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Windows fogging up! Cause? 200 1988

posted by  someone claiming to be Randy Starkie on  Mon Dec 11 12:34 CST 2000 [RELATED]

I had the opportunity to drive what has become my son's car today. It was in the low 50's and raining. The inside of the windows were heavily steamed up during the entire drive- 40 minutes. The heater set to defroster managed to keep the windshield and most of each front window relatively clear and the rear defroster made it possible to see out of the back. However, anytime I turned the blower motor down all the windows fogged up again. The rear seat windows were covered with condensation the entire time. I don't believe the heater core is leaking (no anti freeze smell) but there seems to be an abundance of moisture ready to condense on the cooler windows. Any ideas on what might be the cause of this? When I questioned him about it he said it is a problem he has been experiencing for some time. Thanks for any insight.

Randy







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    Re: Windows fogging up! Cause? 200 1988

    posted by  someone claiming to be Trevor G. on  Mon Dec 11 17:39 CST 2000 [RELATED]

    Carefully check all the carpets to make sure you don't have a leak somewhere. I had this problem in my 140. I found a rust spot in a rear wheel well was letting in wheel spray, making the rear carpet wet, just over the muffler. When the hot muffler heated the floor it evaporated the moisture into the car, which then caused heavy condensation on the windows.







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    Re: Windows fogging up! Cause? 200 1988

    posted by  someone claiming to be Guy W on  Mon Dec 11 13:34 CST 2000 [RELATED]

    I have always had poor performance from my defroster in my 89 244 in similar situations. My defroster ducts do not throw enough air on the windshield to clear it if it fogs up. My only solution has been to use the Anti-Fog products (check your local auto supply) applied to the windows, which helped a lot.







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      Re: Windows fogging up! Cause? 200 1988

      posted by  someone claiming to be Dave Stevens on  Tue Dec 12 08:31 CST 2000 [RELATED]

      Next time you're under the dash, put the fan on high and check for defrost air leaks between the top of the ductwork and the bottom of the defrost vents. Of the handful of 140/240's I've worked on, none was fully sealed there -some worse than others. The solution is to seal around the base of the defrost vent with self-sticking foam insulation. In one case I also needed to re-position the ductwork a bit by bending the mounting brackets and using drywall screws to secure the connection. Unfortunately it does require a bit of gymnastics to get at this. On our '85 240 I'm sure I was able to double the defrost air flow and it was well worth the effort.

      As a note to others on fogging problems. Make sure that the passenger compartment air exit venting is not obstructed. On sedans that means the slots under the rear window, on wagons it's the small grill below the right rear side window. Of course, this will be of little concern to those with rusted out floors.







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    Re: Windows fogging up! Cause? 200 1988

    posted by  someone claiming to be Don Foster on  Mon Dec 11 12:56 CST 2000 [RELATED]

    Randy --

    "Any ideas on what might be the cause of this?"

    Sure — it's simple thermodynamics. It's raining out, so the humidity is high. You brought water into the car and raised the temperature, evaporating more moisture—the RH is near 100% inside the car, and because it's warmer the dew point is higher than the outside dry bulb temperature. The result is abundant condensation on anything slightly cooler than the inside temp (such as the windshield).

    The answer, as Mr. Bain said, is to vent some moisture-laden air outside. Open a window.



    Don Foster (near Cape Cod, MA)







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      Re: Windows fogging up! Cause? 200 1988

      posted by  someone claiming to be Peter Burt on  Mon Dec 11 15:19 CST 2000 [RELATED]

      ok, this kind of pretains to this post and it happened to me this morning, hopefully you might know. The passenger-side windshield defrost turns off when the floor vents are on. But when the floor vents are turned off, the windshield vent on that side DOES turn back on. Is it supposed to do that, to dirrect more air to the floor? Or Do you think there is a "gate" that is confused? Thanks for any suggestions.

      Peter







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        Re: Windows fogging up! Cause? 200 1988

        posted by  someone claiming to be Don Foster on  Mon Dec 11 22:39 CST 2000 [RELATED]

        Peter --

        When you open the floor vents, a considerable volume of the air from the blower is diverted to the floor—which means the defroster vents receive MUCH less. So it may seem as if the windsheild vent has shut off, when it has really just been reduced in flow.

        On the other hand, each vent is controlled by a damper (or "gate," as you said). The floor, defroster, and recirculate "gates," or dampers, are controlled by vacuum signals. These vacuum signals come from the three pushbuttons in the middle of the center console, and are led to the dampers by small hoses.

        It's possible that this plumbing network—from the pushbuttons to the dampers—is, well, hosed. Maybe someone reconnected a few of these hoses to the pushbutton assembly and one or two aren't hooked up right.



        Don Foster (near Cape Cod, MA)







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    Re: Windows fogging up! Cause? 200 1988

    posted by  someone claiming to be wbain on  Mon Dec 11 12:51 CST 2000 [RELATED]

    Did you have the ventiation system in reculate mode?







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      Re: Windows fogging up! Cause? 200 1988

      posted by  someone claiming to be Randy Starkie on  Mon Dec 11 14:49 CST 2000 [RELATED]

      You hit it on the head. I didn't pick up on the recirculating control being on. I knew there was too much moisture in the interior air but just didn't think it through. Thanks everyone for the help.

      Randy







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        Re: Windows fogging up! Cause? 200 1988

        posted by  someone claiming to be WBain on  Tue Dec 12 00:35 CST 2000 [RELATED]

        I know this because it happened to me. My wife, kids and I were driving the 965 in the rain. I had the recirculate button pushed and the windows were all fogged up. Once I reset it the windows cleared. The 965 has an all electronic climate control with vacuum acutators. Very nice, I guess I will be resoldering it in a few years.







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      Re: Windows fogging up! Cause? 200 1988

      posted by  someone claiming to be Jim McDonald on  Mon Dec 11 14:33 CST 2000 [RELATED]

      Good point; even if the button isn't depressed, a failure of the vacuum actuator can leave the "inside/outside" air flap in the "inside" positon,. which will contribute heavily to fogging. BT,DT.

      Turning on the A/C will help a lot.






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