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  #1  
Old 11-16-2002, 05:29 PM
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Question Heater on drugs

I'm sure this question has been asked countless times in the past. However, here I get to ask it again.

The heater on my 300D works very erratically. I assume the heater core's okay; haven't noticed any coolant leaks. However, I have noticed that the heater doesn't work very well. Even in full DEFROST mode, the air actually only blows out hot slightly less than 50% of the time. The rest of the time, it feels like the air's not being heated or cooled at all. Even when the air feels like the heater's warming it up, I don't think the inside temp ever gets anywhere near the temp set on the wheel. Also, on EC mode, every window fogs up real quick and it starts feeling humid in the car.

What should I be looking at? The a/c works pretty happily so I would guess the push-button unit's okay...

Thanks in advance!

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1991 Ford F-350, work in progress
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  #2  
Old 11-16-2002, 06:17 PM
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Monovalve
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  #3  
Old 11-16-2002, 07:06 PM
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I thought that only some cars had the monovalve i.e. cars built before a certain date do and cars built after a certain date don't. I thoughthe earlier cars had the monovalve. Am I wrong?

Thanks!
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2001 VW Jetta TDI, 5 speed, daily driver
1991 Ford F-350, work in progress
1984 Ford F-250 4x4, 6.9l turbo diesel, 5 speed manual
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  #4  
Old 11-16-2002, 07:16 PM
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Monovalve?

Having similar problems but whatz a "monovalve"?

And yes, I am a neophyte.
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  #5  
Old 11-16-2002, 07:23 PM
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Hey, another Bay Area type. Welcome aboard!!

I believe the monovalve controls coolant flow through the heater core. Don't quote me on that, though.

Also, on the monovalve...if that's what it is, I looked on Fastlane and they have a repair kit listed. Can that be used, or do I have to replace the whole bloody thing?

Thanks!
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2001 VW Jetta TDI, 5 speed, daily driver
1991 Ford F-350, work in progress
1984 Ford F-250 4x4, 6.9l turbo diesel, 5 speed manual
Previous oilburners: 1980 IH Scout, 1984 E-350, 1985 M-B 300D, 1979 M-B 300SD, 1983 M-B 300D
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  #6  
Old 11-17-2002, 10:56 AM
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That's what it does and I believe the repair kit is the trick. You also might be able to use the monovalve from a 671TIB Detroit(kidding)
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  #7  
Old 11-17-2002, 10:18 PM
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According to my manual, in the EC mode, the A/C should not operate (Economy Mode= no A/C) so I'm not sure why the windows are fogging up. I concur though on rebuilding the monovalve.
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  #8  
Old 11-17-2002, 11:41 PM
rob_frick
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Tim,
I had this same thing going on with my '82 as soon as it got cold.
I got the monovalve repair kit from Fastlane and in 5 min. it was done. Man what a difference. I thought I had problems with the t-stat and heater core becaust the valve would move in and out like it is supposed to do, but with the new parts in, it blows and and stays blowing hot.
Robert
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  #9  
Old 11-18-2002, 02:06 AM
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Quote:
Originally posted by TXBill
The windows fogged up on EC because the air (most likely humid air) is coming into the closed car. This humid air is warmer than the air outside the car, so you have condensation on the inside of the windows.
That makes sense, except that the air isn't really humid around here. Also, it gets noticeably more humid in the car (not just the windows fogging; the air itself actually starts feeling more humid) when the unit's set to EC, even compared to the unit being off altogether. I'm wondering if there may be water trapped around the heater core or something. Is the windows fogging and air getting humid with the ACC set to EC a common thing with Benzes? I've never seen this on any other vehicle I've had (except my old '65 Chevelle, where the windows would fog up in 90° weather ). I haven't tried putting the anti-fog Rain-X stuff on the insides of the windows yet, although that will be my next step after getting the monovalve done (going to order the parts from Fastlane as soon as I finish this post).

Also, Bill, where is the monovalve located on a 123? I went over to the aforementioned page (thanks for that; I had the page bookmarked before my computer took a dump) and the only description I saw on a location was on a 126.

