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  #1  
Old 08-01-2014, 07:52 PM
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can't shut off hot air

i have an 83 300TD and one of the center vents is blowing very hot air, and i can't seem to make it stop. around the dash also feels hotter then normal i believe.

last night i did a valve adjustment and i'm wondering if i knocked something that controls that? for what its worth i noticed yesterday i had blown a fuse on my horn too...don't know if that matters but thought i'd mention since its an electrical issue.

about to go on a long drive and hoping that coolant is circulating correctly and that the car won't overheat....

any thoughts on what i should look in to?

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Old 08-01-2014, 08:20 PM
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Normally that's the mono valve. If it's voltage goes to zero it will open fully and produce full heat. It's controlled with a PWM signal that controls the amount of heat it allows.

Check the voltage at the connector. On mine the connector got sloppy and tightening the connector contact fixed it.

It could also be a torn diaphragm though this happens slowly over time not generally all at once.

You can bypass it or put a constant 12v on the connector as an emergency fix.

Good luck
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Old 08-01-2014, 09:28 PM
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Quote:
Originally Posted by mach4 View Post
It's controlled with a PWM signal that controls the amount of heat it allows.
There is no "PWM signal." The monovalve is controlled via a switched ground; it's either off or on.

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Originally Posted by mach4 View Post
You can bypass it ...as an emergency fix.

Good luck
If you bypass the monovalve, you will need a lot of luck to stay cool. That will result in continuous heat.
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Old 08-01-2014, 09:49 PM
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ok. in reading about monovalves it seems that they are possibly on the same fuse as the horn (#14) so maybe i have a short somewhere and that is all going out together.

not to be an idiot, but when you said "Check the voltage at the connector" how do I do that...

thanks
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Old 08-01-2014, 09:57 PM
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Originally Posted by tangofox007 View Post
There is no "PWM signal." The monovalve is controlled via a switched ground; it's either off or on.
I agree that it is either on or off. If it's not a PWM signal what allows the heat to be something other than full heat or no heat? I was under the impression that by varying the duty cycle you would control the heat. I was also under the impression that the rate was very slow, on the order of a few HZ.



Looking to learn...

Thanks
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Old 08-01-2014, 10:40 PM
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in reading about mono-valves i found that they are on the same fuse (#14) as the horn. found a little wire right behind the transmission pan that has the tiniest bit of insulation worn away (the kickdown switch wire?). taped it up and replaced the fuse. will see if that was causing a short which was blowing a fuse which was keeping my heat on....
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Old 08-01-2014, 11:27 PM
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I agree that it is either on or off. If it's not a PWM signal what allows the heat to be something other than full heat or no heat? I was under the impression that by varying the duty cycle you would control the heat. I was also under the impression that the rate was very slow, on the order of a few HZ.
Historically, the notion has been put forth that there is a high frequency modulation that holds the monovalve in a somewhat steady position between open and closed. Obviously, that is not what you were suggesting, so it looks like we are in agreement about the basic operation of the monovalve.

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