Odd problem with heat/AC
 

I have a rather odd problem with my acc unit. I thought I knew what the issue was by reading old threads but it didn't help the problem. The issue is I am only getting heat out of the outer vents, no matter where the temp wheel is placed, only hot air. The AC comp still comes on, also, when placed on AUTO, I don't get any other fan speed but low. I have tried another known good acc unit, but the same thing still happens. Any idea?
Thanks

1976 280C
1976 280S
1982 300D
1983 300TD (Ivory)
1983 300TD (gold)
1985 300TD (gray)
1987 190D 2.5
1970 280SEL (sold)
1977 240D 4 speed (sold)
1974 280 (rusted to death)
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1927 Chevy 4 door sedan
1938 Chevy 2 door sedan
1950 Willys Jeepster
1955 Studebaker President 4 door
1977 Ford F250
1979 Glastron 17'
1948 John Deere A

Which one of the FLEET has this problem?????

I will give seven to one odds it's not the Deere!!!

Maybe it's the boat. :D

OMG.....I must be losing my mind.....Its for the ivory 300TD wagon. Thats funny, I could never get the heat to work on the Deere.....haha..
Thanks again! :D

The vacuum valve is bad....need replacing...and to do it it requires that you remove the dash...so if you decide to do it, you may as well replace them all while you're at it, and consider the whole thing....afterall, that's not a job you want to do twice...

This link may help you: http://www.peachparts.com/shopforum/showthread.php?postid=526705#post526705

I know this one will, it shows the piece that's bad on your car....: http://www.peachparts.com/shopforum/showthread.php?t=131908

Good luck, if you want to do it twice, my wifes 300D needs this done.... :)

Is temperature and fan speed controlled by vacuum?

No, I don't think vacuum is the problem. If only the center vents didn't work, then it would be vac, but the heat is on all the time, no matter where the wheel is set, and the fan speeds don't change. I'm thinking resistor, but why won't the heat shut off, bad mono? But why all at once? Luck?

Guess I should have specified that I was referring to the no air from center vents part of the question....

When my Alternator went out a few weeks ago, all I got was hot air and I couldn't turn it off no matter what.

Not saying that's what's wrong with yours, but I had that and other weird things happen till I fixed it.

If the monovalve is defective or not getting 12 volts to the coil you will have coolant flow through the heater core all the time.

Temp sensor?

Do you get high speed fan when you press the top button and low speed when you press the bottom one?

Yes