82 300D front defrost only blows hot? - PeachParts Mercedes-Benz Forum

Dkirk · #1 10-06-2020, 07:06 AM

I was out the other evening and tho it was warm, it was quite humid and I had to turn on the front and rear defrost for a few minutes to clear the windshields.

I was quite pleased to find out the rear defrost works, but the front defrost only works in heat mode. I turned the temp. select dial to full cool, but the temp never went down. After a few minutes I had to shut it off, then turn it back on again as the windshield fogged up. During the off interval, I turned on the AC to see if it worked, and it was fine, so I'm thinking maybe the AC compressor doesn't kick on like it does in newer cars or should I add that to the list of things to see if I can fix?

Sugar Bear · #2 10-06-2020, 11:04 AM

IIIRC full heat in defrost/de-ice is how it is designed to work.

Anyone know for certain?

Diseasel300 · #3 10-06-2020, 11:08 AM

Pushing the "Defrost" button forces the fan on high and max heat. That is how it is designed to operate.

If you're in "Automatic" mode (center button), the defrost vent will crack open if you're in the "white" area of the temp wheel.

Otherwise, you can force the defrost flap to open with temp regulation by pushing the button next to defrost (2 white arrows).

jay_bob · #4 10-06-2020, 11:56 AM

The 123 (and others of the era) climate control has the five buttons. This is what they do:

DEF is maximum heat, maximum fan, no compressor. This is actually the fail safe mode if the controls fail.
2nd button (up/down) is for the situation you were in the other night, air to the windshield and the floor and compressor if you need it, with regulated fan speed and temperature. In those days we did not have the fancy variable displacement compressors like we have today so they used reheat to maintain the desired temperature (the compressor runs wide open and the hot water valve is PWM’d to maintain temperature).
3rd button is the normal mode. Air direction, temperature, and fan speed regulated by temperature set point, compressor comes on if needed to meet set point.
4th button is the eco mode, same as the third, but no compressor.
5th button is system off. Ram air only, no heat or cooling.

Dkirk · #5 10-06-2020, 12:03 PM

Ah, I thought so, but I have no one to ask but the forum.

Thanks very much!

vtmbz · #6 10-08-2020, 09:10 AM

This brings up an interesting point. The only way to get reliable heat in my tempmatic is to use the def button, but the Fan Is annoying. What would it take to get the default all heat setting but without so much fan speed? My vacuum pods all seem to function but in any other setting but def the temperature is glitchy to say the least.

Diseasel300 · #7 10-08-2020, 10:30 AM

Quote:

Originally Posted by vtmbz

This brings up an interesting point. The only way to get reliable heat in my tempmatic is to use the def button, but the Fan Is annoying. What would it take to get the default all heat setting but without so much fan speed? My vacuum pods all seem to function but in any other setting but def the temperature is glitchy to say the least.

You should start your own thread on the Tempmatic system. It shares nearly nothing in common with the W123, 124, 126, and R107 systems.

Usaguy · #8 10-10-2020, 08:31 PM

Quote:

Originally Posted by Dkirk

...t the front defrost only works in heat mode. I turned the temp. select dial to full cool, but the temp never went down...

This is normal

Quote:

Originally Posted by jay_bob

The 123 (and others of the era) climate control has the five buttons. This is what they do:

DEF is maximum heat, maximum fan, no compressor. This is actually the fail safe mode if the controls fail.
2nd button (up/down) is for the situation you were in the other night, air to the windshield and the floor and compressor if you need it, with regulated fan speed and temperature. In those days we did not have the fancy variable displacement compressors like we have today so they used reheat to maintain the desired temperature (the compressor runs wide open and the hot water valve is PWM’d to maintain temperature).
3rd button is the normal mode. Air direction, temperature, and fan speed regulated by temperature set point, compressor comes on if needed to meet set point.
4th button is the eco mode, same as the third, but no compressor.
5th button is system off. Ram air only, no heat or cooling.

Let me make a few corrections:

AC compressor is on in the defrost position, bi-level and normal. (1st, 2nd, 3rd)

Also the compressor doesn't come on "if needed" in the 3rd position or any other of the above mentioned. It always keeps the evaporator near freezing regardless of the position of the rotary temp switch.

The "0" button doesn't give you "Ram air". It puts the recirculation flap into "recirculate" position so that you don't get "Ram air" from the outside.