Thank you all thus far
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2001 VW Jetta TDI, 5 speed, daily driver
1991 Ford F-350, work in progress
1984 Ford F-250 4x4, 6.9l turbo diesel, 5 speed manual
Previous oilburners: 1980 IH Scout, 1984 E-350, 1985 M-B 300D, 1979 M-B 300SD, 1983 M-B 300D
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  #10  
Old 11-18-2002, 08:59 AM
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My .02

The AC pump should come on with the defroster. This is a normal operation in most cars. My Cadillac does this, but I cannot defeat the pump in defrost mode. If I hit EC, the defroster is diverted to heater mode.

I live in San Jose, so my weather is the same as yours, and if I didn't have the AC pump running in defrost mode, I wouldn't be able to see for quite a while.
If you are perhaps confused with the AC pump running while in heat mode, the hot water easily defeats the cooling effect of the AC.
Having the AC dehumidify the air while in heat mode, makes for a much more comfortable environment in a car.

Fix your monovalve first, then report back on the humidity level inside your car. There may be some leak somewhere but that would show up as a level drop.

More likely, the AC drain is plugged. The AC is very adept at removing moisture from the air, but this moisture needs to leave the car by a drain. Check the drain hose (under the car, sorry).
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1981 300CD (Benzina)
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1976 Jaguar XJS-saved a V-12 from the chevy curse, what a great engine!
1988 Cadillac Eldorado (better car than you might think!)
1988 Yamaha Venture (better than a Wing!)
1977 Suzuki GS750B
1976 Yamaha XS 650 (sold)
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  #11  
Old 11-18-2002, 08:22 PM
rob_frick
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Tim,
The monovalve is located on the firewall just to the right (from in front of the car) of the battery. There are (4) black heater hoses converging into the valve body. There are (4) screws that hold the valve solenoid. The rebuild kit includes a new valve piston with new rubber washers. All that is required to re-build is to pull out the old piston, and replace with the new one. Then replace the spring washer that is between the solenoid and the valve body. This is a very straight forward proceedure.

Robert
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  #12  
Old 11-20-2002, 11:43 PM
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My 85 300CD has the problem of heat for a moment, then ambient temp air. If you bypass the monovalve won't the coolant circulate unimpeded (and unregulated of course) through the core? What are the odds of the core packing up...mine doesn't leak, but I am really suspicious of a clogged tube...
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  #13  
Old 11-21-2002, 01:22 AM
MVK MVK is offline
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Monovalve location photo by MVK

Its a monovalve almost certain. Check my past link at

Image- Location of Monovalve on 1985 300D Turbo

Also the auxillary water recirculating pump can be culprit but that is about 5% chances 95% its the Monovalve.
No you dont have to buy the whole valve unless the valve body is cracked( and you will se a lot of coolant leak around it). The mono valve stem is the only thing you need. This is the part of the repair kit. I did mine last year and wow it heats good now.

MVK
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  #14  
Old 11-21-2002, 01:33 AM
MVK MVK is offline
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Humidity from your exhaled air

Ok guys.
In the winter time or cooler weather, the humidity from out exhaled breath( consistes of lot of water vapor) quickly condenses to the wall of glass because the glass is cooler than the body temp.
In summer it takes longer to fog up as the glass is not that cold.

When you turn the defrost on, the AC compressor kicks in and the air in the cabin is dehumidified no matter where you set the temp dial. This principle is true for all the cars.

In MB setting to defrost kicks on AC compressor and automatically overrides the temp dial setting and sends warm air from defrost vents.

MVK
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  #15  
Old 11-21-2002, 08:33 AM
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...swuut I said !

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1981 300CD (Benzina)
1968 250 S (Gina) 266,000 miles!
1983 Alfa Romeo GTV6 (Guido)
1976 Jaguar XJS-saved a V-12 from the chevy curse, what a great engine!
1988 Cadillac Eldorado (better car than you might think!)
1988 Yamaha Venture (better than a Wing!)
1977 Suzuki GS750B
1976 Yamaha XS 650 (sold)
1991 Suzuki GSX1100G (Shafty Gixser)
1981 Yamaha VX920RH (Euro "Virago")
Solex Moped
1975 Dodge P/U camper


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