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  #1  
Old 06-23-2003, 12:14 PM
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Join Date: Nov 2002
Location: Dallas
Posts: 802
'84 300SD W126 - A/C clicks off dead once it reaches dialed in temp

From another thread, someone suggested I ask a tech - help!:

'84 300SD W126 - A/C Question - the saga continues

So here goes:

"So, I finally pulled the stereo, checked to see if the connnections were snug on the back of the thermostat wheel and the Climate Control buttons in the center - still no difference. Basically my A/C is operating like a house system - it cools down to the desired / dialed in temperature, then cuts off completely. When the temperature rises in the cabin and then the system 100% comes back to life. Here is some more detail:

When I dial in the temp between 18*- 20* C and just leave it alone in "Auto" using the normal A/C button (one white arrow), the A/C kicks in fine, will vary the blower speed as needed to cool the cabin, side and center vents blow, then "click" - it shuts down 100%. When I mean 100% you can not get the high or low fan to turn on at all. Just still dead air in the car until the temp rises again, and you hear the "click," and the A/C slowly kicks in again, varies the fan speed as needed, then “click” – it’s off 100% again.

Now while it is on its cool down cycle and the A/C is blowing you CAN cycle the high / auto / low, but once it cools off - "click" - dead still air - the fan speed buttons will not respond. It does not matter if I hit / cycle the "O" off button and then the normal A/C button or any combination of the fan buttons (high/auto/off). No A/C or fan until the temp rises in the cabin past where I have it dialed in on the wheel. While cycling between normal A/C and Off I think I can still hear the compressor engage, just nothing blows.

I did try the following. If I left the fan on high during the cool down cycle, it would cool down on high fan speed, cut off, dead air (temp rises in car), then the A/C would "click" and recover on the "high" setting."

So - Do I gamble and buy a $250 rebulit CCU (or whatever the horizontal pushbutton unit is called between the thermostat wheel and the fan switches)? That's an expensive guess
__________________
~shell
As of 2/2010:
2001 CLK55 0o\=*=/o0
13.6 @ 106mph
10K mi
1984 300SD
260K mi and going and going...
97 S600
46K miles
1991 Sentra SE-R (extremely dorked with)
www.se-r.net
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  #2  
Old 06-23-2003, 06:15 PM
olderdude
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bodyart27

I also have a ‘84SD, and mine would blow hot air out of the defrosting vents as well as other malfunctions no matter what I did – including replacing the “mono valve”. Finally got fed up and pulled the CCU unit (or whatever its called) and took it apart completely. Found where the sides connect to the base circuit board - all the solder joints were cracked. Got a cheap-o soldering iron from Radio Shack and some silver solder and re-did all-solder joints. Has been working as designed since then (around 6months).

It is quite simple too do if one is somewhat mechanically inclined and remembers where everything goes. Making a schematic of how it comes apart helps a lot in re-assembling.

Thomaspin (member of this forum) has an excellant web site on w-126s - check it out

One thing to consider: Once the unit is taken apart, most rebuilders won’t accept it for a core!

Good Luck

Jim
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  #3  
Old 06-23-2003, 06:26 PM
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Join Date: Nov 2002
Location: Dallas
Posts: 802
thought about that

I love Thomaspin's site - it's the best. Unfortunately that is not something he has covered.

I'm *thinking* it's the CCU, but I want to be sure before I swap it for a rebuilt unit (as you mentioned - the core has to go back). I'm tempted to try to solder, but I don't want to risk it (lower pain in the rear factor to swap?).
__________________
~shell
As of 2/2010:
2001 CLK55 0o\=*=/o0
13.6 @ 106mph
10K mi
1984 300SD
260K mi and going and going...
97 S600
46K miles
1991 Sentra SE-R (extremely dorked with)
www.se-r.net
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  #4  
Old 06-23-2003, 06:39 PM
olderdude
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Time-consuming more than anything. Pretty self-explanatory once you remove the radio. Radio has to come out in order too remove two screws that hold the wood covering.

Jim
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  #5  
Old 06-24-2003, 11:34 AM
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Join Date: Nov 2002
Location: Dallas
Posts: 802
so the CCU wins?

I'm hope I have consensus it is the CCU. I'll order one at the end of the week if no one has any other thoughts.... thanks!
__________________
~shell
As of 2/2010:
2001 CLK55 0o\=*=/o0
13.6 @ 106mph
10K mi
1984 300SD
260K mi and going and going...
97 S600
46K miles
1991 Sentra SE-R (extremely dorked with)
www.se-r.net
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  #6  
Old 06-29-2003, 05:00 PM
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Join Date: Nov 2002
Location: Dallas
Posts: 802
bad news after CCU swap - new theory - HELP!!!

So I switched out the CCU (the big electronic controller that has the temp wheel, climate control push buttons and the fan speed) and the problem came back. I say “came back” because for about 25 minutes it seemed to be working perfect. I believe I could hear the system opening and closing vents and redirecting airflow as I was driving. Then about 5 minutes from home the system cut out completely like before - dead air - no response from the fan control. Just like before, I could cycle the system “off” and then back to norm A/C, but with no response from the CCU until the cabin warmed back up above the dialed in temp which would trigger the system back to life with a "click," and it starts cooling / blowing again.

I took the kick panel off below the glove box and wiggled wires like crazy to see if there were any loose connections down there - nothing. I uninstalled / installed the CCU again to see if I could see and broken solders (sp?) on the multi pin connectors that flank the CCU – they looked good.

I did discover that the temp wheel if wiggled would send the CCU into a clicking frenzy. Even with everything mounted up tight - if you give it a little grab and jiggled it during cool down you can make it click like a machine gun. Weird huh.

So - here is my latest theory. When it did work when I first swapped the CCU (hopefully I was not imagining anything), does the system in fact vary airflow through different parts of the dash automatically when cooling? Does the system direct airflow one way for max cooling then switchover to another path for maintaining the temp? If so, I don’t think my system gets that far in the process. In my situation the fan speed does decrease to a lower setting as it cools down, then “click” it shuts down, but the airflow is never changed to other vents.

I'm wondering if it is at that point (when it should change airflow from rapid cool to maintain) perhaps there is resistance when it tries switching the vents, which is shutting the CCU down? If that is probable, what part of the system controls the opening and closing of vents when the A/C is running? Is it a relay?

I'm frustrated as can be - when it was working (though briefly) it was awesome.

For more history – here is the original thread:

'84 300SD W126 - A/C Question - the saga continues
__________________
~shell
As of 2/2010:
2001 CLK55 0o\=*=/o0
13.6 @ 106mph
10K mi
1984 300SD
260K mi and going and going...
97 S600
46K miles
1991 Sentra SE-R (extremely dorked with)
www.se-r.net
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  #7  
Old 06-29-2003, 05:18 PM
Moderator
 
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
The blower speed on your car is controlled by the blower regulator with inputs from the push button controller.

If your blower completely shuts down I would check for a bad blower or ignition switch or fuse.

What I absolutely wouldn't do is replace any good parts. If you can't test then find someone who can. This should be real easy to test with a wiring diagram and a plan.
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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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  #8  
Old 08-12-2005, 04:19 PM
RAYMOND485
 
Join Date: May 2005
Location: CALIF
Posts: 508
A/c Control 300d

1984 Mercedes 300d Turbo
I Replace The Etr Relay Behind The Glove Box $41.00 It Has A Probe
Going Into The Evap Box To Cut Out The A/c From Freeze Temp. Super
Cold Now. **************.com Has A Blower Brush Kit $27.00 With Instructions
Also I Replace The Blower And Temp Control Box Behind Glove Box
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