After searching the website I have not been able to find where any one else has had the same symptoms that I am having with my 300. But similar.

I have a 79 300SD that is blowing very hot air through the A/C vents on the right and left but air does not blow through the center vents. Also hot air blows through the heater vents on the floor. When the A/C is not on hot air still comes through the vents. The faster you drive the hotter it is and when the A/C is on it blows fairly cool at an idle. When you stop the A/C is able to cool the car down again. I live in the Dallas Texas area it has been 100 degrees lately and it is unbearable to drive my car at this time.

The compressor has been replaced with a new one (Not a reman.), receiver/drier, low pressure hoses and recharged with a low pressue of 30 and the high pressure is 300. The monovalve was leaking coolant and I replaced it with one from the salvage yard from 1980 model that fortunately is made of metal instead of plastic. After reading through portions of the forum I took a pair of needle nose vise-grips and effectively shut off the heater hose on the drivers side of the cylinder head and there did not seam to be any change.

The car has been sitting in my grandfathers hay barn for the last 6 years with 135,000 miles. The only thing he was aware of is that it had a leak that I fixed with new hoses.

Your help is appreciated.

Tony

Check all the vacuum lines for leaks. It is obvious that you are getting outside hot air blown in through an open recirculation door, but if the air is too hot to stand then you might be still getting hot coolant through that neddle nose. I found that the best way to avoid the problem with the monovalve is to just bypass it all together with a hose from the left side of the head to the pipe that the monovalve drains to on the other side of the eng. I did this on my 84 300sd (sold)

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87 300d Turbo 241K with original #14 head (blue)
87 300d Turbo 198K will run great again someday (silver)
84 300sd sold ( buyer drove it to Paraguay )

You'll need to locate the big ugly servo unit on the passenger(?) side of the engine, youre going to have to clamp off one of the hoses from it- I'm recalling from memory as none of the cars I've owned has had one, and I've rarely touched 116 body SD's. I've helped a few of my friends with them in the past, but only on 123's and 107's etc....

keep us posted if that works...

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1980 500SE/AMG Euro
1981 500SEL Euro
1982 380SEL
1983 300TD
1983 500SEC/AMG Euro
1984 500SEC
1984 300TD Euro
1986 190E 2.3-16
1986 190E 2.3
1987 300D
1997 C36 AMG
2003 C320T 4matic

past: 1969 280SE 4.5 | 1978 240D | 1978 300D | 1981 300SD | 1981 300SD | 1982 300CD | 1983 300CD | 1983 300SD | 1983 380SEC | 1984 300D | 1984 300D | 1984 300TD | 1984 500SEL | 1984 300SD | 1985 300D | 1986 300E | 1986 560SEL | 1986 560SEL/Carat | 1987 560SEC | 1991 300D 2.5 | 2006 R350

The center vents only blow ambiant temp air when the dial is set at the right point....nothing comes out at other times.....

the '79 300SD does not have a monovalve...it has the evil vacuum servo designed by some guy in Chrysler that spent the remainder of his life in a straightjacket in a padded cell in the 60's. I suspect LSD had a part in its design but I can't prove that.

These will die and would say few from the bone yards will actually work......unless you saw it work on the car it came off of.

There is a related servo amp behind the glove box. One can fry the other....recomend replaceing both at the same time.

__________________
Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
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Section 609 MVAC Certified
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"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

Thank you for the replies.

Just to clarify, it seems like the heat is coming from the heater core. Very hot. Or it is coming from the engine compartment.

What setting on the dial will cause the A/C to blow through the center vents? I have it maxed out now. Is the servo the square looking thing with black plastic on top with vacum lines going into it on the passengerside of the firewall? Can you be more specific about the location of the, "servo amps", behind the glovebox? What do they look like? Are the amps a dealer only item?

Is it possible that the fresh air door is broken?

Thank you

Tony

Also, I did not see the servo work before buying it at the salvage yard. Are there rebuild kits for the servo?

Thank you

Tony

GREAT POST!!!!!!!!!!!!

I hope there are some who noticed that by indicating the model and year, a good post title and a good description of the problem, that Tony has gotten lots of good replies!

Thank you,

You will have to buy a rebuilt one...its not user friendly inside one I hear. Call Phil at fastlane...even if its not in the online catalog he can still get it...do the servo amp at the same time to play it safe. I never took one appart but they are not something joe average could put back together correctly. I paid $350 + core charge for mine about a year ago.....the servo amp is just under $100 I think.

They don't call these the "evil vacume servo" for nothing.


And there is NO setting that will let cold a/c blow through the center vents.....why did they build it that way I can't answer...its just the way it is.

__________________
Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
---------------------
Section 609 MVAC Certified
---------------------
"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

The servo amp has 2 large areas where it appears to be burned and 1 small area on the circuit board. It seems to have a slight smell like it might be burned. After reading some of the post I was wondering if it could be resoldered or should it be replaced as it appears to be burnt?

Since I purchased the servo from a salvage yard and do not know if the amp was burned prior to installing the servo, is it ok to install a new amp and test it first? Or will the new amp end up being burned instantly without an opportunity to even test to see if the servo is ok, making it necessary to replace both the servo and the amp again?

Thank you

Tony

If the servo is siezed up you mightl toast the amp.....its a gamble....the amps not reparible...if its burnt its history.

__________________
Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
---------------------
Section 609 MVAC Certified
---------------------
"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

After installing a new servo amplifier the A/C is working much better. In addition to no longer having heat coming from the side vents and floor vents the center vents are now working. It is actually putting out cool air instead of heat.

I went ahead and took a chance on the salvage yard servo being ok and installed the new amplifier. The store were I bought it said that if the servo ended up frying the amplifier they would warranty it. So there was no way I could lose.

What a relief. This is my first Mercedes and I know very little about these cars. I was starting to think about taking the car to a mechanic. That would have cost even more money.

I was wondering about the amount of oil to put in the compressor. Normally you put 8 ounces of oil in the system when install a new compressor (GM compressor). But a well estblished AC repair shop in the Dallas area, tells me that because of the design of the 300SD it is a good idea to use 10 ounces. Is this true given the design of the compressor being along the bottom area of the engine?

Thank you for all the replies. You have been a big help and I appreciate it.

Tony

Tony - let me know if it holds. I changed my amp (but did not change the servo) about 3 weeks ago due to the same problems you were having. Today, it started back w/ the same thing - hot air out of the side vents, no matter what temp I set it on. Just like before though, it is intermitent. If I turn the A/C off, it sometimes blows cold when I turn it back on.

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Kip

1966 230SL
1969 300SEL 6.3
1985 300TDT
1990 350SDL
1992 500E
2002 SL500
2008 GL320 CDI