Infuriating 124 Defrost Problem
 

I recently picked up a '95 E300D, which I am enjoying tremendously. However, I am starting to dread damp and rainy days in this car because of its absolutely abismal windshield defrost. I have to assume that MB didn't intentially design a system this inadequate, so I am going to assume that there is a problem.

(For ease of identification, I'm going to lable the buttons on the climate control A-E, left to right. Defrost is A and Off is E).

In cold, dry weather it takes forever for the windshield to clear, but it does eventually, even left on D. However, when it is rainy or humid, anything but A usually makes the windshield worse. I have a 75 mile, 1.5 hour commute, so I have a lot of time to mess with the system. :( Today, it was rainy and humid, around 55 degrees (F).

On C or D, there is a slight amount of air felt coming from the defrost ducts. On B, there is more air coming from the defrost ducts. On A, there is a ton of air coming from the ducts. The system seems to route the air correctly, so I do not believe it is a vacuum pod issue.

On A, B, and C, I can see and hear the A/C compressor engaging, so I do not think it is an A/C issue.

On A, the system DEFINATELY cranks at full heat, as it's supposed to. This is how I drove home today, with the rear windows down so I wouldn't pass out from heat stroke.

When attempting to defrost, I never use the recycle button.

A does a reasonable job clearing the window, but even then there is a strip of fog along the top of the windshield. B, C, and D all *increase* the amount of fog on the window. I'm better off switching between E (off) and A (full blast) for the duration of the trip, which isn't really ideal. I am not getting any coolant smell or an oily film on the windshield, so I don't think the heater core is leaking.

Based on this info, I have to guess that I'm getting moisture in the heater box that is getting blown onto the windshield. I assume there are some drains that I can check, but I'm not sure where to look.

Any suggestions on the problem, or where to look for the drains, would be most appreciated. I hate the fact that my '92 GMC pickup (actually, EVERY vehicle I've ever owned) has better defrost on low fan than my MB has on max.

Thanks in advance!

De Frost
 

Comrade,

the Gnomes of Sindilfingen (Etc., Etc.) generally design in Defrost as the
default function [ If nothing else works in the HVAC system, Defrost will]
(To get you safely home!)

I will suspect the vaccum actuator inside the dash is Kaput.

[Triage, by Internet, is always suspect.]

It is also possible your drains are plugged. There are some drain tubes that carry water from condensation or any other source that might make its way into the HVAC system, like rain water coming in with the air. I believe these drains are located on either side of the transmission under the car. Left plugged they will contribute to a musty smell developing in the A/C system. Try a search on the subject if you think you want to explore this further. The drains are discussed relatively frequently. Good luck, Jim

It may be a good idea to pour some water into the fresh air intake and see at what rate it drains through. I’ll try this sometime today on my 94 E320 and report back for comparative purposes. Also, check in the nooks & crannies near the battery and fuse box for composting leaves and other junk, it may be an indication as to what’s going on in the intake channels. Finally, the cabin filters may be so clogged that, effectively, the air is re-circulating all the time.

J B, That has been a source of frustration for me too since I've had my W124 wagon (bought in 1998). It is a lousy climate control system providing very little air flow to the windshield for defrosting. What I do is use button 'B' with the upper center vent closed and the two side door vents open. This mode concentrates the air flow to the windshield more effectively. The 'fog' usually happens on rainy days above 45F. Sometimes popping the moonroof a little helps as it sucks the human respiration responsible for the inside fog out of the car. JimSmith has a good point in the clogged drains but maybe you are not giving button 'B' long enough time to work. Give me a manual system anyday!

I am getting plenty of air out of the vent, and it varies depending on the button pressed, so I don't believe it's a vacuum issue.

Thanks.

I'll do some more searches. I'm not getting a musty smell, but I think it might be an issue.

Thanks.

I'll give this a try as soon as I locate the drains. The car is pretty much spotless; I haven't found any leaves or debris anywhere. You can see pics in this thread.

The cabin filters, though, are something that I hadn't thought of. I'm not sure they've EVER been replaced and I'll check into those right away.

Thanks.

I don't think it's a time thing. I've run on "B" for over an hour and gotten maybe the bottom 6" of the windshield clear. I seem to get good flow, it just doesn't do anything. I'd pop the moodroof, but when it's pouring rain out it's not a good idea. :)

Thanks.

I poured a quart of water throught the fresh air intake grate, it went right through the drain holes immediately.

As I understand the buttons,

"a" directs all air up, full fan, full heat. Power Defrost. :eek:
"b" directs air up and down, with center vents if cooling, A/C compressor enabled.
"c" directs air down, 'leak' air up, center vents if cooling, A/C compressor enabled.
"d" directs air down, 'leak' air up, with center vents if cooling, A/C compressor disabled.
"e" is OFF.

55F weather is hard to defrost, you want 'dry' air coming from the vents. Air is 'dried' when it is cooled below dew point, and then reheated.

I'll suggest using "b" and selecting a heating temperature that will result in reheated, warmer, drier air from the windshield vents. If you select a colder temperature, the glass will be cooled and more water will condense, making the problem worse. :mad:

Try using Defrost to get the glass clear, and then "b" to keep it clear, with perhaps an occasional manual Fast fan speed to stir up the air?

Hope this works, let us know.

Best Regards,
Jim

Check the temperature of the AC hoses while the compressor is running. The high pressure side must be warm or hot, the low pressure side must be cool to cold.

If you have a clogged evaporator valve (or one stuck open), you could have either no cooling or a freezeup of the evaporator, either one will cause the problem you have.

Someone may have also shorted the pressure switch on the reciever/dryer so that the compressor runs with no freon in it -- this will eventually damage the compressor.

Your problem is strictly related to no cooling (and hence no de-humidifying) of the intake air.

Peter

I have the same car and the Defrost functions work fine. I would follow PSFred's recommendations. Make sure that your AC compressor is functioning. [Its supposed to come on in Defrost mode].

If the AC is not working, your windows will not defrost properly in humid weather.

Phil

If I used Defrost (a) and switch to any other button to keep from getting cooked, the winshield immediately refogs.

I will check this. I know that the A/C is cycling when in Defrost mode, but it's been too cold to see if it's blowing cold air.

Thanks.