Does this problem go back to the tricky vacuum pods or is it an element or something like that??
I have already replaced the monovalve.

__________________
http://www.libertyagrifuels.com
1984 300D Turbo
1980 300SD Sold
1983 300TD Sold
Dagger 16 ft. Legend Canoe (for the weekends)
Marin Northside Trail Mtn. Bike

I had to replace my CCU to get my car to go from an inferno, to a nice toasty ride. I tried to solder all the joints on the CCU first - still didn't work.

Also, check to make sure the monovalve is getting proper electrical signal, making it open/close when it should.

__________________
1984 300D: 275,000 miles (soon to be for sale)

In search of my 1987 300D in anything but white.

I would have guessed the monovalve. It fixed mine when I had that problem. You did use a new one for a replacement, right?

__________________
"Knowing is not enough, we must apply.
Willing is not enough, we must do." Goethe

***********************************


1951 Chevy 3100

2003 Indian Chief Roadmaster

1983 GMC 1 ton Dually

1982 Chevy 1 ton Dually, service body (sold)

'90 GMC Suburban 6.2 "SS Veg-Burban"
(single tank WVO\diesel conversion) SOLD

'81 300D ~ Mama's car...my job (now my car)(but still my job) SOLD

'83 300sd ~ rescue car SOLD

2005 Ford Taurus (Mama's new car)(NOT my job!)

Yes I used a new one.
IT is weird b/c maybe 5-10% on the time it will blow cold air.
Another quircky thing about my ccu is that it only blow on high mode. No low mode at all.

__________________
http://www.libertyagrifuels.com
1984 300D Turbo
1980 300SD Sold
1983 300TD Sold
Dagger 16 ft. Legend Canoe (for the weekends)
Marin Northside Trail Mtn. Bike

Doesn't sound to me like a vacuum pod issue. Sounds to me like a CCU/Monovalve issue.

Since you just replaced the monovalve (and I can assume a fairly confident that is not the issue??) I also would lean towards the Push Button Unit.

When I first got my SD, my heat would kick on under heavy load (acceleration) no matter what setting I had (on/off, heat/cool). Turned out to be a faulty aux water pump/CCU combination.

__________________
2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD

I would say that pretty much seals the deal. A faulty CCU (push button unit) it is. It could be a faulty blower amp, but that fact that it only blows heat leads me to believe it is the push button unit vs the blower amp.

If you are planning on keeping the car I would be a rebuilt one. I can't think of the name or website off-hand. Maybe GDL?? Do some searching or someone else will chime in.

__________________
2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD

I have already bought a rebuilt unit off of ebay (last fall) and it did not fix the problem.
Last month I pulled a unit out of an identical w123 and it did not change or fix the problem at all too.
I also replaced the blower brushes.

It might be a fuse?? This is interesting. I never thought of this.

__________________
http://www.libertyagrifuels.com
1984 300D Turbo
1980 300SD Sold
1983 300TD Sold
Dagger 16 ft. Legend Canoe (for the weekends)
Marin Northside Trail Mtn. Bike

Well you can't trust ebay units, but since you have stated that you put 3 different CCUs in and had no change for the better or worse, then yes that does sound like it eliminates it.

Then it sounds like you have seperate issues going on.
1- Blower amp
2- Something messing with the cold/hot air flow. Have you checked the aux water pump? Again on mine, that was causing hot air to be put out under load.

__________________
2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD

Seems like if it were a fuse, it would either work, or not work...not go back and forth. I always thought that a fuse was an "all or nothing" type set up.

If it blows cold when you first start the car, then turns to only blowing hot, perhaps your t-stat is stuck open, thereby always allowing hot coolant to pass through.

Also, check to make sure the plug is, indeed, plugged into the top of the monovalve assembly. I believe that when it's unplugged, the monovalve (and subsequently, CCU) defaults to full heat.

Did you pull the blower motor and test directly to 12v?

EDIT: on the aux pump issue, mine is unplugged all together, but I can still get hot and cold air.

__________________
1984 300D: 275,000 miles (soon to be for sale)

In search of my 1987 300D in anything but white.

I have exactly the same problem. My CCU works fine for a day or two until Fuse 14 blows and then it's just hot. Something on fuse 14 (there are many things on this fuse) has an issue and I can't figure out what's causing it to blow.