My AC/climate control system works great. Original r-12 freon is still in the system and it keeps the inside cool no matter the temp outside (so far).

I do have one problem - in either AC setting the windshield will 'frost' or fog over the defrost vents on the outside of the windshield.

I have put rain-x and some other things on the windshield, and this may be the problem.

Has anyone else experienced this ? It doesn't happen when the outside temp is over 90 deg., but in the afternoons it is really hard to control. IF I turn down the temp wheel to the 'white' zone, this will usually fix it, but then the inside temp is marginal.

thanks

I hope you didn't put RainX INSIDE the car...........


Are your flapper controls functioning correctly?

any cold A/C will fog the outside of the glass on a day with any real humidity....becasue it makes the glass cold....I'm not sure you should have that much A/C air directed through the defrosters normally.

__________________
Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
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Section 609 MVAC Certified
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"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

I agree with BHD (shakes head and moans j/k j/k).

You're starting to have some vacuum issues that are beginning to direct A/C to defrost vents as default position and you're cooling the glass too much. A/C should generally be out the center and side vents.

Likely suspects: cracked rubber diaphragms on vacuum pod(s).

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The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)

First tip: let people know, one way or another, what model and year of car you're asking about. Do it in your post or do it in your sig, but do it.

At least on my '85 300D, the access to the vacuum pod for the defroster vent is pretty good (remove underdash panels on driver's side). One can visually check to see if it's actuating as it should when switching between regular A/C, defrost, and the mixture of the two.

If it's not actuating, then one can hook up that pod to a Mityvac to check for operation and leak tightness. I agree with jbaj007, that's the prime suspect. If it works with the Mityvac, but not under control of the ACC system, then maybe it's a bad switchover valve.

I had a similar fogging problem on mine once, along with very little air flow from the center vents. A new vacuum pod for the defrost flap set things to rights. It still condenses a little moisture on the outside of the windshield under some conditions, but I attribute that to the seals in the defrost flap being ~20 years old - always some air going to the windshield.

Sorry, its' a 1987 300sdl. Should there be any cold air coming through the defrost vents ? (there are two ac positions I think, with the push buttons).

The ac blows pretty strong from the dash vents, and you can hardly 'feel' any cold air coming from the defrost vents (altho there obviously is).

The manual says that there is one setting which should 'defog' the windshield in the ac position, but other than adjusting the temp wheel down to a warmer level, you can't defog.

This is not a huge issue, just kind of aggravating. The wipers will usually remove the fog.

And I didn't put the rain-X on the inside lol. I did try the rain-x with 'defogger' on the outside and it did help quite a bit - just two small stripes above the defrost vents now.

I'm sure that the defrost 'flaps' are not working exactly right, and probably the seals are worn enough to allow a 'leak' through the vents to the windshield.

So far :
Likely suspects: cracked rubber diaphragms on vacuum pod(s).
Switchover Valve.
Worn defrost-vent seals.


Thanks for the info - I do have access to a Mercedes wrecking yard. Not many diesels, but quite a few 126 gassers.

I'm going to follow this thread...............

My 87 SDL has the same 'windshield fogging' issue.

I await the results and a detailed repair procedure with pictures........

Thank You

Stephen

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98 K2500 Suburban 6.5 Luxo Barge

"We cannot change the wind - But we can adjust our sails"

I Found my problem on the 300SDL today!!!!!!!!!!!!

Same situation - no A/C coming out of the center vents.......

Mighty-Vac to the rescue!!!!!!!!!!

Once I disconnected my Headlight Vacuum Adjust(And plugged the open port) - my center vents came back to life...... Must be that darn check valve that I never picked up to finish the headlight project.........

So for the moment, with 90 degree days with 90 percent humidity - I will go without the headlight adjusters - I'll find out tonight where the headlight positions are defaulting to.

Another good day in the driveway.............

Stephen

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98 K2500 Suburban 6.5 Luxo Barge

"We cannot change the wind - But we can adjust our sails"

In some cases, the flaps are working properly but, due to age, are not sealing completely and allow some cold air to leak out of the defrost vents onto the back of the windshield.

This is exactly my problem. I'm kinda new here, and so I'm wondering, how would you go about removing 'underdash panels on driver's side' and fixing this vent problem? Its too hot here in Florida to not have fulll blast on front vents. Thanx. O yea, I have an '82 300D Turbo.