defrost air is really hot - PeachParts Mercedes-Benz Forum

triacon · #1 09-10-2009, 01:08 PM

1985 300SD, the defrost air is super hot no matter how I set the temp dial. I know a valve is missing and vac hoses are plugged. When I removed the glove box liner, I noticed the void and also see a lever that must attach to the valve.

If I figure out what valve that is and order one, install it, will that correct the problem...?

compu_85 · #2 09-10-2009, 01:27 PM

Is it hot all the time or just when you push the maximum defrost button?

-Jason

rrgrassi · #3 09-10-2009, 02:21 PM

Check the monovalve.

triacon · #4 09-10-2009, 02:24 PM

when I start the car cold, the defrost is cold, but if i keep it on while driving and the engine warms up, it becomes hot air in the car.

if climate control is off and I have been driving for awhile, then press defrost button in, I get hot air.

AC works, very cold

vent works, I can vary the temp with vent

triacon · #5 09-10-2009, 02:25 PM

Is the monovalve the valve that I mentioned in first place. The one I am missing?

jimishadow · #6 09-10-2009, 02:30 PM

if you are pushing the end button on left it will always blow full hot.if you want defrost for the car to be cool use the button to the right of full derost and it will blow at temp you set.look in owners manuel

rrgrassi · #7 09-10-2009, 02:31 PM

The mono valve is under the hood.

IF the A/C is cold and the temp varies at the vents with the wheel, your monovalve sounds like it operating as designed.

IIRC, if in defrost mode, it's full fan and full heat.

tangofox007 · #8 09-10-2009, 02:38 PM

Quote:

Originally Posted by rrgrassi

IIRC, if in defrost mode, it's full fan and full heat.

And air conditioning. The temp selector does not function in the defrost mode.

Quote:

Originally Posted by triacon

if climate control is off and I have been driving for awhile, then press defrost button in, I get hot air.

"Defrost" is intended to melt ice off the windshield. Why should the air be anything but hot?

triacon · #9 09-10-2009, 02:43 PM

thanks for the info. I did not get an owners manual with the car.

wow, it's really blowing hot hot air. you can probably cook food in the car with the defroster.....

rrgrassi · #10 09-10-2009, 02:45 PM

Quote:

Originally Posted by tangofox007

And air conditioning. The temp selector does not function in the defrost mode.

Yep, you are right. I forgot about the compressor engaging as well, allowing dehumidifaction of the air.

matthias08 · #11 09-10-2009, 02:51 PM

I CRACKED A WINDSHIELD! (on a C280 4Matic last winter using the defrost)

be careful!

Oldwolf · #12 09-10-2009, 03:01 PM

My 124 manual says that in "Defrost" mode the blower will go to FULL, the heat will go to FULL and only the defrost vents will be open. This is to quickly melt any ice or frost on the windshield.