80' 300SD Heater Valve - PeachParts Mercedes-Benz Forum

mj_sid · #1 04-02-2003, 08:03 PM

Hello All,
The heater valve in my 1980 300SD was bypassed before i bought it. I was reading the posts here on why my heater was always blowing hot air and discovered the bypass. I am planning on going to pick n pull to get a used valve, but have a few questions about replacing the unit.
1. The vacume lines (im assuming they are vacume lines) at the top of the unit look hard to put on/off. One of them on my car is off and i couldnt put it back on. Does anyone have any suggestions on what im doing wrong.
2. I understand i need to check the bellows on the new valve, is there anything else i should look at?
3. Any other suggestions, comments, ideas would be greatly appreciated.
Thanks,
Matt

Fisherman · #2 04-02-2003, 08:15 PM

I think you have the acc servo.
Has about 12 vacuum lines going into a block on the top of the unit?

If this is the case that whole block is released via one phillips screw that is accessed between the lines in the center of the rear of the vacuum block.

I am unsure about your reference to a bellows....

If it is the servo then make certain there are no cracks in the black housing or apparent leaking that may have taken place in the unit. This is the numberr one cause of failure is the plastic mid section cracks and leaks.

There are a number of tests that you can perform on your system prior to just replacing it and I think you might want to try those first...
There is also a doc circulating around that covers many of the simple tests.
Use the search function at the bottom fo the forum to search for "acc servo"

If you do not have the acc servo but instead have the monovalve then I have been typing nonsense...

Just kidding. If you haven't tried the search feature it will give youa wealth of info...
Good luck on the fix.

mj_sid · #3 04-02-2003, 11:57 PM

I'm not sure whats going on. The problem i have is that the heater is always on blowing really hot air through the defrost vents. After searching the message boards i concluded that the problem was the heater servo unit. I didnt have a clue what a heater servo unit was so i looked around on EBay and found one (

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=2409124051&category=33548). I looked at the same thing on my car and found that it was bypassed on the bottom with a piece of hose and some clamps. My conclusion is that this would cause straight hot water to constantly flow through the heating system thus causing my constant hot air. Ok, so i have some questions. First is my theory correct? and Secondly if so i am concerned about the vacuum lines (are they vacuum lines) on the top of the servo unit. On my car one of the lines is unplugged, i tried to plug it back in but i couldnt. How would i go about plugging them all back in?
Thanks for any help,
Matt

Fisherman · #4 04-03-2003, 05:51 PM

Lots of them all differnt colors on the top fo the unit...

Take a phillips screwdriver and remove teh single screw in the backside of the unti that holds the vacuum "block" in the servo. You don't pull the top off or anything just remove the one screw adn the whole block slips out the back of the unit.

Once you get the block loose you can look at how the other lines are inserted and try to get the loose one back in.

The wire looms that connect there via the flat plugs are removed by lifting on the back of them (they are keyed into the servo) and then pulling them free to the rear...