undiscussed acc problem - PeachParts Mercedes-Benz Forum

augusta · #1 12-16-2005, 05:14 PM

I have searched this forum and not found a problem like mine. I hope someone can give me some help.

My car is a 1979 300sd.

In the summer the acc works perfectly.(auto-low and auto-high work ) Thanks to Larry Bible for talking me into keeping it R12 freon...Even in Augusta Ga, in the summer, it will keep me cool.

Now the problem, when I try to use my heater the auto-low and auto-high buttons will not work until I press the defrost or bi-level button. Once I press either of those buttons, the system works as it was designed.

If I shut off the car, I once again have to push the defrost or bi-level button to get the heater to work correctly. It is not a huge problem since I can get the system to work...but I would like it to work as it came out of the factory.

My thoughts are, if there are loose solder connections or if the servo is broken, why would work after pushing the defrost or bi-level buttons.

Thanks
Augusta

whunter · #2 12-16-2005, 08:58 PM

This is a very common issue.

Here is the fix:

Fixing that Damn Climate Control!
http://www.diymbrepair.com/Climate/

boneheaddoctor · #3 12-16-2005, 10:40 PM

Quote:

Originally Posted by whunter

This is a very common issue.

Here is the fix:

Fixing that Damn Climate Control!
http://www.diymbrepair.com/Climate/

wrong one Roy......he has the Chrysler designed monstrosity in his....its a W116 remember. The vacuum servo the Chrysler Imperial used before they sold it to Benz.

http://www.imperialclub.com/Repair/Lit/Master/281/cover.htm

Tymbrymi · #4 12-17-2005, 02:04 AM

Quote:

Originally Posted by augusta

Now the problem, when I try to use my heater the auto-low and auto-high buttons will not work until I press the defrost or bi-level button. Once I press either of those buttons, the system works as it was designed.

The Type II ACC (vertical buttons) has a sensor on the bottom of the Servo housing that senses coolant temperature. Its purpose in life is to disable the fan until the coolant temperature has reached 104F (40C) when the system is in heating mode. The system ignores this sensor when DEF or BI-LEV is pressed, and once those buttons are pressed and the system is operating in AUTO-LOW/HIGH the fan will continue to operate regardless of the sensor.

Gotta ask... are you waiting till the coolant reaches 104F?

If you are, it sure sounds like something in that part of the system is fubar.

A quick test for the servo sensor is to connect the two vacuum lines that go to the bottom of the servo. I'm too tired to dig up more references right now, but if that doesn't turn out to be the problem I'll dig it up.

You could always look into this as well

The Approach, Evaluation, and Exec. Summary are the only docs that really mean anything (and they could be much better... too bad time says no). Whenever the hardware itself is in a more presentable condition, I will be posting a good bit about it

#5 12-17-2005, 02:37 AM

Sounds sorta coolant temp sensor fishy to me...

On the w123 and w126 body with auto climate control replacement of this unit is simple and fast

Its behind the thermostat housing next to the EGR vac switch. Single pin with brass housing.

augusta · #6 12-17-2005, 08:09 AM

Trying to use the heat, the auto-high or auto-low buttons do not work at any tempature. I think Tymbrymi has the answer. I realized there was a tempature sensor somewhere in the heating system, but I had no idea where.

One last question can this sensor be replaced or would I have to replace the servo?.....I think I know the answer to my question...replace the servo. If that is the answer I think I can live with the problem

I am going out and pull the two vacuum lines

All of you, thanks for your help

Augusta

whunter · #7 12-17-2005, 10:47 AM

Quote:

Originally Posted by boneheaddoctor

wrong one Roy......he has the Chrysler designed monstrosity in his....its a W116 remember. The vacuum servo the Chrysler Imperial used before they sold it to Benz.

http://www.imperialclub.com/Repair/Lit/Master/281/cover.htm

Darn.

That is what I get for trying to answer a question when I have a nasty case of the flue.

Proving once again that I am human.

Fisherman · #8 12-17-2005, 12:07 PM

The sensor is integrated into the bottom of the ACC Servo mounted on the passenger front of the engine coompartment. I believe the vacuum lines going to it are yellow and black. Basically what it does is not allow any vacuum until the sensor reaches the correct temp. One line is the vacuum source line (should always have vacuum) and the other is the feed line to the rest of the system. As I remember you can unplug these from teh bottom of the servo and just connect them to each other and the system will function below normal operating temps. Very simple test and it is in fact the first one in the troubleshooting guide for this Servo based system.

That servo system is a real pain. There are so many variables in it (look at the number of vacuum lines and electrical connections going into the thing) that most everyone seems to either end up buying a replacement or bypassing it. However this is usually upon failiure when the body cracks which is the most common failure. If your issue is only that temp sensor then you are way ahead of the game.

boneheaddoctor · #9 12-17-2005, 12:25 PM

Quote:

Originally Posted by whunter

Darn.

That is what I get for trying to answer a question when I have a nasty case of the flue.

Proving once again that I am human.

I make mistakes too...we all do....hope you get over that flu quick....I still am not completely over that pneumonia or whatever it was yet...Seeing a cardiologist tuesday and a pulmonologist Wednesday....2 months now still have chest conjestion.

augusta · #10 12-17-2005, 03:47 PM

I connected them together and it did not change anything. I think I am going to quit while I am ahead. I am afraid if I start messing around with 26 year old vaccum lines that I could create a bigger problem. I guess I should feel lucky that this is the only issue I have with the acc and servo