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Old 06-29-2003, 03:58 PM
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Join Date: Nov 2002
Location: Dallas
Posts: 802
Swapped the CCU, the problem came back - new theory, still need help with a/c diagnos

1984 300SD....

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

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)
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Old 06-29-2003, 04:34 PM
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Join Date: Aug 2001
Location: Evansville, Indiana
Posts: 8,150
Sounds like the resistor network for the blower is bad, or the blower itself isn't working. It will run down to low speed, then come back up as needed. It sounds like your's is simply shutting the fan off -- if the high resistance coil (low speed) is bad, no current to the blower.

Do you get heat if you turn the temp control all the way up, but not to the final "click" that turns on full heat no matter what? If not, I'd suspect either some bad switchover valves (behind the PBCU) or bad servos. The valves switch the air flow around to the appropriate vents (heat to the floor, cool air out the side and center vents, and defrost to the windshield), and if you have a broken vac line or a dead servo, the air may not be going where it is supposed to.

Only way to check is get a hand vac pump and locate the servos and test them. They must hold vac indefinitely.


1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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