I have a 1983 300D and it has been only blowing hot air out of the vents no matter what setting I have the climate control on. No matter if I have the dial set on hot or cold.....AC or defrost.....setting doesn't matter. Well it will change which vents are blowing air between AC and defrost but no matter what it will be hot.

So what I have found is that the number 14 fuse is burning out and that is when this condition starts. The 14 fuse is a white 8A fuse. I can replace it and everything will work fine for about 2 to 3 days and then it will burn out the fuse and then obviously heat out of the vents again.

So 5 days ago I replaced the fuse once again and unhooked the mono valve to see if I could isolate the trouble. So far the fuse has not burned up. I am believing more and more that this is some type of mono valve issue. Does it sound like a mono valve repair kit will help me or do I need to replace the whole thing?

I checked the mono valve diaphram which I assume is the area with the fine filtering screen and it is not tore at all, but it does appear to be dirty.

Any help would be appreciated.

Thanks

unplug kickdown switch behind accelerator pedal.
see what happens/post back.

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1984 300D Turbo - 231k....totalled 11/30/07 RIP

What does the kickdown switch do? Can you describe what it looks like? Also I have been searching the forums and this is the first time I have seen someone mention the kickdown switch possibly being related to this type of issue that I am having.....is the kickdown off of that same fuse number 14?

the kickdown downshifts the trans. yes it is on the fuse 14 circuit. unplug it and see if it solves the heat on all the time issue.

here's a pic

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1984 300D Turbo - 231k....totalled 11/30/07 RIP

did you make any progress sofar?

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1984 300D Turbo - 231k....totalled 11/30/07 RIP

Check the bundled vacuum lines that run next to the batt.

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Toblin

'79 300D, "Liesel von Diesel", 235K

I kid proofed the house....but they still get in

Ok I found out why fuse 14 was burning up by doing further research on these forums. Turns out there is a black wire that runs next to the transmission and it was zip tied to the transmission in two different places. Sure enough at one of of these spots the black plastic had worn out a spot so bare wire was exposed and it was shorting out there. Wrapped it with some electrical tape and now I have solved the fuse 14 problem. So just I put in a new fuse in slot 14 but I am still having the full heat problem. Everything else is working on fuse 14 so I know thats not the issue. What is weird is before I found the bare wire I could still replace fuse 14 and my heat problem went away for 2 to 3 days until the bare wire shorted out fuse 14. So now it appears I have a secondary issue. I replaced the mono valve with a new insert (repair kit) but this didn't help.

Is it possible that the coil winding that goes around the mono valve insert is somehow faulty? I plan on taking a volt meter and checking it tomorrow.

The mono valve repair kit usually solves the opposite problem from the one that you are having, ie: no heat or intermittent heat.
The mono valve is "spring loaded" to the full heat position and is closed (or regulated) electrically. So it sounds like your problem is most likely due to a lack of electrical power to the mono valve. I would start by checking the monovalve connector for voltage.

foam tube behind glove box in good shape?

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1984 300D Turbo - 231k....totalled 11/30/07 RIP

right. it's my understanding that the signal to close the monovalve ie shut heat off is a ground not 12v..
as it defaults to full heat when the #14 fuse blows..

i would also check the plug to the monovalve ie. take the plug itself apart and clean.

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1984 300D Turbo - 231k....totalled 11/30/07 RIP

Here is some other additional information. You know at the connector on top of the mono valve there is a black plastic piece that snaps on top of the mono valve and it houses the two electrical wires on the mono valve? Then on top of this little black housing there is a small top piece that snaps on top of that.......well I lost this little black piece that snaps on top. I still have the piece that the wire ends actually sit in and have just taken electrical tape over the top of this since I lost the black capping piece. But I can't imagine this making a difference because there is nothing electronic in that black cap that I know of. And as long as as those wires are held into place securely that shouldn't be my problem.

yes i know exactly what you're talking about. i would take the wires out and after cleaning, squeeze the end connectors tight together (they loosen up over time), then replace into the holder and plug back in.

one of my problems was that those ends wre corroded and had loosened up so the valve wasn't getting the proper signal to turn 'off'.

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1984 300D Turbo - 231k....totalled 11/30/07 RIP

Thanks for the quick reply. Yeah when I was messing with them yesterday I thought they seemed kinda loose so I will tighten them a bit and clean them up.

Well I crimped the ends and they are definately much tighter on the posts of the mono valve now but it didn't fix it.

Where do the two wires go once they go through the firewall and into the cab of the car? Someone said earlier about checking some foam behind the glove box......what is that about?

Here is another update but I am not sure if this is associated with my full heat problem. Now my engine will not shutdown when I turn the ignition key off which might indicate a vacuum problem. Is there anyway this could be associated with my heater problem?