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  #1  
Old 10-18-2008, 10:39 AM
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123 Heat all the time in defrost

Can anyone tell me where to go about diagnosing this issue? 84 123 with auto climate control. All other settings work perfectly. I get a mixture of outside air and heat when in economy mode. And the variable speeds on the fan throttle up and back down as designed. Good and appropriate Heat or Cold from the correct vents when in all other modes. So I can assume, since it mixes heat and cold well and about appropriately, to the setting of the dial, that the monovalve is doing its thing correctly.

However, when in defrost setting I get strong heat all the time. Does not throttle back no matter where the temperature dial is set. Always blows full speed heat all the time out the defrost vents and never throttles back even when the heat is obviously excessive.

Currently I have to push different button to take it off defrost and then occasionally re-engage it to clear the windshield again. Otherwise it gets too hot in there.

Is this normal? Maybe I should read the manual eh? Never get anything but heat from the Defrost vents and setting at all.

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  #2  
Old 10-18-2008, 10:51 AM
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my 79 acts the same way. I am used to it.
I think you can turn on the a/c to regulate temp.... also aids defogging on the windshield.
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Old 10-18-2008, 10:57 AM
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Do you have the owner's manual with your car? That's normal. When you press the defrost button it activates maximum defrost (IE max heat, max fan, and air con). If you only need some defrosting select the button to the right. Notice the arrow point up? That's for the defroster vents, not the panel vents.

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  #4  
Old 10-18-2008, 11:18 AM
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... I had wondered if it was just operator error. Sounds like it is.

I do have an owners manual.... somewhere.

Thanks for the lesson!
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  #5  
Old 10-18-2008, 01:27 PM
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Quote:
Originally Posted by cphilip View Post
However, when in defrost setting I get strong heat all the time. Does not throttle back no matter where the temperature dial is set. Always blows full speed heat all the time out the defrost vents and never throttles back even when the heat is obviously excessive.
That's the way the system is designed to work. Defrost activates maximum heat, maximum blower speed and engages the a/c compressor. The temp and fan speed controls have no affect when Defrost is selected.

If you think about it, there is no way for the ACC to know when your windshield has been successfully defrosted. How could it determine when to "throttle back?"
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  #6  
Old 10-18-2008, 01:43 PM
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Well... if you try and logic it out...

The systems are all in place to KNOW what temperature to put out. In most cars you can set the temperature of the air being used to defrost. The temperature setting is there. The sensor to detect that temperature is there. All the systems exist in the car to have variable temperature defrost. The ability to know a temperature is reached and throttle the fan back and the mix back is there. Because it exists in the other settings.

They just don't appear to be linked or utilized in that defrost setting. Normal I accept. Logical? I dunno... could have been accomplished.

For one thing there are times you defrost with cold (or cool) dry air from the AC. Most cars now a days can do this. In defrost setting. In some cases a blast of Hot air at the wrong time will make things much worse. Often on a Hot day if you run into a cold rain storm you want cool air to hit that windshield in full defrost mode.
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  #7  
Old 10-18-2008, 01:56 PM
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The system was designed to work that way for safety reasons. It puts out max heat with max fan and a/c (to dehumidify the air) to ensure that you have a clear windshield in minimum time. Once the windshield is clear so you can see, you can switch to one of the other buttons, which provide a small "leak" of air to the windshield.

Given that these cars are all over 20 years old and in various states of repair (as we read here every day) and new owners in various states of ignorance (ditto), it's not surprising that there are so many questions. All of us have had to learn how these things work. In good repair and used as intended, they are still excellent systems.

Caveat: when modified to have center-vent heat, of course.

Jeremy
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  #8  
Old 10-18-2008, 01:58 PM
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Quote:
Originally Posted by cphilip View Post

For one thing there are times you defrost with cold (or cool) dry air from the AC. Most cars now a days can do this. In defrost setting. In some cases a blast of Hot air at the wrong time will make things much worse. Often on a Hot day if you run into a cold rain storm you want cool air to hit that windshield in full defrost mode.
You just need to know which button to push. Try the one next to Defrost is you don't want hot air.

And if you are looking for a car with "now a days" technology, an '84 model is bound to be a disappointment, regardless of what the used car salesman said.
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  #9  
Old 10-18-2008, 02:04 PM
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Your system is working as it should. If you wanted cool air to defrost the windshield, use the bi-level or normal mode, it partially emits cool air from the windshield vents to keep them clear.

All hot air here...
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  #10  
Old 10-18-2008, 07:02 PM
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Oh yea... I get cha...

When the Germans meant "Defrost" they meant Defrost for sure.

Defog is another concept all together. And my bad for thinking along those lines... I guess.

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1983 300D-Turbo - Deep Blue w Palomino MB Tex (total loss in fire 1/5/09 RIP)
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