrost flap. Remember that "safe mode" on the ACC is that the defroster defaults to open if it loses vacuum or control power. I suspect your defrost flap might not be shutting all the way even when the ACC is telling it to.

I will say in central SC where we have high humidity that this is a common problem for us. Just running the ac can drop the surface temperature of the glass below the dew point (and our summer morning dew points can be in the 70s) and cause the entire car to fog up all at once.

I've noticed the problem on some W124 cars, and not on others.
I suspect a weak flap or possibly door insulation not presnt.
When on the non-defrost mode, you shouldn't have enough bleed to create the moisture outside the glass.

Jim

Phillytwotank · #4 07-17-2013, 11:30 AM

I can't say if it is normal or not for your w124 car. I can say that I do experience this in my '78 CD. It is usually not that much of a problem but if I want it to go away I just put a couple lilttle rags up there to block off all air coming from the defrost vents.

compu_85 · #5 07-17-2013, 11:38 AM

I'd say your defroster flap is open more than it should be...

-J

engatwork · #6 07-17-2013, 01:42 PM

It should not be there for reason compu said. Do this, find the vacuum "splitter" under the hood near brake booster and see if the lines continuing on into the firewall leak down. If they do then you will need to fix what is leaking. If it does not leak down the you have a vacuum leak issue on the engine compartment side of the "splitter".

compu_85 · #7 07-17-2013, 01:55 PM

At least cars with a glove box instead of an airbag have easier access to the vacuum manifold. That's where the problem was in my car.

-J

Phillytwotank · #8 05-17-2015, 01:48 PM

JamesDean
Did you ever find a resolution to your problem?
I'm in a w124 now and experiencing the same situation.
My defrost vents seem to be always 100% open. Probably a bad vac pod in there. Are the difficult to access on the w124?

JamesDean · #9 05-17-2015, 04:42 PM

I do recall replacing my defroster pod. It was partially bad. I also capped off all my other leaky pod's at the switchover valve. I've only got 2 working at the moment (defroster and floor).

I don't recall having any fogging problems. I should note that I've not been driving my 300D much lately. It down for some maintenance so I've only driven it in 2-3 days of A/C.usage.

jbach36 · #10 05-17-2015, 10:19 PM

It's caused by the pods not closing tightly. I think to get to them the dashboard has to come out and it's a big job.

Instead of doing that, you can put something in that dash area to keep the a/c overflow from coming out of that area, and/or diverting it a bit. You might be able to use those "draft dodgers" used underneath doors of a house in the winter or anything else that would fit in easily and not cause damage or cause you to get in an accident if it falls off the dash while driving.

JamesDean · #11 05-17-2015, 11:27 PM

The defroster pod is accessible by removing the glove box. It and the floor pod are the only two that you can get without removing the dash.

I think the best approach (short of removing the dash) is to just disable the pods that don't work or close fully.

My shop quoted $900 to pull the dash and replace the pods, with me providing the pods.

gatorblue92 · #12 05-18-2015, 05:58 AM

My 300TE does the same thing as did the 300E before the a/c died last summer. Rebuilding all of the HVAC system is on the list for the TE.

iroka · #13 10-25-2020, 07:49 AM

I as driving today in from Atlanta to Alabama and low and behold a rainstorm came about causing 10 meter visibility, my windows began to fog as there was no AC running. I turned on the rear defroster, then the defroster on the ac panel. suddenly my entire front windshield turned white! I WAS BLINDED! in 80+ MPH traffic! Thankfully I was in the leftmost lane with a shoulder and was able to pull over. I had to wipe my windshield for 10 miles stuff my defrost vents with paper towels i had to minimize the fogging. I managed to find a restaurant that had lemons so i could put the juice on the window to prevent further fogging... Some care from my central vents... I was worried about fire and shut the car down. on restart the blowers were blowing full force... CONFUSED AF!!!